ZOO 3200 Lecture 8: Sensory Physiology
Introduction
○
Classification of sensory receptors
○
Transduction of sensory signals
○
Stimulus encoding
○
General properties of sensory reception:
!
Olfaction: an example of chemoreception
!
Thermoreceptors
○
Infrared receptors
○
Thermoreception
!
Outline:
Reception of signal (exteroreceptors and interoreceptors)
1.
Transduction of signal
2.
Amplification of signal
3.
Transmission of the signal to the integrating centre
(afferent/sensory neurons)
4.
Perception of the stimulus at the integrating center (CNS)
5.
Sensory reception is a process:
!
Dorsal root ---> sensory nerve
○
Ventral root --> motor nerve
○
*see dorsal view of the central nervous system
!
Introduction:
Chemoreceptors -specific chemicals
○
Mechanoreceptors -mechanical energy
○
Photoreceptors -light (electromagnetic radiation)
○
Thermoreceptors -temperature
○
Nociceptors -noxious chemical, mechanical and thermal stimuli
○
Electroreceptors -electric fields
○
Magnetoreceptors -magnetic fields
○
Sensory receptors can be categorized by the type of stimulus energy to
which they respond:
!
Each receptor is highly selective for a specific kind of energy
!
Modality specialization favours higher sensitivity
!
Classification of Sensory Receptors:
Sensory receptor is the ion channel, it receives and
transduces the signal
!
Mechano/thermo/electro and some taste receptors use an inotropic
transduction mechanisms
○
Stimulus induces a conformation change in specific
membrane receptor --> activates GPCR --> adenylate
cyclase (ATP-->cAMP) --> opens ion channel -->
depolarization
!
Photo/olfactory and some taste receptors use metabotropic
transduction mechanisms (like hormone signalling)
○
Transduction: the process by which sensory receptors change stimulus
energy into electrical signals
!
Transduction of Sensory Signal:
Sensory receptor cells are either primary afferent neurons (e.g. olfactory
neurons), or epithelial sensory cells (e.g. photoreceptors)
!
In primary afferent neurons, the stimulus elicits a graded generator
potential and all-or-none action potentials
!
In epithelial sensory cells, the stimulus elicits a graded receptor potential,
transmitter release, post-synaptic graded potential, and all-or-non action
potentials
!
*see slide
!
Transmission of Sensory Signals:
For a sensory signal to be interpreted by the CNS in a coherent way, the
receptor must encode stimulus modality, location, intensity and duration
!
The labeled-lines principle: since sensory receptors are maximally
sensitive to one type of stimulus and different sensory neurons project to
different regions of the CNS, the origin of affect neurons encodes both
stimulus modality and location
!
Sensory receptor cells encode stimuli over a limited range
!
Threshold intensity -receptor saturation = dynamic range
○
Instead, the frequency of AP is adjusted with stimulus
intensity
!
Action potentials are all-or-none events, so can't encode intensity
directly via magnitude
○
The upper limit of the dynamic range in epithelial sensory
cells is determined by the saturation of receptor proteins
when the maximum rate of release of neurotransmitters is
reached
!
The upper limit of the dynamic range in primary afferent
neurons is determined by the membrane potential at which
the maximum frequency of action potentials is reached
!
A given sensory receptor cell is sensitive to a specific level of
stimulus intensity (dynamic range)
○
Action potentials code stimulus intensity through changes in frequency
!
Receptor potential amplitude is proportional to the logarithm
of stimulus intensity
!
Higher sensitivity at low stimulus intensity
!
Logarythmic coding
○
Receptors sensitive to a different range of intensities work
together to provide discrimination across a wide range of
intensities
!
Range fractionation
○
Two strategies are used by the sensory system to expand the range of
stimuli that can be detected:
!
Sensory adaptation occurs in all senses, with the possible exception
of the sense of pain
○
Sensory adaptation occurs when sensory receptors change their
sensitivity to the stimulus
!
Ex. Wearing a shirt
!
Tonic receptors continue to be depolarized throughout the duration
of stimulus and adapt slowly
○
Phasic receptors depolarize primarily at the beginning of a
stimulus and adapt rapidly
○
Only fire when stimulus is changing
!
Most receptors undergo receptor adaptation when stimulus
intensity is maintained at a constant level
○
Tonic and phasic receptors encode stimulus duration:
!
Stimulus Encoding:
Clones the first 18 of ~1000 genes from the OR family
!
The recipients of the 2004 Nobel Prize in Physiology and Medicine
have made a significant contribution in solving this problem
○
How do we recognize and remember ~10,000 different odors?
!
Odorants bind to receptors
○
Olfactory receptor cells are activated and send electric signals
○
The signals are relayed in glomeruli
○
The signals are transmitted to higher regions of the brain
○
Organization of the olfactory system:
!
3% of the mammalian genome codes for different odorant
receptors
○
Most odors are composed of multiple odorant molecules, and each
molecule activates a unique subset of odorant receptors
○
Odorant receptors are used in a combinatorial manner to detect
odorants and encode their identities
○
5 = 5 goat
!
5+2+6 = 13 flowers
!
….add a few more and you're smelling cheese
!
Examples:
○
In both mammals and insects, but at vastly different scales,
the organization of the peripheral olfactory circuit is
conserved
!
*see mouse vs. fly
"
In mammals, the area of olfactory epithelium and the
number of odorant receptor types reflect reliance on the
sense of smell and degree of specialization
!
Species differences:
○
Odorant binding causes conformational change1.
Activated G protein stimulates adenylate cyclase2.
cAMP production3.
Opening of cation channels4.
Ca2+ and Na+ entry elicits generator potential 5.
Increased depolarization with Cl-channel opening6.
Generator potential opens voltage-gated Na+ channels and
triggers action potential
7.
Signal transduction in an olfactory receptor cell of mammals:
○
How olfactory receptors encode odors:
!
Olfaction: an example of chemoreception
11/21/17
Warm-sensitive --> respond to above skin temperature1.
Cold-sensitive --> respond to below skin temperature2.
Thermal nociceptors --> respond to extreme heat/cold3.
Peripheral thermoreceptors include:
○
The hypothalamus integrates info from the central and peripheral
thermoreceptors and coordinates adjustments in heat gain and loss
in a negative-feedback fashion
○
Vertebrates possess central thermoreceptors that monitor core body
temperature and peripheral thermoreceptors that monitor environmental
temperature
!
Hypothalamus acts as a thermostat and receives nerve impulses
from heat and cold thermoreceptors in the skin
○
Detects changes in blood temperature
!
There are also receptors in the hypothalamus = central
thermoreceptors
○
Body temperature control:
!
These can be internal (endogenous) or external (exogenous)
!
Pyrogens = substance that induces fever
○
All endogenous pyrogens are cytokines, molecules that are
all part of the innate immune system
!
They are produced by phagocytic cells and cause the
increase in the thermoregulatory set-point in the
hypothalamus
!
Interleukin-1 (alpha and beta)
"
Interleukin-6
"
Tumor necrosis factor (alpha)
"
Major endogenous pyrogens:
!
Endogenous:
○
Bacterial lipopolysaccharide (LPS), present in the cell wall
of some bacteria is an example of an exogenous pyrogen
!
Exogenous:
○
A trigger of fever (=pyrogen) causes a release of
prostaglandin E2 (PGE2)
!
PGE2 then acts on the hypothalamus, which generates a
systemic response back to the rest of the body, causing heat-
creating effects to match a new temperature level
!
Increased mobility of leukocytes
"
Enhanced leukocyte phagocytosis
"
Endotoxin effects decreased
"
Fever assists the healing process via:
!
When infected, survival was highest at 32.7C
(100%) vs. 25.5C (20%)
!
Fever in goldfish:
"
When infected, highest survival at 42C (~95%)
vs. 34C (0%)
!
High temperature is selected to help fight
disease
!
Fever in lizard:
"
*note: there are a variety of pyrogens across vertebrate taxa
!
Fever:
○
Pyrogens
!
Thermoreceptors are heat-gated or cold-gated ion channels
specialized for detecting distinct temperature ranged and involved
in converting thermal energy into electrical signals
○
They are part of the transient receptor potential (TRP) family of
cation channels
○
Modulated by endogenous lipid agonists derived from
glycerophospholipids via phospholipase A2
"
Repeated stimulation with menthol results in a
reduced response (-->adaptation to stimulus;
attenuates)
!
Regulated by phophatidylinositol (-->desensitized
without)
"
Transient receptor potential melastatin 8 (TRPM8) is a
calcium permeable nonselective cation channel activated by
cold
!
Many TRP channels are also chemoreceptors and a number of
plant-derived chemicals (menthol, capsaicin, allyl isothioncyanate)
are potential activators of specific TRP channels
○
Thermoreceptors:
!
Infrared receptors can detect heat radiating from objects at a
distance
○
The receptors are specialized to detect a portion of the
electromagnetic spectrum
○
Abdominal infrared pit organs of Australian fire-beetle
!
Metathorax infrared pit organs of black pine beetle
!
Lower lip infrared organs of green tree python
!
Infrared pit organs of western diamond back rattlensnake
!
Infrared pit organs of common vampire bats
!
Typically these receptors are concentrated in specialized organs:
○
100+ receptors in pit membrane -respond to temperature changes
<0.003C
○
Thermal image is superimposed on visual image
○
Localize endothermic prey in the dark
○
Pit membrane contains TRPA1 receptors that send
information via TG fibres
!
Rat snake does not feed on mammals specifically
"
Rattlesnake --> sample thermal environment
"
Determined the relative heat response profiles of rattlesnake
and rat snake channels expressed in oocytes of frogs
!
Ligand: allyl isothiocyanate = AITC
"
Used calcium imaging to identify TRPA1 (wasabi
receptor) channels as infrared receptors on sensory
nerve fibers that innervate the pit organ
"
Responds to temperatures similar to that of mammals
(prey)
"
Snake TRPA1 is a heat activated channel --> rattlesnake
!
Mechanism:
○
Another study found that a blind Northern Pacific rattlesnake, can
accurately strike as long as pit organs ere uncovered
○
Island is densely population with up to 3 tiger snakes in
every 25 square meters
!
Blinded by seagulls defending chicks
!
Would prey on abundant immobile prey (that can't escape
approach)
!
Ex. Blind tigers (rattlesnakes) of Carnac Island
○
Infrared receptors:
!
Thermoreception:
11/21/17
Olfaction and salmon migration
○
Pheromones and the vomeronasal organ
○
Chemoreceptors
!
Detecting touch and pressure
○
Detecting motion, position and soundwaves
○
Mechanoreceptors
!
Outline:
4+ basic taste perceptions: sweet, sour, salty, bitter, umami
○
If the olfactory system is blocked, the perception of taste is
sharply reduced
!
Taste and smell interact
○
Chemoreceptors in taste buds or nasal cavity are triggered when specific
molecules bind and trigger action potential
!
Sensory nerve fibers move through connective tissue of
tongue
!
Tongue --> papillae --> taste buds --> single taste bud
○
Causes conformational change in ion channels --> Na+
opens
!
Causes depolarization --> Ca2+ influx
!
Which stimulates release of neurotransmitters that enter
sensory neuron to create an action potential
!
Ex. Sugar molecule enters taste bud --> signal transduction
pathway
○
Signals to PLCbeta2 via PIP2 causing release of IP3 (+
DAG)
!
Causes Ca2+ to enter from Ca2+ store or external sources
!
Na2+ then enters cell from TrpM5
!
Ca2+ also results in production of ATP
!
G-protein coupled receptor are specific to sweet, bitter or umami
tastant
○
Enters cells (diffusion) and converts into H+ which then
binds to a proton-sensitive channel
!
Sour tastant:
○
Binds to ENaC (epithelial sodium channel), causing Na+ to
enter cell --> depolarization
!
Salt tastant:
○
Umami -T1R1 + T1R3
!
Sweet -T1R2 + T1R3
!
Bitter -~30 T2Rs
!
Sodium -ENaC
!
Sour and Carbonation Cells -PKS2L1 & CA IV
!
Mammalian taste receptors (see slide):
○
Less with just glutamate
!
Discovered in 1913 that dried bonito flakes contained
another umami substance (ribonucleotide IMP)
!
In 1957, realized that ribonucleotide GMP present in shiitake
mushrooms also confirmed umami taste --> synergistic
effect between ribonucleotides and glutamate
!
Glutamate + GMP --> Umami
○
Taste
!
How do salmon form the olfactory memories that guide them
to their home stream after their oceanic journeys
!
Freshwater residence (0-3years): emergence to
seaward migration
A)
Ocean distribution (4-6years)B)
Homing migration to Iliamna lake and natal site for
spawning (4-6years)
C)
Ex. Sockeye salmon of Iliamna Lake
!
Thyroid hormone primes the fish for imprinting
shortly after hatching and during spring downstream
migration
"
*see slide
"
Olfactory imprinting to the odor of the streambed where
salmon hatched involves an increased sensitivity of the
olfactory neurons to specific compounds (memory)
!
In olfactory receptor cells, stimulation of guanylyl
cyclase activity by the odorant PEA is greater in the
olfactory cilia of mature salmon previously PEA-
imprinted than in PEA-naïve fish
"
*see signal transduction in an olfactory cell of salmon
"
Pathway stimulation during imprinting results in
enhanced odorant sensitivity during homing migration
"
Olfactory memories (long term potentiation) are at least
partly mediated by enhanced cGMP signaling:
!
Olfaction and Salmon Migration:
○
Found in amphibians, reptiles and non-primate
mammals
"
The vomeronasal organ is distinct from the olfactory
epithelium
!
From the AOB, the information is transmitted to the
hypothalamus and results in modification of
behaviours and endocrine status
"
Vomeronasal neurons have distinct receptors that
differ from odorant receptors and respond to
pheromones
"
Vomeronasal neurons project to the accessory olfactory
bulbs (AOB)
!
Odour particles are captured from the air by the tongue
"
The vomeronasal organ is found on the roof of the
buccal cavity
"
During tongue retraction, odour particles are mized
with fluids in the mouth and are delivered to the ducts
leading to the vomeronasal organs
"
Ex. In Snakes
!
Presence and functionality in humans in controversial
"
Organ regresses during fetal development
"
Many genes essential for VNO function in animals
(such as TRPC2) are non-functional in humans
"
Chemical communication does appear to occur among
humans, but this does not necessarily imply that the
human vomeronasal organ is functional
"
Vomeronasal organ (aka. Jacobson's organ) -auxiliary
olfactory sense organ
!
Flehmen response --> bearing upper teeth
!
Pheromones and the Vomeronasal Organ
○
Olfaction:
!
Chemoreception:
11/23/17
*see slide
○
Hair receptor --> hair movement and very gentle touch
○
Merkel's disc --> light, sustained touch
○
Pacinian corpuscle --> vibrations and deep pressure
○
Reffini endings --> deep pressure
○
Meissner's corpuscle --> light, fluttering touch
○
Skin tactile receptors function as isolated sensory cells and include free
nerve endings or various enclosed accessory structures:
!
Mechanoreceptors -application of the mechanical stimulus produces
deformity in the receptors --> stretch of the membrane --> open Na+
channels --> Na+ influx --> depolarization
!
Tonic (slow) detection of pressure within very small receptive field
for fine tactile discrimination
○
Releases serotonin
○
*see slide
○
Merkel's disk:
!
Phasic (fast acting) detection of pressure
○
Pressure on the skin is transmitted to the corpuscle in the dermis
○
The shape of the corpuscle is changed causing sodium channels in
the neurone membrane to open
○
Sodium ions diffuse down the concentration gradient, depolarizing
the membrane (no calcium!!) = generator potential
○
The greater the pressure, the more sodium channels open causing a
bigger generator potential
○
If the threshold of that neuron is reaches, an action potential
develops and is transmitted along the sensory neuron
○
Pacinian corpuscle:
!
Deep in the skin, as well as in joint ligaments and joint capsules
○
Tonic (slow)
!
Large field
!
Detects skin stretch
○
Cutaneous or proprioceptive
○
Cigar shaped, encapsulated and contains longitudinal strands of
collagenous fibers
○
Ruddini ending:
!
Mechanoreceptors: detecting touch and pressure
Pacinian
Corpuscle
(vibration)
Meissner's
Corpuscle
(touch)
Merkel's
Disc
(touch)
Ruffini's
Ending
(stretch)
Receptive
fields
Large, vague
borders
Small, sharp
borders
Small,
sharp
borders
Large, vague
borders
Response Fast-adapting Fast-adapting Slow-
adapting
Slow-
adapting
Receptor Sensation Adaptation
Rate
Receptive
Field
Free nerve
endings
Itch, tickle, pain Tonic or
phasic
Large or
small
Ruffini
endings
Stretching of skin,
deep pressure
Tonic
(prolonged)
Large
Merkel discs Fine touch and
pressure
Tonic Small
Meissner
corpuscle
Fine touch, pressure,
slow vibration
Phasic -
moderate
Small
Hair follicle Crude touch,
movement of hairs
Phasic -
moderate
Small
Krause bulbs Fast vibration Phasic -fast Small
Pacinian
corpuscle
Pressure, fast
vibration, tickling
Phasic -
fastest
Large
Summary of cutaneous mechanoreceptors:
!
Depending on the stimulus, the hair cell and either
increase/decrease the AP frequency in the afferent sensory fiber
(inhibition/excitation depending on direction of movement)
○
K+ channels on the stereocilia are linked by elastic filaments
!
Displacement of cilia opens or closes K+ channels
!
Note: K+ entry generally causes hyperpolarization
(normally K+ is less outside of cell)
"
Endolymph has high concentration of K+ (low Na+),
compared to perilymph
"
K+ entering the cell causes depolarization
!
Depolarization causes voltage gated Ca2+ channels to open
!
Ca2+ triggers the release of neurotransmitter
!
Depolarization of hair cells:
○
*see slide
!
Direction of movement of stereocilia --> inhibition or
excitation (via release of aspartate/glutamate)
!
Detect motion, position and sound
"
3 different regions: macula, cristae (motion),
vestibular system (positional information) + cochlea
(sound)
"
Functions:
!
Hair cell function:
○
Crista ampullaris lateralis
!
Crista ampullaris posterior
!
Crista ampullaris superior
!
Organ of corti
!
Macula utriculi
!
Macula sacculi
!
Hair cell locations:
○
Semicircular canal --> cupula (position/orientation)
!
Sacculus --> otolithic membrane (motion)
!
Cochlea --> tectorial & basilar membrane (sound)
!
Arrangement of hair cells:
○
Hair cells: modified epithelial cells that transduce mechanical stimuli
into electrical signals with extraordinary sensitivity
!
They overlie the macular sensory epithelium of the gravity
receptors of most vertebrates and are required for optimal
stimulus input of linear acceleration and gravity
!
The greater relative mass of the membrane (due to presence
of otoconia), causes it to lag behind the macula temporarily,
leading to the transient displacement of the hair bundle
!
Note: fish have a large crystal = otolith
!
Otoconia are crystals of calcium carbonate and make the otolitic
membrane heavier than the structures and fluids surrounding it
○
Linear acceleration of head or changes of head position1.
Shift of position of otolithic membrane2.
Deflect stereocilia to or away from kinocilium3.
Results in stimulation or inhibition 4.
Mechanism of stimulation in utricle and saccule:
○
3 fluid-filled semi-circular canals --> equilibrium
!
Utriculus with utricular otolith (ear-stone) --> equilibrium,
gravity detector
!
Sacculus with otolith --> sound detection
!
Lagena with otolith --> sound detection
!
Inner ear of fish:
○
Uses cupula with sensory hair cells that connect to afferent
nerve through neuromast
!
As water flows, opening experience higher or lower
pressure
"
**note: K+ in water is low (K+ may be elevated in
tubule)
"
Tube is consistent with the water from environment
!
Consists of hair cells encased in gelatinous cap
specialized for detecting water movements
"
Found in the skin and typically grouped into structures
such as the lateral line or dispersed over the anterior of
the body
"
Object/predator avoidance
!
Prey detection
!
Ability to swim in school
!
Fish behaviours linked to the laternal line invludes:
"
Neuromasts:
!
Note: lateral line stitches in frogs
!
The lateral line system of fish and amphibians is involved in
motion detection:
○
Detection of motion:
!
Uses mechanoreceptors of the inner ear
○
Are important for balance
"
The vestibular organs detect position and motion of the head
!
Endolymph movement in the semicircular canals causes
deflection of hair cells
!
CNS integration from ampullae permits precise
determination of head movement direction
!
Vestibular organs and equilibrium:
○
*see slide
!
Evolution of semicircular canals:
○
Detecting position:
!
Pitch -depends on frequency
!
Intensity -depends on amplitude
!
Timbre -depends on overtones
!
Sound is characterized by its pitch (tone), intensity (loudness) and
timbre (quality)
○
Outer ear air pressure waves are converted into cochlea
liquid pressure waves by the middle ear ossicles
!
Cochlea pressure waves cause site-specific and pitch-
dependent displacement of the basilar membrane
!
Displacement of basilar membrane bends hair cells
!
Spatial coding is maintained in auditory nerve and cortex
!
Amplitude of pressure waves determined magnitude of
displacement and frequency of AP in afferent neurons
(intensity of sound)
!
*see slide
!
Cochlea and Hearing (in land animals)
○
*see slides
!
Staples --> oval window --> perilymph in cochlea
!
Amplitude of wave = loudness
"
Low frequency sound waves take longer (move further
through cochlea) before they are detected (by basilar
membrane)
!
Inner and outer hair cells
"
Organ of corti -contains clustered hair cells
!
Anatomy of Cochlea:
○
Sound waves arrives at tympanic membrane
!
Movement of tympanic membrane causes displacement of
the auditory ossicles
!
Movement of the stapes at the oval window establishes
pressure waves in the periplymph of the vestibular duct
!
The pressure waves distort the basilar membrane on their
way to the round window of the tympanic duct
!
Vibration of the basilar membrane causes vibration of hair
cells against the tectorial membrane
!
Information about the region and intensity of stimulation is
relayed to the CNS over the cochlear branch of cranial nerve
VIII
!
Events involved in hearing:
○
Ear canal with cellular debris prevents air from
entering ear
"
Sound waves are received through fat-filled lower jaw
"
*see slide
"
Ex. Whales
!
Fish have bones in the inner ear (=otoliths), which are
much denser than water and the fish's body
"
As a result, these ear bones move more slowly in
response to sound waves than the rest of the fish
"
The difference between the motion of the fish's bosy
and the otoliths bend/displace the cilia on the hair cells
that are located in the inner ear
"
This differential movement between the sensory cells
and the otolith is interpreted by the brain as sound
"
Otoliths are made of calcium carbonate and their
size/shape is highly variable among species
"
Bodies of fish are approximately the same density as water,
so sound passes through their bodies
!
Sound detection in marine animals:
○
The hearing range (frequency) various widely across taxa
○
Detecting sound:
!
Mechanoreceptors: detecting motion, position and sound waves
Remarkably, prestin aminio-acid sequences of echolocating
dolphins have converged to resemble those of distantly related
echolocating bats
○
The motor protein prestin confers sensitive and selective hearing in
mammals
!
Melon -fat structure involved in creation of 'clicks'
!
Marine mammals use similar mechanism
○
Bat: uses sonar and returning sound waves from prey
!
Solute linked carrier -associated with high frequency hearing
○
Prestin SLC26 protein family:
!
This appears to be an essential component of a mechanism tuning
mechanism that is unique to the mammalian ear
○
Prestin molecule changes in shape during hyperpolarization and
depolarization
○
The mammalian ear has more specialized cells which detect and converte
sound in the normal way, but then convert the electrical signals into
changes in cell length
!
Prestin is the motor protein of the outer hair cells of the inner ear
of the mammalian cochlea
○
Immunolocalization shows prestin is expressed in the laternal
plasma membrane of the outer hair cells, the region where
electromotility occurs
○
*uses chloride binding sites
○
Prestin is a protein that in humans is coded by the SLC26A5 gene
!
*see prestin gene phylogeny
!
Molecular Evolution: Gene Convergence in Echolocating Mammals
11/28/17
Detecting weak electric fields
!
Electrogenesis
!
Electroreception and electrogenesis:
Although flatfish bury themselves in the sand, sharks can still
detect them
○
Vision (live flatfish) --> no
!
Olfaction (dead flatfish) --> no
!
Electroreception (battery) --> YES
!
Possible reasoning:
○
Natural food for many sharks is flatfish
!
Sense electric currents produced by active muscles of their prey
○
Sharks are attracted to prey over long distances by smell but use electric
sense over short distances to attack
!
Prey detection using electroreception: sharks
Can detect weak electrical signals in the environment (< 1 uV/cm)
○
Sense organs specialized for electroreception have only been found
among vertebrates (8600 sp)
○
Through convergent evolution, several vertebrate groups have developed
ability to perceive electric signals
!
Passive electroreception: detection of electric fields generated by other
organisms
!
Active electroreception: using self-generated electric fields for
electrolocation (detection co-specifics, prey, objects) and in some species
for electrocommunication
!
High frequency E: ~14,000 active electrosensory receptor organs
○
Low frequency E: ~700 passive electrosensory receptor organs
○
Mechano: ~250 mechanosensory receptor organs
○
Apteronotus
!
Electroreception:
Electric sensing was an early development in the course of
vertebrate evolution
○
The electrosensory system is closely related to the lateral line
system of fish and hearing/balance in terrestrial animals
○
Evolution:
!
*see slide for equation
○
Only useful over short distances
○
If distance of an object is doubled, its size must be quadrupled and
the energy expended by the fish increased by 8-fold if it is still to
be detected
○
Limitations of electrolocation:
!
Lower frequency
!
Ampullary
○
Higher frequency
!
Tuberous
○
Have different morphology
!
Have evolved from earliest ampullary electroreceptors with
loss of electroreception capacity in many (still have
mechanoreceptors)
!
No need
"
Electricity does not travel well through air
"
All mammals (except monotremes) do not have
electroreception
!
*both can occur in the same individual
○
Types of electroreceptors:
!
Closes eyes, ears and nose when it swims at night
○
Tail flip causes field of 1mV at 5cm
!
Sensitivity is 10uV/cm
!
Eats shrimp
○
40,000 electroreceptors and 40,000 mechanoreceptors on bill
○
Ex. Duck-billed Platypus
!
Detects prey by their electric fields
○
Feed by straining zooplankton with their filtering apparatus
○
Vision -feed in dark
!
Feeding on like plankton with nares plugged
"
Olfaction -no feeding with plankton extract
!
Mechanical -feed on plankton encapulsated in agarose
!
Hydrodynamic -feed in turbulent water conditions
!
Other senses:
○
Conclusion: electric sense is sufficient for prey detection and
capture
○
Ampulla of Lorenzini dispersed over rostum
○
Assists in prey detection, orientation and navigation
○
Paddlefish will strike at dipoles that produce artificial electric
fields that stimulate natural plankton
○
Rostrum is specialized for hunting electric signals in the near-field
evironment
○
Voltage gradient of 0.01uV/cm causes change in EEG
"
Voltage gradient of 1-10uV/cm causes escape when
swimming
!
To stimulate fields that fish can detect, need to
separate electrodes by 5km
"
1.5V battery connected to electrodes 50m apart sets of a field
of 300uV/cm
!
Sensitivity:
○
Ex. Paddlefish
!
Scalloped hammerhead vs. sandbar shark
○
Approximately 70% of orientations were initiated to stimuli of
<0.1uV/cm for both species
○
Note: 0.1uV/cm = flash light battery conencted to electrodes
over 16,000km apart
!
Both species initiated approximately 35-40% of orientations to
stimuli of <0.01uV/cm
○
Shark sensitivity:
!
*see slides
○
Canals connect ampullae of lorenzini to pores on the shark's
rostrum
○
The gel is a glyco-protein based substance with
semiconductor properties
!
Electrical impulses travel through the canal to stimulate
ampullae
!
Nerve impulses then send the information directly to the
brain
!
Canal of lorenzini is a jelly-filled tubule
○
*note: also determines direction in 3D
○
Room temperature proton conductivity of the jelly is very
high at 2 mS/cm
!
This conductivity is 40-fold lower then Nafion (highest
reported for a biological material)
!
Proton conductivity in ampullae of Lorenzini jelly:
○
Influx of positive charge causes the cell to release
neurotransmitters at synapses (or contact points) with nerves
to the brain, stimulation them to fire
!
The firing rate indicates strength and polarity of the external
field, and the field's location relative to the shark is thought
to be determined by the positions of the activated pores on
its body
!
The cells return to their original electrical state afterward by
opening a second time of membrane channel that permits
positively charged potassium ions to exit
!
A sensing cell reacts when an external electric field producing a
small electric potential across its membrane, leading channels to
allow positively charged calcium ions to rush in
○
Elasmobranch electroreception: ampullae of lorenzini
!
Electroreceptors:
Active electroreception, communication, defense & prey capture
!
*see slide
○
Electroreceptors, CNS and the electric organ interact
!
Use for prey capture (see slide for examples)
!
Fresh water and sea-water have difference in arrangement of
cells that produce electricity due to differences in
conductivity of the medium
!
Strongly electric fish (several hundred volts)
○
Prey detection and communication (see slide for examples)
!
Weakly electric fish (millivolts -few volts)
○
*10 families, over 500 sp --> convergent evolution
○
Skates: have electric organs in tail
○
Electric organs:
!
Electric rays: torpedoes, numbfish
!
See slide
○
Isopotential lines and current flow around an electric fish
○
Conductive objects concentrate the field
!
Resistive objects spread the field
!
Active electrolocation
○
Electric field emanates from an electric organ in the tail
region
!
It is sensed by the electroreceptive skin areas, using two
electric pits (foveas) to actively search and inspect objects
!
*see slide --> conductive vs. resistive objects
!
Ex. Elephantfish
○
Active electroreception:
!
Contain electrocytes = modified muscle cells
○
Non-innnervated surface does not change membrane
potential
!
Posterior surface generates an action potential
!
At rest = 0
!
Voltage gated sodium channels open to depolarize
posterior size of membrane (-90--> + 60 mV)
"
When stimulated = 150mV
!
Potential differences across membrane
○
*see slide
○
Electric eel
!
Electrocytes arranged in series increases the discharge voltage,
whereas an arrangement in parallel increases total current flow
(amperage)
○
In low-conductivity water the caudal electric organ is long and thin
(more electrocyte columns, fewer rows) which serves to increase
voltage in order to maximize the power output of the EOD
○
In high-conductivity water, electric organ is short and deep (more
electrocyte rows, fewer colums) which serves to increase amperage
(current)
○
Note: seawater is more conductive than freshwater
○
Electrocytes stacked from tail to head increase voltage
!
Multiple rows of electrocytes increase current
!
Overall:
○
Series vs. Parallel
!
This temporarily creates an additional charge across the cell
membrane on that side of the cell of about 0.065V
○
Now, instead of having a negative inside and positive outside, the
cell temporarily has a 0.085V difference across the convoluted
side, and a similarly oriented charge of about 0.065V on the
smooth side
○
These chargers are essentially stacked in series, so that the end
result is a brief charge across the entire cell of about 0.15V (0.085+
0.065)
○
When nerve fibers send a signal to an electrocyte, ion channels on the
smooth side of the cell open, allowing positive ions to rush into the cell
!
Electrophorus has ~ 5000 cells/column
○
Therefore, voltage = 5000 cells*150mV/cell = 755 V
○
Note: when electrocytes are arranged in a series in a column, the voltages
sum
!
--> electric organ discharge (EOD)
!
Pacemaker neurons --> relay neurons --> motor neurons -->
muscle
○
If a normal nerve signal went out from the brain to each
electrocyte, the signal would reach the first cells in the stack
before it reached the cells at the end of the stack
!
By the time the cells a the end firest, the ones at the
beginning would have already shut off again
!
Nerve fibers closer to the head are smaller than
those near the tail
!
Nerves closer to head also tend to take more of a
winding path than nerves near tail
!
Slower chemical signals are used in nerve fibers
closer to the head
!
Involves:
"
The electric eel has to synchronize the firing of thousands of
electrocytes in a stack so they all turn on at the same and add
together to create the big voltage needed to shock the fishe's
prey
!
Make pathlengths equal
"
Vary conduction velocity
"
Vary electrocyte stalk length (conducts faster than
nerve)
"
Overall:
!
Compensatory delay:
○
Generating electric organ discharge:
!
Brains tend to be larger than average fish --> more sensory
information
○
*see slide (do not need to memorize names)
○
Pathways in electroreception:
!
See slide
!
V = I (current) *R(resistance)
!
Ohm's Law
○
*see slide
!
Same current --> decreased voltage
"
Wet skin has decreased resistance
!
Electrocution
○
The current path must usually include either the heart or
brain to be fatal
!
Whether an electric current is fatal is dependent on the path it takes
through the body, which depends in turn on the points at which the
current enters and leaves the body
○
Electricity for prey capture and protection (strongly electric fish):
!
Electrogenesis:
Works because the normal response of a fish to direct electric current
(=galvanotaxis) is to swim towards the positive (anode) electrode
!
As in all vertebrate species, fish muscles are controlled by electrical
signals sent through the nervous system from the brain
!
The brain is believed to be negatively charged, therefore orientating the
fish toward the positively charged anode
!
The electric current interrupts the neurological pathway, causing the fish
to involuntary swim towards the anode
!
When fish are placed across the electric field, the bodies curve toward
the positive pole because the motor spinal nerves facing the negative
pole are inhibited while those facing the positive pole are stimulated
!
Electrofishing:
11/30/17
Inductive magnetoreception
!
Mechanically induced mechanoreception
!
Magnetoreception (detecting the Earth's magnetic field)
Flight as long as 1800km have been recorded by birds in
competitive pigeon racing
○
Because of this skill, homing pigeons were used to carry messages
as messenger pigeons
○
Homing pigeon is a variety of domestic pigeon derived from the rock
pigeon, selectively bred for its ability to find its way home over
extremely long distances using mechanoreception
!
Captive-raised monarchs appear capable of migrating to
overwintering sites in Mexico
○
Have spring and fall migrations
○
*see slide
○
Monarch butterfly:
!
Newly hatched loggerhead turtles use a magnetic map for
orientation along their migratory route in the North Atlantic
subtropical gyre
○
Hatchling turtles were placed in artificial magnetic fields that
replicate three locations along the migratory route
○
Results indicate that the turtles preferentially orient to stay within
the gyre (i.e. to follow a specific migratory route)
○
Sea turtles:
!
Species Examples:
Bacteria
○
Crustaceans (spiny lobsters)
○
Insects (butterflies, bees, flies)
○
Fish (salmon)
○
Elasmobranchs (sharks, skates, rays)
○
Amphibians (cave salamanders)
○
Reptiles (sea turtles)
○
Birds (homing pigeons, migratory birds)
○
Mammals (dolphins, whales)
○
Magnetic compasses are phylogenetically widespread:
!
Declination (--> north)
○
Inclination
○
Field intensity
○
Information obtained by earth's magnetic fields:
!
Direction (polarity compass)
○
*north pole is here and I am ___ away from it
Dip angle (inclination compass)
○
Animals can sense two qualities of earth's magnetic field:
!
Behavioural experiments have shown that many animals can sense the Earth's
magnetic field and use it for guiding movements (nature's GPS)
*use electric fields
○
Changing field induces a current in the jelly filled canals of the
ampullae of Lorenzini which changes the electric potential in the
ampulla
○
The voltage is amplified in the ampulla due to ion-channel
mediated interactions between the apical and basal membrane
○
There is an induced electromagnetic field in the
secondary coil (producing an induced current) due to
the changing magnetic field through this coil
"
When the switch is closed, the ammeter deflects
momentarily
!
Steady magnetic fields cannot produce a current
"
When there is a steady current in the primary coil, the
ammeter reads zero current
!
Experiment:
○
If the animal moves so that rotation occurs around an axis in
the plane of a semi-circular canal, there will be no
displacement of the endolymph but electromagnetic
induction could occur
!
Depending on intensity and orientation of the external
magnetic field, this will induce an electromotive force in the
conductive endolymph
!
This results in the separation of charged within the circuit,
inducing cation influx through highly sensitive voltage-gated
ion channels
!
Depicted is one semicircular canal of a vertebrate, filled with
cation-rich endolymph, and sensory cells located on either side of
the cupula
○
Inductive1.
*see slide for mechanism and biology
○
Fe3O4 -one of the oxides of iron
!
Is ferrimagnetic
!
They could site between ciliated olfactory and sensory
cells
"
Magnetite has been found in many animal tissues, but in fish
some evidence points to the nose as the home for
magnetoreceptors
!
Magnetite tugged by Earth's field would mechanically
control neural circuits by opening ion channels in cell
membranes
!
Magnetite:
○
Magnetitie field changers trigger movement of a SPM
mangetite cluster, which in turn induces derformation
in the plasma membrane
"
This event induces the opening of a non-selective
mechanosensitive ion channel
"
Mechanosensitive ions channels are located in the membrane
but do not have a physical link with the magnetite
A)
At rest, the ion channel is blocked
"
Change in the magnetite field relieves the blockade,
allowing sodium or calcium entry into the cells
"
Magnetite is physically connected to the ion channel B)
The messenger binds to the ion channel and opens it
"
Magnetite movement releases second messenger, either
directly or through the creation of tension in the membrane
C)
Cell containing magnetite linked to a neighbouring
nerve terminal through a 'tip link'
"
The ion channel is linked via a molecular motor to
actin filaments
"
Movement in the magnetite-containing compartment
stretch the link and open the ion channel
"
After opening, a molecular motor could reposition the
ion channel reducing the tension in the link and
allowing the system to be stimulated again
"
Mechanism based on auditory hair cell
mechanotransduction**
D)
Proposed scenario:
○
Mechanical2.
Magnetosensing rod-like protein complex identified in
drosophila
!
Exhibits spontaneous alignment in magnetic fields, including
that of Earth
!
Protein complex may form the basis of magnetoreception in
animals
!
A magnetic protein biocompass:
○
They are found in plants and animals
!
Cryptochromes are involved in the circadian rhythms of
plants and animals, and in the sensing of magnetic fields in a
number of species
!
Cry-deficient drosophilia does not show
magnetosensitive behaviour
"
Response of Cry to magnetic fields via radical pairs
may be used to perceive inclination information from a
geomagnetic field
"
Evidence:
!
Cry perceive geomagnetic information via the
quantum spin dynamics of radical-pair of FAD
reaction initiated by light (from sun or moon)
"
*see slide
!
Cyptochromes are a class of flavoproteins that are sensitive to blue
light
○
Magnetosensing protein crystals orient in a magnetic field
○
In nerve fiber layer --> to optic nerve
!
Gangion cell layer
!
Outer nuclear layer --> photoreceptor cell
!
Cry Magr protein complex is located…
○
Cryptochromes may lie in mysterious double cone cells
!
The ratio of chemical products from each cone could
determine magnetic orientation, which the brain might
process as light and dark patches on the visual field
!
Light turns the cyptochrome into a radical pair molecular,
with two unpaired electrons that flip between parallel and
anti-parallel states
!
Study:
○
11-cis retinal absorbs light and isomerized into all-trans
retinal
!
All-trans retinal dissociates from opsin
!
Activated opsin activates G protein transducin
!
Transducin activates PDE, which converts cGMP to GMP
!
The decreased cGMP closes a Na+ channel
!
Na+ entry decreases --> hyperpolarization
!
Mechanism:
○
Hypothesis: visual field of a bird may be modified through the
magnetic filter function
○
Specifically, many studies have shown that birds can only
orient if blue light is present
!
The avian compass is also an inclination-only compass,
meaning that it can sense changes in the inclination of
magnetic field lines but is not sensitive to the polarity of the
field lines
!
A bird can only sense the magnetic field if certain wavelengths of
light are available
○
Cry and MagR proteins are found widely distributed across taxa
○
Light-based 3.
Three types of magnetoreception:
Passive magnetoreception
!
Polarity via magnetite containing cells
○
Inclination via light sensitive magnetoreception
○
Active magnetoreception
!
Summary:
Sensory Physiology
#$%&'()*+,-./0120&, 34+,5637
3854,9:
Introduction
○
Classification of sensory receptors
○
Transduction of sensory signals
○
Stimulus encoding
○
General properties of sensory reception:
!
Olfaction: an example of chemoreception
!
Thermoreceptors
○
Infrared receptors
○
Thermoreception
!
Outline:
Reception of signal (exteroreceptors and interoreceptors)1.
Transduction of signal2.
Amplification of signal3.
Transmission of the signal to the integrating centre
(afferent/sensory neurons)
4.
Perception of the stimulus at the integrating center (CNS)5.
Sensory reception is a process:
!
Dorsal root ---> sensory nerve
○
Ventral root --> motor nerve
○
*see dorsal view of the central nervous system
!
Introduction:
Chemoreceptors -specific chemicals
○
Mechanoreceptors -mechanical energy
○
Photoreceptors -light (electromagnetic radiation)
○
Thermoreceptors -temperature
○
Nociceptors -noxious chemical, mechanical and thermal stimuli
○
Electroreceptors -electric fields
○
Magnetoreceptors -magnetic fields
○
Sensory receptors can be categorized by the type of stimulus energy to
which they respond:
!
Each receptor is highly selective for a specific kind of energy
!
Modality specialization favours higher sensitivity
!
Classification of Sensory Receptors:
Sensory receptor is the ion channel, it receives and
transduces the signal
!
Mechano/thermo/electro and some taste receptors use an inotropic
transduction mechanisms
○
Stimulus induces a conformation change in specific
membrane receptor --> activates GPCR --> adenylate
cyclase (ATP-->cAMP) --> opens ion channel -->
depolarization
!
Photo/olfactory and some taste receptors use metabotropic
transduction mechanisms (like hormone signalling)
○
Transduction: the process by which sensory receptors change stimulus
energy into electrical signals
!
Transduction of Sensory Signal:
Sensory receptor cells are either primary afferent neurons (e.g. olfactory
neurons), or epithelial sensory cells (e.g. photoreceptors)
!
In primary afferent neurons, the stimulus elicits a graded generator
potential and all-or-none action potentials
!
In epithelial sensory cells, the stimulus elicits a graded receptor potential,
transmitter release, post-synaptic graded potential, and all-or-non action
potentials
!
*see slide
!
Transmission of Sensory Signals:
For a sensory signal to be interpreted by the CNS in a coherent way, the
receptor must encode stimulus modality, location, intensity and duration
!
The labeled-lines principle: since sensory receptors are maximally
sensitive to one type of stimulus and different sensory neurons project to
different regions of the CNS, the origin of affect neurons encodes both
stimulus modality and location
!
Sensory receptor cells encode stimuli over a limited range
!
Threshold intensity -receptor saturation = dynamic range
○
Instead, the frequency of AP is adjusted with stimulus
intensity
!
Action potentials are all-or-none events, so can't encode intensity
directly via magnitude
○
The upper limit of the dynamic range in epithelial sensory
cells is determined by the saturation of receptor proteins
when the maximum rate of release of neurotransmitters is
reached
!
The upper limit of the dynamic range in primary afferent
neurons is determined by the membrane potential at which
the maximum frequency of action potentials is reached
!
A given sensory receptor cell is sensitive to a specific level of
stimulus intensity (dynamic range)
○
Action potentials code stimulus intensity through changes in frequency
!
Receptor potential amplitude is proportional to the logarithm
of stimulus intensity
!
Higher sensitivity at low stimulus intensity
!
Logarythmic coding
○
Receptors sensitive to a different range of intensities work
together to provide discrimination across a wide range of
intensities
!
Range fractionation
○
Two strategies are used by the sensory system to expand the range of
stimuli that can be detected:
!
Sensory adaptation occurs in all senses, with the possible exception
of the sense of pain
○
Sensory adaptation occurs when sensory receptors change their
sensitivity to the stimulus
!
Ex. Wearing a shirt
!
Tonic receptors continue to be depolarized throughout the duration
of stimulus and adapt slowly
○
Phasic receptors depolarize primarily at the beginning of a
stimulus and adapt rapidly
○
Only fire when stimulus is changing
!
Most receptors undergo receptor adaptation when stimulus
intensity is maintained at a constant level
○
Tonic and phasic receptors encode stimulus duration:
!
Stimulus Encoding:
Clones the first 18 of ~1000 genes from the OR family
!
The recipients of the 2004 Nobel Prize in Physiology and Medicine
have made a significant contribution in solving this problem
○
How do we recognize and remember ~10,000 different odors?
!
Odorants bind to receptors
○
Olfactory receptor cells are activated and send electric signals
○
The signals are relayed in glomeruli
○
The signals are transmitted to higher regions of the brain
○
Organization of the olfactory system:
!
3% of the mammalian genome codes for different odorant
receptors
○
Most odors are composed of multiple odorant molecules, and each
molecule activates a unique subset of odorant receptors
○
Odorant receptors are used in a combinatorial manner to detect
odorants and encode their identities
○
5 = 5 goat
!
5+2+6 = 13 flowers
!
….add a few more and you're smelling cheese
!
Examples:
○
In both mammals and insects, but at vastly different scales,
the organization of the peripheral olfactory circuit is
conserved
!
*see mouse vs. fly
"
In mammals, the area of olfactory epithelium and the
number of odorant receptor types reflect reliance on the
sense of smell and degree of specialization
!
Species differences:
○
Odorant binding causes conformational change1.
Activated G protein stimulates adenylate cyclase2.
cAMP production3.
Opening of cation channels4.
Ca2+ and Na+ entry elicits generator potential 5.
Increased depolarization with Cl-channel opening6.
Generator potential opens voltage-gated Na+ channels and
triggers action potential
7.
Signal transduction in an olfactory receptor cell of mammals:
○
How olfactory receptors encode odors:
!
Olfaction: an example of chemoreception
11/21/17
Warm-sensitive --> respond to above skin temperature1.
Cold-sensitive --> respond to below skin temperature2.
Thermal nociceptors --> respond to extreme heat/cold3.
Peripheral thermoreceptors include:
○
The hypothalamus integrates info from the central and peripheral
thermoreceptors and coordinates adjustments in heat gain and loss
in a negative-feedback fashion
○
Vertebrates possess central thermoreceptors that monitor core body
temperature and peripheral thermoreceptors that monitor environmental
temperature
!
Hypothalamus acts as a thermostat and receives nerve impulses
from heat and cold thermoreceptors in the skin
○
Detects changes in blood temperature
!
There are also receptors in the hypothalamus = central
thermoreceptors
○
Body temperature control:
!
These can be internal (endogenous) or external (exogenous)
!
Pyrogens = substance that induces fever
○
All endogenous pyrogens are cytokines, molecules that are
all part of the innate immune system
!
They are produced by phagocytic cells and cause the
increase in the thermoregulatory set-point in the
hypothalamus
!
Interleukin-1 (alpha and beta)
"
Interleukin-6
"
Tumor necrosis factor (alpha)
"
Major endogenous pyrogens:
!
Endogenous:
○
Bacterial lipopolysaccharide (LPS), present in the cell wall
of some bacteria is an example of an exogenous pyrogen
!
Exogenous:
○
A trigger of fever (=pyrogen) causes a release of
prostaglandin E2 (PGE2)
!
PGE2 then acts on the hypothalamus, which generates a
systemic response back to the rest of the body, causing heat-
creating effects to match a new temperature level
!
Increased mobility of leukocytes
"
Enhanced leukocyte phagocytosis
"
Endotoxin effects decreased
"
Fever assists the healing process via:
!
When infected, survival was highest at 32.7C
(100%) vs. 25.5C (20%)
!
Fever in goldfish:
"
When infected, highest survival at 42C (~95%)
vs. 34C (0%)
!
High temperature is selected to help fight
disease
!
Fever in lizard:
"
*note: there are a variety of pyrogens across vertebrate taxa
!
Fever:
○
Pyrogens
!
Thermoreceptors are heat-gated or cold-gated ion channels
specialized for detecting distinct temperature ranged and involved
in converting thermal energy into electrical signals
○
They are part of the transient receptor potential (TRP) family of
cation channels
○
Modulated by endogenous lipid agonists derived from
glycerophospholipids via phospholipase A2
"
Repeated stimulation with menthol results in a
reduced response (-->adaptation to stimulus;
attenuates)
!
Regulated by phophatidylinositol (-->desensitized
without)
"
Transient receptor potential melastatin 8 (TRPM8) is a
calcium permeable nonselective cation channel activated by
cold
!
Many TRP channels are also chemoreceptors and a number of
plant-derived chemicals (menthol, capsaicin, allyl isothioncyanate)
are potential activators of specific TRP channels
○
Thermoreceptors:
!
Infrared receptors can detect heat radiating from objects at a
distance
○
The receptors are specialized to detect a portion of the
electromagnetic spectrum
○
Abdominal infrared pit organs of Australian fire-beetle
!
Metathorax infrared pit organs of black pine beetle
!
Lower lip infrared organs of green tree python
!
Infrared pit organs of western diamond back rattlensnake
!
Infrared pit organs of common vampire bats
!
Typically these receptors are concentrated in specialized organs:
○
100+ receptors in pit membrane -respond to temperature changes
<0.003C
○
Thermal image is superimposed on visual image
○
Localize endothermic prey in the dark
○
Pit membrane contains TRPA1 receptors that send
information via TG fibres
!
Rat snake does not feed on mammals specifically
"
Rattlesnake --> sample thermal environment
"
Determined the relative heat response profiles of rattlesnake
and rat snake channels expressed in oocytes of frogs
!
Ligand: allyl isothiocyanate = AITC
"
Used calcium imaging to identify TRPA1 (wasabi
receptor) channels as infrared receptors on sensory
nerve fibers that innervate the pit organ
"
Responds to temperatures similar to that of mammals
(prey)
"
Snake TRPA1 is a heat activated channel --> rattlesnake
!
Mechanism:
○
Another study found that a blind Northern Pacific rattlesnake, can
accurately strike as long as pit organs ere uncovered
○
Island is densely population with up to 3 tiger snakes in
every 25 square meters
!
Blinded by seagulls defending chicks
!
Would prey on abundant immobile prey (that can't escape
approach)
!
Ex. Blind tigers (rattlesnakes) of Carnac Island
○
Infrared receptors:
!
Thermoreception:
11/21/17
Olfaction and salmon migration
○
Pheromones and the vomeronasal organ
○
Chemoreceptors
!
Detecting touch and pressure
○
Detecting motion, position and soundwaves
○
Mechanoreceptors
!
Outline:
4+ basic taste perceptions: sweet, sour, salty, bitter, umami
○
If the olfactory system is blocked, the perception of taste is
sharply reduced
!
Taste and smell interact
○
Chemoreceptors in taste buds or nasal cavity are triggered when specific
molecules bind and trigger action potential
!
Sensory nerve fibers move through connective tissue of
tongue
!
Tongue --> papillae --> taste buds --> single taste bud
○
Causes conformational change in ion channels --> Na+
opens
!
Causes depolarization --> Ca2+ influx
!
Which stimulates release of neurotransmitters that enter
sensory neuron to create an action potential
!
Ex. Sugar molecule enters taste bud --> signal transduction
pathway
○
Signals to PLCbeta2 via PIP2 causing release of IP3 (+
DAG)
!
Causes Ca2+ to enter from Ca2+ store or external sources
!
Na2+ then enters cell from TrpM5
!
Ca2+ also results in production of ATP
!
G-protein coupled receptor are specific to sweet, bitter or umami
tastant
○
Enters cells (diffusion) and converts into H+ which then
binds to a proton-sensitive channel
!
Sour tastant:
○
Binds to ENaC (epithelial sodium channel), causing Na+ to
enter cell --> depolarization
!
Salt tastant:
○
Umami -T1R1 + T1R3
!
Sweet -T1R2 + T1R3
!
Bitter -~30 T2Rs
!
Sodium -ENaC
!
Sour and Carbonation Cells -PKS2L1 & CA IV
!
Mammalian taste receptors (see slide):
○
Less with just glutamate
!
Discovered in 1913 that dried bonito flakes contained
another umami substance (ribonucleotide IMP)
!
In 1957, realized that ribonucleotide GMP present in shiitake
mushrooms also confirmed umami taste --> synergistic
effect between ribonucleotides and glutamate
!
Glutamate + GMP --> Umami
○
Taste
!
How do salmon form the olfactory memories that guide them
to their home stream after their oceanic journeys
!
Freshwater residence (0-3years): emergence to
seaward migration
A)
Ocean distribution (4-6years)B)
Homing migration to Iliamna lake and natal site for
spawning (4-6years)
C)
Ex. Sockeye salmon of Iliamna Lake
!
Thyroid hormone primes the fish for imprinting
shortly after hatching and during spring downstream
migration
"
*see slide
"
Olfactory imprinting to the odor of the streambed where
salmon hatched involves an increased sensitivity of the
olfactory neurons to specific compounds (memory)
!
In olfactory receptor cells, stimulation of guanylyl
cyclase activity by the odorant PEA is greater in the
olfactory cilia of mature salmon previously PEA-
imprinted than in PEA-naïve fish
"
*see signal transduction in an olfactory cell of salmon
"
Pathway stimulation during imprinting results in
enhanced odorant sensitivity during homing migration
"
Olfactory memories (long term potentiation) are at least
partly mediated by enhanced cGMP signaling:
!
Olfaction and Salmon Migration:
○
Found in amphibians, reptiles and non-primate
mammals
"
The vomeronasal organ is distinct from the olfactory
epithelium
!
From the AOB, the information is transmitted to the
hypothalamus and results in modification of
behaviours and endocrine status
"
Vomeronasal neurons have distinct receptors that
differ from odorant receptors and respond to
pheromones
"
Vomeronasal neurons project to the accessory olfactory
bulbs (AOB)
!
Odour particles are captured from the air by the tongue
"
The vomeronasal organ is found on the roof of the
buccal cavity
"
During tongue retraction, odour particles are mized
with fluids in the mouth and are delivered to the ducts
leading to the vomeronasal organs
"
Ex. In Snakes
!
Presence and functionality in humans in controversial
"
Organ regresses during fetal development
"
Many genes essential for VNO function in animals
(such as TRPC2) are non-functional in humans
"
Chemical communication does appear to occur among
humans, but this does not necessarily imply that the
human vomeronasal organ is functional
"
Vomeronasal organ (aka. Jacobson's organ) -auxiliary
olfactory sense organ
!
Flehmen response --> bearing upper teeth
!
Pheromones and the Vomeronasal Organ
○
Olfaction:
!
Chemoreception:
11/23/17
*see slide
○
Hair receptor --> hair movement and very gentle touch
○
Merkel's disc --> light, sustained touch
○
Pacinian corpuscle --> vibrations and deep pressure
○
Reffini endings --> deep pressure
○
Meissner's corpuscle --> light, fluttering touch
○
Skin tactile receptors function as isolated sensory cells and include free
nerve endings or various enclosed accessory structures:
!
Mechanoreceptors -application of the mechanical stimulus produces
deformity in the receptors --> stretch of the membrane --> open Na+
channels --> Na+ influx --> depolarization
!
Tonic (slow) detection of pressure within very small receptive field
for fine tactile discrimination
○
Releases serotonin
○
*see slide
○
Merkel's disk:
!
Phasic (fast acting) detection of pressure
○
Pressure on the skin is transmitted to the corpuscle in the dermis
○
The shape of the corpuscle is changed causing sodium channels in
the neurone membrane to open
○
Sodium ions diffuse down the concentration gradient, depolarizing
the membrane (no calcium!!) = generator potential
○
The greater the pressure, the more sodium channels open causing a
bigger generator potential
○
If the threshold of that neuron is reaches, an action potential
develops and is transmitted along the sensory neuron
○
Pacinian corpuscle:
!
Deep in the skin, as well as in joint ligaments and joint capsules
○
Tonic (slow)
!
Large field
!
Detects skin stretch
○
Cutaneous or proprioceptive
○
Cigar shaped, encapsulated and contains longitudinal strands of
collagenous fibers
○
Ruddini ending:
!
Mechanoreceptors: detecting touch and pressure
Pacinian
Corpuscle
(vibration)
Meissner's
Corpuscle
(touch)
Merkel's
Disc
(touch)
Ruffini's
Ending
(stretch)
Receptive
fields
Large, vague
borders
Small, sharp
borders
Small,
sharp
borders
Large, vague
borders
Response Fast-adapting Fast-adapting Slow-
adapting
Slow-
adapting
Receptor Sensation Adaptation
Rate
Receptive
Field
Free nerve
endings
Itch, tickle, pain Tonic or
phasic
Large or
small
Ruffini
endings
Stretching of skin,
deep pressure
Tonic
(prolonged)
Large
Merkel discs Fine touch and
pressure
Tonic Small
Meissner
corpuscle
Fine touch, pressure,
slow vibration
Phasic -
moderate
Small
Hair follicle Crude touch,
movement of hairs
Phasic -
moderate
Small
Krause bulbs Fast vibration Phasic -fast Small
Pacinian
corpuscle
Pressure, fast
vibration, tickling
Phasic -
fastest
Large
Summary of cutaneous mechanoreceptors:
!
Depending on the stimulus, the hair cell and either
increase/decrease the AP frequency in the afferent sensory fiber
(inhibition/excitation depending on direction of movement)
○
K+ channels on the stereocilia are linked by elastic filaments
!
Displacement of cilia opens or closes K+ channels
!
Note: K+ entry generally causes hyperpolarization
(normally K+ is less outside of cell)
"
Endolymph has high concentration of K+ (low Na+),
compared to perilymph
"
K+ entering the cell causes depolarization
!
Depolarization causes voltage gated Ca2+ channels to open
!
Ca2+ triggers the release of neurotransmitter
!
Depolarization of hair cells:
○
*see slide
!
Direction of movement of stereocilia --> inhibition or
excitation (via release of aspartate/glutamate)
!
Detect motion, position and sound
"
3 different regions: macula, cristae (motion),
vestibular system (positional information) + cochlea
(sound)
"
Functions:
!
Hair cell function:
○
Crista ampullaris lateralis
!
Crista ampullaris posterior
!
Crista ampullaris superior
!
Organ of corti
!
Macula utriculi
!
Macula sacculi
!
Hair cell locations:
○
Semicircular canal --> cupula (position/orientation)
!
Sacculus --> otolithic membrane (motion)
!
Cochlea --> tectorial & basilar membrane (sound)
!
Arrangement of hair cells:
○
Hair cells: modified epithelial cells that transduce mechanical stimuli
into electrical signals with extraordinary sensitivity
!
They overlie the macular sensory epithelium of the gravity
receptors of most vertebrates and are required for optimal
stimulus input of linear acceleration and gravity
!
The greater relative mass of the membrane (due to presence
of otoconia), causes it to lag behind the macula temporarily,
leading to the transient displacement of the hair bundle
!
Note: fish have a large crystal = otolith
!
Otoconia are crystals of calcium carbonate and make the otolitic
membrane heavier than the structures and fluids surrounding it
○
Linear acceleration of head or changes of head position1.
Shift of position of otolithic membrane2.
Deflect stereocilia to or away from kinocilium3.
Results in stimulation or inhibition 4.
Mechanism of stimulation in utricle and saccule:
○
3 fluid-filled semi-circular canals --> equilibrium
!
Utriculus with utricular otolith (ear-stone) --> equilibrium,
gravity detector
!
Sacculus with otolith --> sound detection
!
Lagena with otolith --> sound detection
!
Inner ear of fish:
○
Uses cupula with sensory hair cells that connect to afferent
nerve through neuromast
!
As water flows, opening experience higher or lower
pressure
"
**note: K+ in water is low (K+ may be elevated in
tubule)
"
Tube is consistent with the water from environment
!
Consists of hair cells encased in gelatinous cap
specialized for detecting water movements
"
Found in the skin and typically grouped into structures
such as the lateral line or dispersed over the anterior of
the body
"
Object/predator avoidance
!
Prey detection
!
Ability to swim in school
!
Fish behaviours linked to the laternal line invludes:
"
Neuromasts:
!
Note: lateral line stitches in frogs
!
The lateral line system of fish and amphibians is involved in
motion detection:
○
Detection of motion:
!
Uses mechanoreceptors of the inner ear
○
Are important for balance
"
The vestibular organs detect position and motion of the head
!
Endolymph movement in the semicircular canals causes
deflection of hair cells
!
CNS integration from ampullae permits precise
determination of head movement direction
!
Vestibular organs and equilibrium:
○
*see slide
!
Evolution of semicircular canals:
○
Detecting position:
!
Pitch -depends on frequency
!
Intensity -depends on amplitude
!
Timbre -depends on overtones
!
Sound is characterized by its pitch (tone), intensity (loudness) and
timbre (quality)
○
Outer ear air pressure waves are converted into cochlea
liquid pressure waves by the middle ear ossicles
!
Cochlea pressure waves cause site-specific and pitch-
dependent displacement of the basilar membrane
!
Displacement of basilar membrane bends hair cells
!
Spatial coding is maintained in auditory nerve and cortex
!
Amplitude of pressure waves determined magnitude of
displacement and frequency of AP in afferent neurons
(intensity of sound)
!
*see slide
!
Cochlea and Hearing (in land animals)
○
*see slides
!
Staples --> oval window --> perilymph in cochlea
!
Amplitude of wave = loudness
"
Low frequency sound waves take longer (move further
through cochlea) before they are detected (by basilar
membrane)
!
Inner and outer hair cells
"
Organ of corti -contains clustered hair cells
!
Anatomy of Cochlea:
○
Sound waves arrives at tympanic membrane
!
Movement of tympanic membrane causes displacement of
the auditory ossicles
!
Movement of the stapes at the oval window establishes
pressure waves in the periplymph of the vestibular duct
!
The pressure waves distort the basilar membrane on their
way to the round window of the tympanic duct
!
Vibration of the basilar membrane causes vibration of hair
cells against the tectorial membrane
!
Information about the region and intensity of stimulation is
relayed to the CNS over the cochlear branch of cranial nerve
VIII
!
Events involved in hearing:
○
Ear canal with cellular debris prevents air from
entering ear
"
Sound waves are received through fat-filled lower jaw
"
*see slide
"
Ex. Whales
!
Fish have bones in the inner ear (=otoliths), which are
much denser than water and the fish's body
"
As a result, these ear bones move more slowly in
response to sound waves than the rest of the fish
"
The difference between the motion of the fish's bosy
and the otoliths bend/displace the cilia on the hair cells
that are located in the inner ear
"
This differential movement between the sensory cells
and the otolith is interpreted by the brain as sound
"
Otoliths are made of calcium carbonate and their
size/shape is highly variable among species
"
Bodies of fish are approximately the same density as water,
so sound passes through their bodies
!
Sound detection in marine animals:
○
The hearing range (frequency) various widely across taxa
○
Detecting sound:
!
Mechanoreceptors: detecting motion, position and sound waves
Remarkably, prestin aminio-acid sequences of echolocating
dolphins have converged to resemble those of distantly related
echolocating bats
○
The motor protein prestin confers sensitive and selective hearing in
mammals
!
Melon -fat structure involved in creation of 'clicks'
!
Marine mammals use similar mechanism
○
Bat: uses sonar and returning sound waves from prey
!
Solute linked carrier -associated with high frequency hearing
○
Prestin SLC26 protein family:
!
This appears to be an essential component of a mechanism tuning
mechanism that is unique to the mammalian ear
○
Prestin molecule changes in shape during hyperpolarization and
depolarization
○
The mammalian ear has more specialized cells which detect and converte
sound in the normal way, but then convert the electrical signals into
changes in cell length
!
Prestin is the motor protein of the outer hair cells of the inner ear
of the mammalian cochlea
○
Immunolocalization shows prestin is expressed in the laternal
plasma membrane of the outer hair cells, the region where
electromotility occurs
○
*uses chloride binding sites
○
Prestin is a protein that in humans is coded by the SLC26A5 gene
!
*see prestin gene phylogeny
!
Molecular Evolution: Gene Convergence in Echolocating Mammals
11/28/17
Detecting weak electric fields
!
Electrogenesis
!
Electroreception and electrogenesis:
Although flatfish bury themselves in the sand, sharks can still
detect them
○
Vision (live flatfish) --> no
!
Olfaction (dead flatfish) --> no
!
Electroreception (battery) --> YES
!
Possible reasoning:
○
Natural food for many sharks is flatfish
!
Sense electric currents produced by active muscles of their prey
○
Sharks are attracted to prey over long distances by smell but use electric
sense over short distances to attack
!
Prey detection using electroreception: sharks
Can detect weak electrical signals in the environment (< 1 uV/cm)
○
Sense organs specialized for electroreception have only been found
among vertebrates (8600 sp)
○
Through convergent evolution, several vertebrate groups have developed
ability to perceive electric signals
!
Passive electroreception: detection of electric fields generated by other
organisms
!
Active electroreception: using self-generated electric fields for
electrolocation (detection co-specifics, prey, objects) and in some species
for electrocommunication
!
High frequency E: ~14,000 active electrosensory receptor organs
○
Low frequency E: ~700 passive electrosensory receptor organs
○
Mechano: ~250 mechanosensory receptor organs
○
Apteronotus
!
Electroreception:
Electric sensing was an early development in the course of
vertebrate evolution
○
The electrosensory system is closely related to the lateral line
system of fish and hearing/balance in terrestrial animals
○
Evolution:
!
*see slide for equation
○
Only useful over short distances
○
If distance of an object is doubled, its size must be quadrupled and
the energy expended by the fish increased by 8-fold if it is still to
be detected
○
Limitations of electrolocation:
!
Lower frequency
!
Ampullary
○
Higher frequency
!
Tuberous
○
Have different morphology
!
Have evolved from earliest ampullary electroreceptors with
loss of electroreception capacity in many (still have
mechanoreceptors)
!
No need
"
Electricity does not travel well through air
"
All mammals (except monotremes) do not have
electroreception
!
*both can occur in the same individual
○
Types of electroreceptors:
!
Closes eyes, ears and nose when it swims at night
○
Tail flip causes field of 1mV at 5cm
!
Sensitivity is 10uV/cm
!
Eats shrimp
○
40,000 electroreceptors and 40,000 mechanoreceptors on bill
○
Ex. Duck-billed Platypus
!
Detects prey by their electric fields
○
Feed by straining zooplankton with their filtering apparatus
○
Vision -feed in dark
!
Feeding on like plankton with nares plugged
"
Olfaction -no feeding with plankton extract
!
Mechanical -feed on plankton encapulsated in agarose
!
Hydrodynamic -feed in turbulent water conditions
!
Other senses:
○
Conclusion: electric sense is sufficient for prey detection and
capture
○
Ampulla of Lorenzini dispersed over rostum
○
Assists in prey detection, orientation and navigation
○
Paddlefish will strike at dipoles that produce artificial electric
fields that stimulate natural plankton
○
Rostrum is specialized for hunting electric signals in the near-field
evironment
○
Voltage gradient of 0.01uV/cm causes change in EEG
"
Voltage gradient of 1-10uV/cm causes escape when
swimming
!
To stimulate fields that fish can detect, need to
separate electrodes by 5km
"
1.5V battery connected to electrodes 50m apart sets of a field
of 300uV/cm
!
Sensitivity:
○
Ex. Paddlefish
!
Scalloped hammerhead vs. sandbar shark
○
Approximately 70% of orientations were initiated to stimuli of
<0.1uV/cm for both species
○
Note: 0.1uV/cm = flash light battery conencted to electrodes
over 16,000km apart
!
Both species initiated approximately 35-40% of orientations to
stimuli of <0.01uV/cm
○
Shark sensitivity:
!
*see slides
○
Canals connect ampullae of lorenzini to pores on the shark's
rostrum
○
The gel is a glyco-protein based substance with
semiconductor properties
!
Electrical impulses travel through the canal to stimulate
ampullae
!
Nerve impulses then send the information directly to the
brain
!
Canal of lorenzini is a jelly-filled tubule
○
*note: also determines direction in 3D
○
Room temperature proton conductivity of the jelly is very
high at 2 mS/cm
!
This conductivity is 40-fold lower then Nafion (highest
reported for a biological material)
!
Proton conductivity in ampullae of Lorenzini jelly:
○
Influx of positive charge causes the cell to release
neurotransmitters at synapses (or contact points) with nerves
to the brain, stimulation them to fire
!
The firing rate indicates strength and polarity of the external
field, and the field's location relative to the shark is thought
to be determined by the positions of the activated pores on
its body
!
The cells return to their original electrical state afterward by
opening a second time of membrane channel that permits
positively charged potassium ions to exit
!
A sensing cell reacts when an external electric field producing a
small electric potential across its membrane, leading channels to
allow positively charged calcium ions to rush in
○
Elasmobranch electroreception: ampullae of lorenzini
!
Electroreceptors:
Active electroreception, communication, defense & prey capture
!
*see slide
○
Electroreceptors, CNS and the electric organ interact
!
Use for prey capture (see slide for examples)
!
Fresh water and sea-water have difference in arrangement of
cells that produce electricity due to differences in
conductivity of the medium
!
Strongly electric fish (several hundred volts)
○
Prey detection and communication (see slide for examples)
!
Weakly electric fish (millivolts -few volts)
○
*10 families, over 500 sp --> convergent evolution
○
Skates: have electric organs in tail
○
Electric organs:
!
Electric rays: torpedoes, numbfish
!
See slide
○
Isopotential lines and current flow around an electric fish
○
Conductive objects concentrate the field
!
Resistive objects spread the field
!
Active electrolocation
○
Electric field emanates from an electric organ in the tail
region
!
It is sensed by the electroreceptive skin areas, using two
electric pits (foveas) to actively search and inspect objects
!
*see slide --> conductive vs. resistive objects
!
Ex. Elephantfish
○
Active electroreception:
!
Contain electrocytes = modified muscle cells
○
Non-innnervated surface does not change membrane
potential
!
Posterior surface generates an action potential
!
At rest = 0
!
Voltage gated sodium channels open to depolarize
posterior size of membrane (-90--> + 60 mV)
"
When stimulated = 150mV
!
Potential differences across membrane
○
*see slide
○
Electric eel
!
Electrocytes arranged in series increases the discharge voltage,
whereas an arrangement in parallel increases total current flow
(amperage)
○
In low-conductivity water the caudal electric organ is long and thin
(more electrocyte columns, fewer rows) which serves to increase
voltage in order to maximize the power output of the EOD
○
In high-conductivity water, electric organ is short and deep (more
electrocyte rows, fewer colums) which serves to increase amperage
(current)
○
Note: seawater is more conductive than freshwater
○
Electrocytes stacked from tail to head increase voltage
!
Multiple rows of electrocytes increase current
!
Overall:
○
Series vs. Parallel
!
This temporarily creates an additional charge across the cell
membrane on that side of the cell of about 0.065V
○
Now, instead of having a negative inside and positive outside, the
cell temporarily has a 0.085V difference across the convoluted
side, and a similarly oriented charge of about 0.065V on the
smooth side
○
These chargers are essentially stacked in series, so that the end
result is a brief charge across the entire cell of about 0.15V (0.085+
0.065)
○
When nerve fibers send a signal to an electrocyte, ion channels on the
smooth side of the cell open, allowing positive ions to rush into the cell
!
Electrophorus has ~ 5000 cells/column
○
Therefore, voltage = 5000 cells*150mV/cell = 755 V
○
Note: when electrocytes are arranged in a series in a column, the voltages
sum
!
--> electric organ discharge (EOD)
!
Pacemaker neurons --> relay neurons --> motor neurons -->
muscle
○
If a normal nerve signal went out from the brain to each
electrocyte, the signal would reach the first cells in the stack
before it reached the cells at the end of the stack
!
By the time the cells a the end firest, the ones at the
beginning would have already shut off again
!
Nerve fibers closer to the head are smaller than
those near the tail
!
Nerves closer to head also tend to take more of a
winding path than nerves near tail
!
Slower chemical signals are used in nerve fibers
closer to the head
!
Involves:
"
The electric eel has to synchronize the firing of thousands of
electrocytes in a stack so they all turn on at the same and add
together to create the big voltage needed to shock the fishe's
prey
!
Make pathlengths equal
"
Vary conduction velocity
"
Vary electrocyte stalk length (conducts faster than
nerve)
"
Overall:
!
Compensatory delay:
○
Generating electric organ discharge:
!
Brains tend to be larger than average fish --> more sensory
information
○
*see slide (do not need to memorize names)
○
Pathways in electroreception:
!
See slide
!
V = I (current) *R(resistance)
!
Ohm's Law
○
*see slide
!
Same current --> decreased voltage
"
Wet skin has decreased resistance
!
Electrocution
○
The current path must usually include either the heart or
brain to be fatal
!
Whether an electric current is fatal is dependent on the path it takes
through the body, which depends in turn on the points at which the
current enters and leaves the body
○
Electricity for prey capture and protection (strongly electric fish):
!
Electrogenesis:
Works because the normal response of a fish to direct electric current
(=galvanotaxis) is to swim towards the positive (anode) electrode
!
As in all vertebrate species, fish muscles are controlled by electrical
signals sent through the nervous system from the brain
!
The brain is believed to be negatively charged, therefore orientating the
fish toward the positively charged anode
!
The electric current interrupts the neurological pathway, causing the fish
to involuntary swim towards the anode
!
When fish are placed across the electric field, the bodies curve toward
the positive pole because the motor spinal nerves facing the negative
pole are inhibited while those facing the positive pole are stimulated
!
Electrofishing:
11/30/17
Inductive magnetoreception
!
Mechanically induced mechanoreception
!
Magnetoreception (detecting the Earth's magnetic field)
Flight as long as 1800km have been recorded by birds in
competitive pigeon racing
○
Because of this skill, homing pigeons were used to carry messages
as messenger pigeons
○
Homing pigeon is a variety of domestic pigeon derived from the rock
pigeon, selectively bred for its ability to find its way home over
extremely long distances using mechanoreception
!
Captive-raised monarchs appear capable of migrating to
overwintering sites in Mexico
○
Have spring and fall migrations
○
*see slide
○
Monarch butterfly:
!
Newly hatched loggerhead turtles use a magnetic map for
orientation along their migratory route in the North Atlantic
subtropical gyre
○
Hatchling turtles were placed in artificial magnetic fields that
replicate three locations along the migratory route
○
Results indicate that the turtles preferentially orient to stay within
the gyre (i.e. to follow a specific migratory route)
○
Sea turtles:
!
Species Examples:
Bacteria
○
Crustaceans (spiny lobsters)
○
Insects (butterflies, bees, flies)
○
Fish (salmon)
○
Elasmobranchs (sharks, skates, rays)
○
Amphibians (cave salamanders)
○
Reptiles (sea turtles)
○
Birds (homing pigeons, migratory birds)
○
Mammals (dolphins, whales)
○
Magnetic compasses are phylogenetically widespread:
!
Declination (--> north)
○
Inclination
○
Field intensity
○
Information obtained by earth's magnetic fields:
!
Direction (polarity compass)
○
*north pole is here and I am ___ away from it
Dip angle (inclination compass)
○
Animals can sense two qualities of earth's magnetic field:
!
Behavioural experiments have shown that many animals can sense the Earth's
magnetic field and use it for guiding movements (nature's GPS)
*use electric fields
○
Changing field induces a current in the jelly filled canals of the
ampullae of Lorenzini which changes the electric potential in the
ampulla
○
The voltage is amplified in the ampulla due to ion-channel
mediated interactions between the apical and basal membrane
○
There is an induced electromagnetic field in the
secondary coil (producing an induced current) due to
the changing magnetic field through this coil
"
When the switch is closed, the ammeter deflects
momentarily
!
Steady magnetic fields cannot produce a current
"
When there is a steady current in the primary coil, the
ammeter reads zero current
!
Experiment:
○
If the animal moves so that rotation occurs around an axis in
the plane of a semi-circular canal, there will be no
displacement of the endolymph but electromagnetic
induction could occur
!
Depending on intensity and orientation of the external
magnetic field, this will induce an electromotive force in the
conductive endolymph
!
This results in the separation of charged within the circuit,
inducing cation influx through highly sensitive voltage-gated
ion channels
!
Depicted is one semicircular canal of a vertebrate, filled with
cation-rich endolymph, and sensory cells located on either side of
the cupula
○
Inductive1.
*see slide for mechanism and biology
○
Fe3O4 -one of the oxides of iron
!
Is ferrimagnetic
!
They could site between ciliated olfactory and sensory
cells
"
Magnetite has been found in many animal tissues, but in fish
some evidence points to the nose as the home for
magnetoreceptors
!
Magnetite tugged by Earth's field would mechanically
control neural circuits by opening ion channels in cell
membranes
!
Magnetite:
○
Magnetitie field changers trigger movement of a SPM
mangetite cluster, which in turn induces derformation
in the plasma membrane
"
This event induces the opening of a non-selective
mechanosensitive ion channel
"
Mechanosensitive ions channels are located in the membrane
but do not have a physical link with the magnetite
A)
At rest, the ion channel is blocked
"
Change in the magnetite field relieves the blockade,
allowing sodium or calcium entry into the cells
"
Magnetite is physically connected to the ion channel B)
The messenger binds to the ion channel and opens it
"
Magnetite movement releases second messenger, either
directly or through the creation of tension in the membrane
C)
Cell containing magnetite linked to a neighbouring
nerve terminal through a 'tip link'
"
The ion channel is linked via a molecular motor to
actin filaments
"
Movement in the magnetite-containing compartment
stretch the link and open the ion channel
"
After opening, a molecular motor could reposition the
ion channel reducing the tension in the link and
allowing the system to be stimulated again
"
Mechanism based on auditory hair cell
mechanotransduction**
D)
Proposed scenario:
○
Mechanical2.
Magnetosensing rod-like protein complex identified in
drosophila
!
Exhibits spontaneous alignment in magnetic fields, including
that of Earth
!
Protein complex may form the basis of magnetoreception in
animals
!
A magnetic protein biocompass:
○
They are found in plants and animals
!
Cryptochromes are involved in the circadian rhythms of
plants and animals, and in the sensing of magnetic fields in a
number of species
!
Cry-deficient drosophilia does not show
magnetosensitive behaviour
"
Response of Cry to magnetic fields via radical pairs
may be used to perceive inclination information from a
geomagnetic field
"
Evidence:
!
Cry perceive geomagnetic information via the
quantum spin dynamics of radical-pair of FAD
reaction initiated by light (from sun or moon)
"
*see slide
!
Cyptochromes are a class of flavoproteins that are sensitive to blue
light
○
Magnetosensing protein crystals orient in a magnetic field
○
In nerve fiber layer --> to optic nerve
!
Gangion cell layer
!
Outer nuclear layer --> photoreceptor cell
!
Cry Magr protein complex is located…
○
Cryptochromes may lie in mysterious double cone cells
!
The ratio of chemical products from each cone could
determine magnetic orientation, which the brain might
process as light and dark patches on the visual field
!
Light turns the cyptochrome into a radical pair molecular,
with two unpaired electrons that flip between parallel and
anti-parallel states
!
Study:
○
11-cis retinal absorbs light and isomerized into all-trans
retinal
!
All-trans retinal dissociates from opsin
!
Activated opsin activates G protein transducin
!
Transducin activates PDE, which converts cGMP to GMP
!
The decreased cGMP closes a Na+ channel
!
Na+ entry decreases --> hyperpolarization
!
Mechanism:
○
Hypothesis: visual field of a bird may be modified through the
magnetic filter function
○
Specifically, many studies have shown that birds can only
orient if blue light is present
!
The avian compass is also an inclination-only compass,
meaning that it can sense changes in the inclination of
magnetic field lines but is not sensitive to the polarity of the
field lines
!
A bird can only sense the magnetic field if certain wavelengths of
light are available
○
Cry and MagR proteins are found widely distributed across taxa
○
Light-based 3.
Three types of magnetoreception:
Passive magnetoreception
!
Polarity via magnetite containing cells
○
Inclination via light sensitive magnetoreception
○
Active magnetoreception
!
Summary:
Sensory Physiology
#$%&'()*+,-./0120&, 34+,5637 3854,9:
Introduction
○
Classification of sensory receptors
○
Transduction of sensory signals
○
Stimulus encoding
○
General properties of sensory reception:
!
Olfaction: an example of chemoreception
!
Thermoreceptors
○
Infrared receptors
○
Thermoreception
!
Outline:
Reception of signal (exteroreceptors and interoreceptors)1.
Transduction of signal2.
Amplification of signal3.
Transmission of the signal to the integrating centre
(afferent/sensory neurons)
4.
Perception of the stimulus at the integrating center (CNS)5.
Sensory reception is a process:
!
Dorsal root ---> sensory nerve
○
Ventral root --> motor nerve
○
*see dorsal view of the central nervous system
!
Introduction:
Chemoreceptors -specific chemicals
○
Mechanoreceptors -mechanical energy
○
Photoreceptors -light (electromagnetic radiation)
○
Thermoreceptors -temperature
○
Nociceptors -noxious chemical, mechanical and thermal stimuli
○
Electroreceptors -electric fields
○
Magnetoreceptors -magnetic fields
○
Sensory receptors can be categorized by the type of stimulus energy to
which they respond:
!
Each receptor is highly selective for a specific kind of energy
!
Modality specialization favours higher sensitivity
!
Classification of Sensory Receptors:
Sensory receptor is the ion channel, it receives and
transduces the signal
!
Mechano/thermo/electro and some taste receptors use an inotropic
transduction mechanisms
○
Stimulus induces a conformation change in specific
membrane receptor --> activates GPCR --> adenylate
cyclase (ATP-->cAMP) --> opens ion channel -->
depolarization
!
Photo/olfactory and some taste receptors use metabotropic
transduction mechanisms (like hormone signalling)
○
Transduction: the process by which sensory receptors change stimulus
energy into electrical signals
!
Transduction of Sensory Signal:
Sensory receptor cells are either primary afferent neurons (e.g. olfactory
neurons), or epithelial sensory cells (e.g. photoreceptors)
!
In primary afferent neurons, the stimulus elicits a graded generator
potential and all-or-none action potentials
!
In epithelial sensory cells, the stimulus elicits a graded receptor potential,
transmitter release, post-synaptic graded potential, and all-or-non action
potentials
!
*see slide
!
Transmission of Sensory Signals:
For a sensory signal to be interpreted by the CNS in a coherent way, the
receptor must encode stimulus modality, location, intensity and duration
!
The labeled-lines principle: since sensory receptors are maximally
sensitive to one type of stimulus and different sensory neurons project to
different regions of the CNS, the origin of affect neurons encodes both
stimulus modality and location
!
Sensory receptor cells encode stimuli over a limited range
!
Threshold intensity -receptor saturation = dynamic range
○
Instead, the frequency of AP is adjusted with stimulus
intensity
!
Action potentials are all-or-none events, so can't encode intensity
directly via magnitude
○
The upper limit of the dynamic range in epithelial sensory
cells is determined by the saturation of receptor proteins
when the maximum rate of release of neurotransmitters is
reached
!
The upper limit of the dynamic range in primary afferent
neurons is determined by the membrane potential at which
the maximum frequency of action potentials is reached
!
A given sensory receptor cell is sensitive to a specific level of
stimulus intensity (dynamic range)
○
Action potentials code stimulus intensity through changes in frequency
!
Receptor potential amplitude is proportional to the logarithm
of stimulus intensity
!
Higher sensitivity at low stimulus intensity
!
Logarythmic coding
○
Receptors sensitive to a different range of intensities work
together to provide discrimination across a wide range of
intensities
!
Range fractionation
○
Two strategies are used by the sensory system to expand the range of
stimuli that can be detected:
!
Sensory adaptation occurs in all senses, with the possible exception
of the sense of pain
○
Sensory adaptation occurs when sensory receptors change their
sensitivity to the stimulus
!
Ex. Wearing a shirt
!
Tonic receptors continue to be depolarized throughout the duration
of stimulus and adapt slowly
○
Phasic receptors depolarize primarily at the beginning of a
stimulus and adapt rapidly
○
Only fire when stimulus is changing
!
Most receptors undergo receptor adaptation when stimulus
intensity is maintained at a constant level
○
Tonic and phasic receptors encode stimulus duration:
!
Stimulus Encoding:
Clones the first 18 of ~1000 genes from the OR family
!
The recipients of the 2004 Nobel Prize in Physiology and Medicine
have made a significant contribution in solving this problem
○
How do we recognize and remember ~10,000 different odors?
!
Odorants bind to receptors
○
Olfactory receptor cells are activated and send electric signals
○
The signals are relayed in glomeruli
○
The signals are transmitted to higher regions of the brain
○
Organization of the olfactory system:
!
3% of the mammalian genome codes for different odorant
receptors
○
Most odors are composed of multiple odorant molecules, and each
molecule activates a unique subset of odorant receptors
○
Odorant receptors are used in a combinatorial manner to detect
odorants and encode their identities
○
5 = 5 goat
!
5+2+6 = 13 flowers
!
….add a few more and you're smelling cheese
!
Examples:
○
In both mammals and insects, but at vastly different scales,
the organization of the peripheral olfactory circuit is
conserved
!
*see mouse vs. fly
"
In mammals, the area of olfactory epithelium and the
number of odorant receptor types reflect reliance on the
sense of smell and degree of specialization
!
Species differences:
○
Odorant binding causes conformational change1.
Activated G protein stimulates adenylate cyclase2.
cAMP production3.
Opening of cation channels4.
Ca2+ and Na+ entry elicits generator potential 5.
Increased depolarization with Cl-channel opening6.
Generator potential opens voltage-gated Na+ channels and
triggers action potential
7.
Signal transduction in an olfactory receptor cell of mammals:
○
How olfactory receptors encode odors:
!
Olfaction: an example of chemoreception
11/21/17
Warm-sensitive --> respond to above skin temperature1.
Cold-sensitive --> respond to below skin temperature2.
Thermal nociceptors --> respond to extreme heat/cold3.
Peripheral thermoreceptors include:
○
The hypothalamus integrates info from the central and peripheral
thermoreceptors and coordinates adjustments in heat gain and loss
in a negative-feedback fashion
○
Vertebrates possess central thermoreceptors that monitor core body
temperature and peripheral thermoreceptors that monitor environmental
temperature
!
Hypothalamus acts as a thermostat and receives nerve impulses
from heat and cold thermoreceptors in the skin
○
Detects changes in blood temperature
!
There are also receptors in the hypothalamus = central
thermoreceptors
○
Body temperature control:
!
These can be internal (endogenous) or external (exogenous)
!
Pyrogens = substance that induces fever
○
All endogenous pyrogens are cytokines, molecules that are
all part of the innate immune system
!
They are produced by phagocytic cells and cause the
increase in the thermoregulatory set-point in the
hypothalamus
!
Interleukin-1 (alpha and beta)
"
Interleukin-6
"
Tumor necrosis factor (alpha)
"
Major endogenous pyrogens:
!
Endogenous:
○
Bacterial lipopolysaccharide (LPS), present in the cell wall
of some bacteria is an example of an exogenous pyrogen
!
Exogenous:
○
A trigger of fever (=pyrogen) causes a release of
prostaglandin E2 (PGE2)
!
PGE2 then acts on the hypothalamus, which generates a
systemic response back to the rest of the body, causing heat-
creating effects to match a new temperature level
!
Increased mobility of leukocytes
"
Enhanced leukocyte phagocytosis
"
Endotoxin effects decreased
"
Fever assists the healing process via:
!
When infected, survival was highest at 32.7C
(100%) vs. 25.5C (20%)
!
Fever in goldfish:
"
When infected, highest survival at 42C (~95%)
vs. 34C (0%)
!
High temperature is selected to help fight
disease
!
Fever in lizard:
"
*note: there are a variety of pyrogens across vertebrate taxa
!
Fever:
○
Pyrogens
!
Thermoreceptors are heat-gated or cold-gated ion channels
specialized for detecting distinct temperature ranged and involved
in converting thermal energy into electrical signals
○
They are part of the transient receptor potential (TRP) family of
cation channels
○
Modulated by endogenous lipid agonists derived from
glycerophospholipids via phospholipase A2
"
Repeated stimulation with menthol results in a
reduced response (-->adaptation to stimulus;
attenuates)
!
Regulated by phophatidylinositol (-->desensitized
without)
"
Transient receptor potential melastatin 8 (TRPM8) is a
calcium permeable nonselective cation channel activated by
cold
!
Many TRP channels are also chemoreceptors and a number of
plant-derived chemicals (menthol, capsaicin, allyl isothioncyanate)
are potential activators of specific TRP channels
○
Thermoreceptors:
!
Infrared receptors can detect heat radiating from objects at a
distance
○
The receptors are specialized to detect a portion of the
electromagnetic spectrum
○
Abdominal infrared pit organs of Australian fire-beetle
!
Metathorax infrared pit organs of black pine beetle
!
Lower lip infrared organs of green tree python
!
Infrared pit organs of western diamond back rattlensnake
!
Infrared pit organs of common vampire bats
!
Typically these receptors are concentrated in specialized organs:
○
100+ receptors in pit membrane -respond to temperature changes
<0.003C
○
Thermal image is superimposed on visual image
○
Localize endothermic prey in the dark
○
Pit membrane contains TRPA1 receptors that send
information via TG fibres
!
Rat snake does not feed on mammals specifically
"
Rattlesnake --> sample thermal environment
"
Determined the relative heat response profiles of rattlesnake
and rat snake channels expressed in oocytes of frogs
!
Ligand: allyl isothiocyanate = AITC
"
Used calcium imaging to identify TRPA1 (wasabi
receptor) channels as infrared receptors on sensory
nerve fibers that innervate the pit organ
"
Responds to temperatures similar to that of mammals
(prey)
"
Snake TRPA1 is a heat activated channel --> rattlesnake
!
Mechanism:
○
Another study found that a blind Northern Pacific rattlesnake, can
accurately strike as long as pit organs ere uncovered
○
Island is densely population with up to 3 tiger snakes in
every 25 square meters
!
Blinded by seagulls defending chicks
!
Would prey on abundant immobile prey (that can't escape
approach)
!
Ex. Blind tigers (rattlesnakes) of Carnac Island
○
Infrared receptors:
!
Thermoreception:
11/21/17
Olfaction and salmon migration
○
Pheromones and the vomeronasal organ
○
Chemoreceptors
!
Detecting touch and pressure
○
Detecting motion, position and soundwaves
○
Mechanoreceptors
!
Outline:
4+ basic taste perceptions: sweet, sour, salty, bitter, umami
○
If the olfactory system is blocked, the perception of taste is
sharply reduced
!
Taste and smell interact
○
Chemoreceptors in taste buds or nasal cavity are triggered when specific
molecules bind and trigger action potential
!
Sensory nerve fibers move through connective tissue of
tongue
!
Tongue --> papillae --> taste buds --> single taste bud
○
Causes conformational change in ion channels --> Na+
opens
!
Causes depolarization --> Ca2+ influx
!
Which stimulates release of neurotransmitters that enter
sensory neuron to create an action potential
!
Ex. Sugar molecule enters taste bud --> signal transduction
pathway
○
Signals to PLCbeta2 via PIP2 causing release of IP3 (+
DAG)
!
Causes Ca2+ to enter from Ca2+ store or external sources
!
Na2+ then enters cell from TrpM5
!
Ca2+ also results in production of ATP
!
G-protein coupled receptor are specific to sweet, bitter or umami
tastant
○
Enters cells (diffusion) and converts into H+ which then
binds to a proton-sensitive channel
!
Sour tastant:
○
Binds to ENaC (epithelial sodium channel), causing Na+ to
enter cell --> depolarization
!
Salt tastant:
○
Umami -T1R1 + T1R3
!
Sweet -T1R2 + T1R3
!
Bitter -~30 T2Rs
!
Sodium -ENaC
!
Sour and Carbonation Cells -PKS2L1 & CA IV
!
Mammalian taste receptors (see slide):
○
Less with just glutamate
!
Discovered in 1913 that dried bonito flakes contained
another umami substance (ribonucleotide IMP)
!
In 1957, realized that ribonucleotide GMP present in shiitake
mushrooms also confirmed umami taste --> synergistic
effect between ribonucleotides and glutamate
!
Glutamate + GMP --> Umami
○
Taste
!
How do salmon form the olfactory memories that guide them
to their home stream after their oceanic journeys
!
Freshwater residence (0-3years): emergence to
seaward migration
A)
Ocean distribution (4-6years)B)
Homing migration to Iliamna lake and natal site for
spawning (4-6years)
C)
Ex. Sockeye salmon of Iliamna Lake
!
Thyroid hormone primes the fish for imprinting
shortly after hatching and during spring downstream
migration
"
*see slide
"
Olfactory imprinting to the odor of the streambed where
salmon hatched involves an increased sensitivity of the
olfactory neurons to specific compounds (memory)
!
In olfactory receptor cells, stimulation of guanylyl
cyclase activity by the odorant PEA is greater in the
olfactory cilia of mature salmon previously PEA-
imprinted than in PEA-naïve fish
"
*see signal transduction in an olfactory cell of salmon
"
Pathway stimulation during imprinting results in
enhanced odorant sensitivity during homing migration
"
Olfactory memories (long term potentiation) are at least
partly mediated by enhanced cGMP signaling:
!
Olfaction and Salmon Migration:
○
Found in amphibians, reptiles and non-primate
mammals
"
The vomeronasal organ is distinct from the olfactory
epithelium
!
From the AOB, the information is transmitted to the
hypothalamus and results in modification of
behaviours and endocrine status
"
Vomeronasal neurons have distinct receptors that
differ from odorant receptors and respond to
pheromones
"
Vomeronasal neurons project to the accessory olfactory
bulbs (AOB)
!
Odour particles are captured from the air by the tongue
"
The vomeronasal organ is found on the roof of the
buccal cavity
"
During tongue retraction, odour particles are mized
with fluids in the mouth and are delivered to the ducts
leading to the vomeronasal organs
"
Ex. In Snakes
!
Presence and functionality in humans in controversial
"
Organ regresses during fetal development
"
Many genes essential for VNO function in animals
(such as TRPC2) are non-functional in humans
"
Chemical communication does appear to occur among
humans, but this does not necessarily imply that the
human vomeronasal organ is functional
"
Vomeronasal organ (aka. Jacobson's organ) -auxiliary
olfactory sense organ
!
Flehmen response --> bearing upper teeth
!
Pheromones and the Vomeronasal Organ
○
Olfaction:
!
Chemoreception:
11/23/17
*see slide
○
Hair receptor --> hair movement and very gentle touch
○
Merkel's disc --> light, sustained touch
○
Pacinian corpuscle --> vibrations and deep pressure
○
Reffini endings --> deep pressure
○
Meissner's corpuscle --> light, fluttering touch
○
Skin tactile receptors function as isolated sensory cells and include free
nerve endings or various enclosed accessory structures:
!
Mechanoreceptors -application of the mechanical stimulus produces
deformity in the receptors --> stretch of the membrane --> open Na+
channels --> Na+ influx --> depolarization
!
Tonic (slow) detection of pressure within very small receptive field
for fine tactile discrimination
○
Releases serotonin
○
*see slide
○
Merkel's disk:
!
Phasic (fast acting) detection of pressure
○
Pressure on the skin is transmitted to the corpuscle in the dermis
○
The shape of the corpuscle is changed causing sodium channels in
the neurone membrane to open
○
Sodium ions diffuse down the concentration gradient, depolarizing
the membrane (no calcium!!) = generator potential
○
The greater the pressure, the more sodium channels open causing a
bigger generator potential
○
If the threshold of that neuron is reaches, an action potential
develops and is transmitted along the sensory neuron
○
Pacinian corpuscle:
!
Deep in the skin, as well as in joint ligaments and joint capsules
○
Tonic (slow)
!
Large field
!
Detects skin stretch
○
Cutaneous or proprioceptive
○
Cigar shaped, encapsulated and contains longitudinal strands of
collagenous fibers
○
Ruddini ending:
!
Mechanoreceptors: detecting touch and pressure
Pacinian
Corpuscle
(vibration)
Meissner's
Corpuscle
(touch)
Merkel's
Disc
(touch)
Ruffini's
Ending
(stretch)
Receptive
fields
Large, vague
borders
Small, sharp
borders
Small,
sharp
borders
Large, vague
borders
Response Fast-adapting Fast-adapting Slow-
adapting
Slow-
adapting
Receptor Sensation Adaptation
Rate
Receptive
Field
Free nerve
endings
Itch, tickle, pain Tonic or
phasic
Large or
small
Ruffini
endings
Stretching of skin,
deep pressure
Tonic
(prolonged)
Large
Merkel discs Fine touch and
pressure
Tonic Small
Meissner
corpuscle
Fine touch, pressure,
slow vibration
Phasic -
moderate
Small
Hair follicle Crude touch,
movement of hairs
Phasic -
moderate
Small
Krause bulbs Fast vibration Phasic -fast Small
Pacinian
corpuscle
Pressure, fast
vibration, tickling
Phasic -
fastest
Large
Summary of cutaneous mechanoreceptors:
!
Depending on the stimulus, the hair cell and either
increase/decrease the AP frequency in the afferent sensory fiber
(inhibition/excitation depending on direction of movement)
○
K+ channels on the stereocilia are linked by elastic filaments
!
Displacement of cilia opens or closes K+ channels
!
Note: K+ entry generally causes hyperpolarization
(normally K+ is less outside of cell)
"
Endolymph has high concentration of K+ (low Na+),
compared to perilymph
"
K+ entering the cell causes depolarization
!
Depolarization causes voltage gated Ca2+ channels to open
!
Ca2+ triggers the release of neurotransmitter
!
Depolarization of hair cells:
○
*see slide
!
Direction of movement of stereocilia --> inhibition or
excitation (via release of aspartate/glutamate)
!
Detect motion, position and sound
"
3 different regions: macula, cristae (motion),
vestibular system (positional information) + cochlea
(sound)
"
Functions:
!
Hair cell function:
○
Crista ampullaris lateralis
!
Crista ampullaris posterior
!
Crista ampullaris superior
!
Organ of corti
!
Macula utriculi
!
Macula sacculi
!
Hair cell locations:
○
Semicircular canal --> cupula (position/orientation)
!
Sacculus --> otolithic membrane (motion)
!
Cochlea --> tectorial & basilar membrane (sound)
!
Arrangement of hair cells:
○
Hair cells: modified epithelial cells that transduce mechanical stimuli
into electrical signals with extraordinary sensitivity
!
They overlie the macular sensory epithelium of the gravity
receptors of most vertebrates and are required for optimal
stimulus input of linear acceleration and gravity
!
The greater relative mass of the membrane (due to presence
of otoconia), causes it to lag behind the macula temporarily,
leading to the transient displacement of the hair bundle
!
Note: fish have a large crystal = otolith
!
Otoconia are crystals of calcium carbonate and make the otolitic
membrane heavier than the structures and fluids surrounding it
○
Linear acceleration of head or changes of head position1.
Shift of position of otolithic membrane2.
Deflect stereocilia to or away from kinocilium3.
Results in stimulation or inhibition 4.
Mechanism of stimulation in utricle and saccule:
○
3 fluid-filled semi-circular canals --> equilibrium
!
Utriculus with utricular otolith (ear-stone) --> equilibrium,
gravity detector
!
Sacculus with otolith --> sound detection
!
Lagena with otolith --> sound detection
!
Inner ear of fish:
○
Uses cupula with sensory hair cells that connect to afferent
nerve through neuromast
!
As water flows, opening experience higher or lower
pressure
"
**note: K+ in water is low (K+ may be elevated in
tubule)
"
Tube is consistent with the water from environment
!
Consists of hair cells encased in gelatinous cap
specialized for detecting water movements
"
Found in the skin and typically grouped into structures
such as the lateral line or dispersed over the anterior of
the body
"
Object/predator avoidance
!
Prey detection
!
Ability to swim in school
!
Fish behaviours linked to the laternal line invludes:
"
Neuromasts:
!
Note: lateral line stitches in frogs
!
The lateral line system of fish and amphibians is involved in
motion detection:
○
Detection of motion:
!
Uses mechanoreceptors of the inner ear
○
Are important for balance
"
The vestibular organs detect position and motion of the head
!
Endolymph movement in the semicircular canals causes
deflection of hair cells
!
CNS integration from ampullae permits precise
determination of head movement direction
!
Vestibular organs and equilibrium:
○
*see slide
!
Evolution of semicircular canals:
○
Detecting position:
!
Pitch -depends on frequency
!
Intensity -depends on amplitude
!
Timbre -depends on overtones
!
Sound is characterized by its pitch (tone), intensity (loudness) and
timbre (quality)
○
Outer ear air pressure waves are converted into cochlea
liquid pressure waves by the middle ear ossicles
!
Cochlea pressure waves cause site-specific and pitch-
dependent displacement of the basilar membrane
!
Displacement of basilar membrane bends hair cells
!
Spatial coding is maintained in auditory nerve and cortex
!
Amplitude of pressure waves determined magnitude of
displacement and frequency of AP in afferent neurons
(intensity of sound)
!
*see slide
!
Cochlea and Hearing (in land animals)
○
*see slides
!
Staples --> oval window --> perilymph in cochlea
!
Amplitude of wave = loudness
"
Low frequency sound waves take longer (move further
through cochlea) before they are detected (by basilar
membrane)
!
Inner and outer hair cells
"
Organ of corti -contains clustered hair cells
!
Anatomy of Cochlea:
○
Sound waves arrives at tympanic membrane
!
Movement of tympanic membrane causes displacement of
the auditory ossicles
!
Movement of the stapes at the oval window establishes
pressure waves in the periplymph of the vestibular duct
!
The pressure waves distort the basilar membrane on their
way to the round window of the tympanic duct
!
Vibration of the basilar membrane causes vibration of hair
cells against the tectorial membrane
!
Information about the region and intensity of stimulation is
relayed to the CNS over the cochlear branch of cranial nerve
VIII
!
Events involved in hearing:
○
Ear canal with cellular debris prevents air from
entering ear
"
Sound waves are received through fat-filled lower jaw
"
*see slide
"
Ex. Whales
!
Fish have bones in the inner ear (=otoliths), which are
much denser than water and the fish's body
"
As a result, these ear bones move more slowly in
response to sound waves than the rest of the fish
"
The difference between the motion of the fish's bosy
and the otoliths bend/displace the cilia on the hair cells
that are located in the inner ear
"
This differential movement between the sensory cells
and the otolith is interpreted by the brain as sound
"
Otoliths are made of calcium carbonate and their
size/shape is highly variable among species
"
Bodies of fish are approximately the same density as water,
so sound passes through their bodies
!
Sound detection in marine animals:
○
The hearing range (frequency) various widely across taxa
○
Detecting sound:
!
Mechanoreceptors: detecting motion, position and sound waves
Remarkably, prestin aminio-acid sequences of echolocating
dolphins have converged to resemble those of distantly related
echolocating bats
○
The motor protein prestin confers sensitive and selective hearing in
mammals
!
Melon -fat structure involved in creation of 'clicks'
!
Marine mammals use similar mechanism
○
Bat: uses sonar and returning sound waves from prey
!
Solute linked carrier -associated with high frequency hearing
○
Prestin SLC26 protein family:
!
This appears to be an essential component of a mechanism tuning
mechanism that is unique to the mammalian ear
○
Prestin molecule changes in shape during hyperpolarization and
depolarization
○
The mammalian ear has more specialized cells which detect and converte
sound in the normal way, but then convert the electrical signals into
changes in cell length
!
Prestin is the motor protein of the outer hair cells of the inner ear
of the mammalian cochlea
○
Immunolocalization shows prestin is expressed in the laternal
plasma membrane of the outer hair cells, the region where
electromotility occurs
○
*uses chloride binding sites
○
Prestin is a protein that in humans is coded by the SLC26A5 gene
!
*see prestin gene phylogeny
!
Molecular Evolution: Gene Convergence in Echolocating Mammals
11/28/17
Detecting weak electric fields
!
Electrogenesis
!
Electroreception and electrogenesis:
Although flatfish bury themselves in the sand, sharks can still
detect them
○
Vision (live flatfish) --> no
!
Olfaction (dead flatfish) --> no
!
Electroreception (battery) --> YES
!
Possible reasoning:
○
Natural food for many sharks is flatfish
!
Sense electric currents produced by active muscles of their prey
○
Sharks are attracted to prey over long distances by smell but use electric
sense over short distances to attack
!
Prey detection using electroreception: sharks
Can detect weak electrical signals in the environment (< 1 uV/cm)
○
Sense organs specialized for electroreception have only been found
among vertebrates (8600 sp)
○
Through convergent evolution, several vertebrate groups have developed
ability to perceive electric signals
!
Passive electroreception: detection of electric fields generated by other
organisms
!
Active electroreception: using self-generated electric fields for
electrolocation (detection co-specifics, prey, objects) and in some species
for electrocommunication
!
High frequency E: ~14,000 active electrosensory receptor organs
○
Low frequency E: ~700 passive electrosensory receptor organs
○
Mechano: ~250 mechanosensory receptor organs
○
Apteronotus
!
Electroreception:
Electric sensing was an early development in the course of
vertebrate evolution
○
The electrosensory system is closely related to the lateral line
system of fish and hearing/balance in terrestrial animals
○
Evolution:
!
*see slide for equation
○
Only useful over short distances
○
If distance of an object is doubled, its size must be quadrupled and
the energy expended by the fish increased by 8-fold if it is still to
be detected
○
Limitations of electrolocation:
!
Lower frequency
!
Ampullary
○
Higher frequency
!
Tuberous
○
Have different morphology
!
Have evolved from earliest ampullary electroreceptors with
loss of electroreception capacity in many (still have
mechanoreceptors)
!
No need
"
Electricity does not travel well through air
"
All mammals (except monotremes) do not have
electroreception
!
*both can occur in the same individual
○
Types of electroreceptors:
!
Closes eyes, ears and nose when it swims at night
○
Tail flip causes field of 1mV at 5cm
!
Sensitivity is 10uV/cm
!
Eats shrimp
○
40,000 electroreceptors and 40,000 mechanoreceptors on bill
○
Ex. Duck-billed Platypus
!
Detects prey by their electric fields
○
Feed by straining zooplankton with their filtering apparatus
○
Vision -feed in dark
!
Feeding on like plankton with nares plugged
"
Olfaction -no feeding with plankton extract
!
Mechanical -feed on plankton encapulsated in agarose
!
Hydrodynamic -feed in turbulent water conditions
!
Other senses:
○
Conclusion: electric sense is sufficient for prey detection and
capture
○
Ampulla of Lorenzini dispersed over rostum
○
Assists in prey detection, orientation and navigation
○
Paddlefish will strike at dipoles that produce artificial electric
fields that stimulate natural plankton
○
Rostrum is specialized for hunting electric signals in the near-field
evironment
○
Voltage gradient of 0.01uV/cm causes change in EEG
"
Voltage gradient of 1-10uV/cm causes escape when
swimming
!
To stimulate fields that fish can detect, need to
separate electrodes by 5km
"
1.5V battery connected to electrodes 50m apart sets of a field
of 300uV/cm
!
Sensitivity:
○
Ex. Paddlefish
!
Scalloped hammerhead vs. sandbar shark
○
Approximately 70% of orientations were initiated to stimuli of
<0.1uV/cm for both species
○
Note: 0.1uV/cm = flash light battery conencted to electrodes
over 16,000km apart
!
Both species initiated approximately 35-40% of orientations to
stimuli of <0.01uV/cm
○
Shark sensitivity:
!
*see slides
○
Canals connect ampullae of lorenzini to pores on the shark's
rostrum
○
The gel is a glyco-protein based substance with
semiconductor properties
!
Electrical impulses travel through the canal to stimulate
ampullae
!
Nerve impulses then send the information directly to the
brain
!
Canal of lorenzini is a jelly-filled tubule
○
*note: also determines direction in 3D
○
Room temperature proton conductivity of the jelly is very
high at 2 mS/cm
!
This conductivity is 40-fold lower then Nafion (highest
reported for a biological material)
!
Proton conductivity in ampullae of Lorenzini jelly:
○
Influx of positive charge causes the cell to release
neurotransmitters at synapses (or contact points) with nerves
to the brain, stimulation them to fire
!
The firing rate indicates strength and polarity of the external
field, and the field's location relative to the shark is thought
to be determined by the positions of the activated pores on
its body
!
The cells return to their original electrical state afterward by
opening a second time of membrane channel that permits
positively charged potassium ions to exit
!
A sensing cell reacts when an external electric field producing a
small electric potential across its membrane, leading channels to
allow positively charged calcium ions to rush in
○
Elasmobranch electroreception: ampullae of lorenzini
!
Electroreceptors:
Active electroreception, communication, defense & prey capture
!
*see slide
○
Electroreceptors, CNS and the electric organ interact
!
Use for prey capture (see slide for examples)
!
Fresh water and sea-water have difference in arrangement of
cells that produce electricity due to differences in
conductivity of the medium
!
Strongly electric fish (several hundred volts)
○
Prey detection and communication (see slide for examples)
!
Weakly electric fish (millivolts -few volts)
○
*10 families, over 500 sp --> convergent evolution
○
Skates: have electric organs in tail
○
Electric organs:
!
Electric rays: torpedoes, numbfish
!
See slide
○
Isopotential lines and current flow around an electric fish
○
Conductive objects concentrate the field
!
Resistive objects spread the field
!
Active electrolocation
○
Electric field emanates from an electric organ in the tail
region
!
It is sensed by the electroreceptive skin areas, using two
electric pits (foveas) to actively search and inspect objects
!
*see slide --> conductive vs. resistive objects
!
Ex. Elephantfish
○
Active electroreception:
!
Contain electrocytes = modified muscle cells
○
Non-innnervated surface does not change membrane
potential
!
Posterior surface generates an action potential
!
At rest = 0
!
Voltage gated sodium channels open to depolarize
posterior size of membrane (-90--> + 60 mV)
"
When stimulated = 150mV
!
Potential differences across membrane
○
*see slide
○
Electric eel
!
Electrocytes arranged in series increases the discharge voltage,
whereas an arrangement in parallel increases total current flow
(amperage)
○
In low-conductivity water the caudal electric organ is long and thin
(more electrocyte columns, fewer rows) which serves to increase
voltage in order to maximize the power output of the EOD
○
In high-conductivity water, electric organ is short and deep (more
electrocyte rows, fewer colums) which serves to increase amperage
(current)
○
Note: seawater is more conductive than freshwater
○
Electrocytes stacked from tail to head increase voltage
!
Multiple rows of electrocytes increase current
!
Overall:
○
Series vs. Parallel
!
This temporarily creates an additional charge across the cell
membrane on that side of the cell of about 0.065V
○
Now, instead of having a negative inside and positive outside, the
cell temporarily has a 0.085V difference across the convoluted
side, and a similarly oriented charge of about 0.065V on the
smooth side
○
These chargers are essentially stacked in series, so that the end
result is a brief charge across the entire cell of about 0.15V (0.085+
0.065)
○
When nerve fibers send a signal to an electrocyte, ion channels on the
smooth side of the cell open, allowing positive ions to rush into the cell
!
Electrophorus has ~ 5000 cells/column
○
Therefore, voltage = 5000 cells*150mV/cell = 755 V
○
Note: when electrocytes are arranged in a series in a column, the voltages
sum
!
--> electric organ discharge (EOD)
!
Pacemaker neurons --> relay neurons --> motor neurons -->
muscle
○
If a normal nerve signal went out from the brain to each
electrocyte, the signal would reach the first cells in the stack
before it reached the cells at the end of the stack
!
By the time the cells a the end firest, the ones at the
beginning would have already shut off again
!
Nerve fibers closer to the head are smaller than
those near the tail
!
Nerves closer to head also tend to take more of a
winding path than nerves near tail
!
Slower chemical signals are used in nerve fibers
closer to the head
!
Involves:
"
The electric eel has to synchronize the firing of thousands of
electrocytes in a stack so they all turn on at the same and add
together to create the big voltage needed to shock the fishe's
prey
!
Make pathlengths equal
"
Vary conduction velocity
"
Vary electrocyte stalk length (conducts faster than
nerve)
"
Overall:
!
Compensatory delay:
○
Generating electric organ discharge:
!
Brains tend to be larger than average fish --> more sensory
information
○
*see slide (do not need to memorize names)
○
Pathways in electroreception:
!
See slide
!
V = I (current) *R(resistance)
!
Ohm's Law
○
*see slide
!
Same current --> decreased voltage
"
Wet skin has decreased resistance
!
Electrocution
○
The current path must usually include either the heart or
brain to be fatal
!
Whether an electric current is fatal is dependent on the path it takes
through the body, which depends in turn on the points at which the
current enters and leaves the body
○
Electricity for prey capture and protection (strongly electric fish):
!
Electrogenesis:
Works because the normal response of a fish to direct electric current
(=galvanotaxis) is to swim towards the positive (anode) electrode
!
As in all vertebrate species, fish muscles are controlled by electrical
signals sent through the nervous system from the brain
!
The brain is believed to be negatively charged, therefore orientating the
fish toward the positively charged anode
!
The electric current interrupts the neurological pathway, causing the fish
to involuntary swim towards the anode
!
When fish are placed across the electric field, the bodies curve toward
the positive pole because the motor spinal nerves facing the negative
pole are inhibited while those facing the positive pole are stimulated
!
Electrofishing:
11/30/17
Inductive magnetoreception
!
Mechanically induced mechanoreception
!
Magnetoreception (detecting the Earth's magnetic field)
Flight as long as 1800km have been recorded by birds in
competitive pigeon racing
○
Because of this skill, homing pigeons were used to carry messages
as messenger pigeons
○
Homing pigeon is a variety of domestic pigeon derived from the rock
pigeon, selectively bred for its ability to find its way home over
extremely long distances using mechanoreception
!
Captive-raised monarchs appear capable of migrating to
overwintering sites in Mexico
○
Have spring and fall migrations
○
*see slide
○
Monarch butterfly:
!
Newly hatched loggerhead turtles use a magnetic map for
orientation along their migratory route in the North Atlantic
subtropical gyre
○
Hatchling turtles were placed in artificial magnetic fields that
replicate three locations along the migratory route
○
Results indicate that the turtles preferentially orient to stay within
the gyre (i.e. to follow a specific migratory route)
○
Sea turtles:
!
Species Examples:
Bacteria
○
Crustaceans (spiny lobsters)
○
Insects (butterflies, bees, flies)
○
Fish (salmon)
○
Elasmobranchs (sharks, skates, rays)
○
Amphibians (cave salamanders)
○
Reptiles (sea turtles)
○
Birds (homing pigeons, migratory birds)
○
Mammals (dolphins, whales)
○
Magnetic compasses are phylogenetically widespread:
!
Declination (--> north)
○
Inclination
○
Field intensity
○
Information obtained by earth's magnetic fields:
!
Direction (polarity compass)
○
*north pole is here and I am ___ away from it
Dip angle (inclination compass)
○
Animals can sense two qualities of earth's magnetic field:
!
Behavioural experiments have shown that many animals can sense the Earth's
magnetic field and use it for guiding movements (nature's GPS)
*use electric fields
○
Changing field induces a current in the jelly filled canals of the
ampullae of Lorenzini which changes the electric potential in the
ampulla
○
The voltage is amplified in the ampulla due to ion-channel
mediated interactions between the apical and basal membrane
○
There is an induced electromagnetic field in the
secondary coil (producing an induced current) due to
the changing magnetic field through this coil
"
When the switch is closed, the ammeter deflects
momentarily
!
Steady magnetic fields cannot produce a current
"
When there is a steady current in the primary coil, the
ammeter reads zero current
!
Experiment:
○
If the animal moves so that rotation occurs around an axis in
the plane of a semi-circular canal, there will be no
displacement of the endolymph but electromagnetic
induction could occur
!
Depending on intensity and orientation of the external
magnetic field, this will induce an electromotive force in the
conductive endolymph
!
This results in the separation of charged within the circuit,
inducing cation influx through highly sensitive voltage-gated
ion channels
!
Depicted is one semicircular canal of a vertebrate, filled with
cation-rich endolymph, and sensory cells located on either side of
the cupula
○
Inductive1.
*see slide for mechanism and biology
○
Fe3O4 -one of the oxides of iron
!
Is ferrimagnetic
!
They could site between ciliated olfactory and sensory
cells
"
Magnetite has been found in many animal tissues, but in fish
some evidence points to the nose as the home for
magnetoreceptors
!
Magnetite tugged by Earth's field would mechanically
control neural circuits by opening ion channels in cell
membranes
!
Magnetite:
○
Magnetitie field changers trigger movement of a SPM
mangetite cluster, which in turn induces derformation
in the plasma membrane
"
This event induces the opening of a non-selective
mechanosensitive ion channel
"
Mechanosensitive ions channels are located in the membrane
but do not have a physical link with the magnetite
A)
At rest, the ion channel is blocked
"
Change in the magnetite field relieves the blockade,
allowing sodium or calcium entry into the cells
"
Magnetite is physically connected to the ion channel B)
The messenger binds to the ion channel and opens it
"
Magnetite movement releases second messenger, either
directly or through the creation of tension in the membrane
C)
Cell containing magnetite linked to a neighbouring
nerve terminal through a 'tip link'
"
The ion channel is linked via a molecular motor to
actin filaments
"
Movement in the magnetite-containing compartment
stretch the link and open the ion channel
"
After opening, a molecular motor could reposition the
ion channel reducing the tension in the link and
allowing the system to be stimulated again
"
Mechanism based on auditory hair cell
mechanotransduction**
D)
Proposed scenario:
○
Mechanical2.
Magnetosensing rod-like protein complex identified in
drosophila
!
Exhibits spontaneous alignment in magnetic fields, including
that of Earth
!
Protein complex may form the basis of magnetoreception in
animals
!
A magnetic protein biocompass:
○
They are found in plants and animals
!
Cryptochromes are involved in the circadian rhythms of
plants and animals, and in the sensing of magnetic fields in a
number of species
!
Cry-deficient drosophilia does not show
magnetosensitive behaviour
"
Response of Cry to magnetic fields via radical pairs
may be used to perceive inclination information from a
geomagnetic field
"
Evidence:
!
Cry perceive geomagnetic information via the
quantum spin dynamics of radical-pair of FAD
reaction initiated by light (from sun or moon)
"
*see slide
!
Cyptochromes are a class of flavoproteins that are sensitive to blue
light
○
Magnetosensing protein crystals orient in a magnetic field
○
In nerve fiber layer --> to optic nerve
!
Gangion cell layer
!
Outer nuclear layer --> photoreceptor cell
!
Cry Magr protein complex is located…
○
Cryptochromes may lie in mysterious double cone cells
!
The ratio of chemical products from each cone could
determine magnetic orientation, which the brain might
process as light and dark patches on the visual field
!
Light turns the cyptochrome into a radical pair molecular,
with two unpaired electrons that flip between parallel and
anti-parallel states
!
Study:
○
11-cis retinal absorbs light and isomerized into all-trans
retinal
!
All-trans retinal dissociates from opsin
!
Activated opsin activates G protein transducin
!
Transducin activates PDE, which converts cGMP to GMP
!
The decreased cGMP closes a Na+ channel
!
Na+ entry decreases --> hyperpolarization
!
Mechanism:
○
Hypothesis: visual field of a bird may be modified through the
magnetic filter function
○
Specifically, many studies have shown that birds can only
orient if blue light is present
!
The avian compass is also an inclination-only compass,
meaning that it can sense changes in the inclination of
magnetic field lines but is not sensitive to the polarity of the
field lines
!
A bird can only sense the magnetic field if certain wavelengths of
light are available
○
Cry and MagR proteins are found widely distributed across taxa
○
Light-based 3.
Three types of magnetoreception:
Passive magnetoreception
!
Polarity via magnetite containing cells
○
Inclination via light sensitive magnetoreception
○
Active magnetoreception
!
Summary:
Sensory Physiology
#$%&'()*+,-./0120&, 34+,5637 3854,9:
Document Summary
Transmission of the signal to the integrating centre (afferent/sensory neurons) Perception of the stimulus at the integrating center (cns) *see dorsal view of the central nervous system. Sensory receptors can be categorized by the type of stimulus energy to which they respond: Nociceptors - noxious chemical, mechanical and thermal stimuli. Each receptor is highly selective for a specific kind of energy. Transduction: the process by which sensory receptors change stimulus energy into electrical signals. Mechano/thermo/electro and some taste receptors use an inotropic transduction mechanisms. Sensory receptor is the ion channel, it receives and transduces the signal. Photo/olfactory and some taste receptors use metabotropic transduction mechanisms (like hormone signalling) Stimulus induces a conformation change in specific membrane receptor --> activates gpcr --> adenylate cyclase (atp-->camp) --> opens ion channel --> depolarization. Sensory receptor cells are either primary afferent neurons (e. g. olfactory neurons), or epithelial sensory cells (e. g. photoreceptors)