BMS1052 Lecture Notes - Lecture 10: Rhodopsin, Trpv1, Lamellar Corpuscle
30 May 2018
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Week 4. Principle of sensory systems, Somatosensation
and Vision I
PRINCIPLES OF SENSORY SYSTEMS
• Sensation: process of encoding event and stimuli by the nervous system (how things are
encoded)
-depends on low level physical, biochemical and neural events
eg. collection of lines
• Perception: process by which the brain interprets sensory information
eg. cube pointing left or right
• Eg. when looking at red and green grating, you only perceive one image at a time
• Neurons are identical in visual and auditory sensory but very different perception
• Transduction:
o Is the conversion of one form of energy into another
o In all sensory receptors, physical stimulus causes a change in membrane potential
(receptor potential)
o Eg. light energy = vision, molecules and ions = taste and smell
o Not all sensory receptors generate action potentials, some neurons are non-spiking so
their rate of ntm release is proportional to their membrane potential
(more depolarised = increase positive MP = increased ntm release)
-> Because neurons geerally sall, do’t eed to e spikig
• Transmission and transduction:
Mechanoreceptors
o Somatosensory system
o Sensitive to different levels of pressure and vibrations
o Contain mechanically gated ion channels
eg. stretch sensitive channels open in response to deformation of cell
membrane -> Na+ channels
o Stretching -> channel opens -> Na+ ions influx
(easy way to convert physical signal into charge in MP)
Hair cells
o Auditory system
o Sound energy = pressure waves
o Pressure wave -> pinna collects over large area -> auditory canal ->
vibrate tympanic membrane (ear drum) -> move ossicles (mechanical
advantage) ->vibrations of oval window -> cause movements of fluid
in cochlea (where cells are) and displacement of tympanic membrane
o Size of tympanic membrane and oval window provide hydraulic
advantage
o Hair cells:
Auditory receptor cells
Vibrations of frequencies -> graded MP
Contain mechanically gated ion channels
find more resources at oneclass.com
find more resources at oneclass.com
Non-spiking
Linked at top
Physically opening channels
Photoreceptors
o Visual system
o Eyelids, cornea, lens and pupil control amount of light entering eye
and image focus
o Photoreceptors:
Absorb photons of light to produce a graded
hyperpolarisation in MP
Rods: sensitive to low light levels
Cones: sensitive to high light levels (colour)
-in low light, no colour perception
Blind spot: no photoreceptors where optic nerve is
Optic nerve: where axons leave eyeball heading go brain
where blood vessels enter
-> physical limitations that constrain evolution as optic nerve
is only few mm in diameter, in order to increase resolution of
visual system, need to make optic nerve head thicker (but
the eye a’t rotate
Are furthest away – light has to travel backwards through
retina
Chemical receptors
o Taste and smell
o Receptors for salty, sweet, sour, bitter and unami taste sensations
o Salty sensations -> Na+ channels
o Sour sensations -> H+ channels/presence of H+ ions (H+ ion influx
closes H-gated K+ channels)
-protons hydrogenised -> increase MP slightly
o Other sensations -> special GPCR
• Muller thought there was something special about the quality of nerved (i.e. something intrinsic
about visual nerves that make them special for visual information) however this is not the case
o A nerve is a nerve
o AP’s are the unit of currency for all sensory systems – there is nothing physically
different between the neurone in different sensory systems
o Information from different sensory receptors are however segregated
• Somatosensory system – different neurons are associated with conveying somatosensory
information from each finger, each toe. This helps localisation, or stimulus detection and
identification
find more resources at oneclass.com
find more resources at oneclass.com
• Coding:
Photoreceptors in the visual system
o Three types of cone are sensitive to different
wavelengths
(blue, green and red)
o Photons activate G ptotein cascade that closes Na+
o Photoreceptors are non-spiking
Single neuron rate code
o Theory suggests precise tiig of AP does’t atter –
only number of spikes within a certain time window is
important
o Usually assume it is rate coding
o Single neurons can encode information in their rate of
AP’s
o Neurons may be tune for visual position, orientation,
speed, auditory pitch, intensity, location, touch location,
pressure etc
Single neuron temporal code
o Theory suggests: precise timing of action potential is
informative
o Relative timing of spikes in the two neurons could
indicate the quality of the stimulus -> which neuron fires
first indicates what type of stimulus was presented
• Stimulus properties are encoded by populations of neurons:
o Each neuron prefers a different stimulus
o The responses of any individual is highly variable
-> a single neurons firing rate gives an unreliable estimate of a stimulus property
o By comparing the responses of multiple neurons (a population) the stimulus can be
estimate more accurately
-> averaging the responses gives a more reliable estimate
• Maps in sensory cortex:
Map is a spatial representation of physical information or relationships (important sensory
qualities)
-> systematic and orderly way of representing structure
->adjacent physical features are adjacent in the map
Topographic maps in cortex:
Somatosensory cortex contains a somatotopic
map
o Surface area is proportional to sensitivity
(largest = most sensitive eg. finger tips, lips)
o Adjacent groups of neurons in S1 respond to
touch of adjacent regions of skin surface
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
Principle of sensory systems, somatosensation and vision i. Principles of sensory systems: sensation: process of encoding event and stimuli by the nervous system (how things are encoded) Is the conversion of one form of energy into another. > because neurons ge(cid:374)erally s(cid:373)all, do(cid:374)"t (cid:374)eed to (cid:271)e spiki(cid:374)g: transmission and transduction: Mechanoreceptors: somatosensory system, sensitive to different levels of pressure and vibrations, contain mechanically gated ion channels. Photoreceptors: visual system, eyelids, cornea, lens and pupil control amount of light entering eye and image focus, photoreceptors: Absorb photons of light to produce a graded hyperpolarisation in mp. Cones: sensitive to high light levels (colour) Blind spot: no photoreceptors where optic nerve is. Optic nerve: where axons leave eyeball heading go brain where blood vessels enter. > physical limitations that constrain evolution as optic nerve is only few mm in diameter, in order to increase resolution of visual system, need to make optic nerve head thicker (but the(cid:374) eye (cid:272)a(cid:374)"t rotate(cid:895)
