BMS1052 Lecture Notes - Lecture 10: Binocular Rivalry, Sensory Cortex, Sound Pressure
5 May 2018
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Lecture 10 – Principles of sensory systems
Learning objectives
All sensory systems have a number of common anatomical and functional properties. At the
end of this lecture, you should be able to:
• Describe the factors that affect sensory transmission and transduction in a range of
sensory systems
• Describe how stimulus information is encoded in action potentials
• Describe the functional organisation of sensory neurons into topographic maps
Lecture outline
• The difference between sensation versus perception
• Transduction- converting a physical stimulus into an electrical signal
• “esor euros are tued – they represent or encode discrete stimulus qualities
• Maps in sensory cortex
The difference between sensation versus perception
• Transduction- converting a physical stimulus into an electrical signal
• “esor euros are tued – they represent or encode discrete stimulus qualities
• Maps in sensory cortex
Sensation and Perception are different
Sensation is the process of encoding of events and stimuli by the nervous system
Sensation depends on low level physical, biochemical and neural events
Binocular rivalry illustrates the difference between sensation and perception
The retina in each eyes encode the red(right) and green(left) grating.
This visual information is passed to the primary visual cortex.
You do not simultaneously perceive a red and green grating – you only perceive one image
at a time.
Transduction- converting a physical stimulus into an electrical signal
Transduction is the conversion of one form of energy into another
In all of the sensory receptors, a physical stimulus causes a change in membrane potential (a
receptor potential)
e.g.
Light energy – vision
Sound pressure waves – hearing
Molecules and ions – taste and smell
Note that not all sensory receptors generate action potentials – some neurons are non-
spiking, so their rate of neurotransmitter release is proportional to their membrane
potential.
find more resources at oneclass.com
find more resources at oneclass.com
Transmission and Transduction
1) Mechanoreceptors in somatosensory system
There are a range of mechanoreceptors sensitive to different levels of pressure and
vibration.
Mechanoreceptors contain mechanically-gated ion channels.
e.g. stretch sensitive channels open in response to deformation of the cell membrane
Typically, the stretch-sensitive channels are Na+ channels,
leading to membrane depolarisation and action potentials.
Transmission and Transduction
2) Hair cells in the auditory system
Sound energy is a pressure wave
The pressure wave enters the auditory canal and vibrates the tympanic membrane (ear
drum).
This causes movement of the ossicles in the middle ear.
These cause vibrations of the oval window, at the entrance to the cochlea.
Transmission and Amplification by the outer and middle ear:
1) Pinna collects sound vibrations over a large area – funnelling them to the auditory
canal
2) The middle ear bones (ossicles) provide a mechanical advantage / lever effect
3) The relative size of the tympanic membrane and oval window provides a hydraulic
advantage
Transmission and Transduction
3) Photoreceptors in the visual system
Eyelids, cornea, lens and pupil control:
- amount of light entering the eye
- image focus
Photoreceptors absorb photons of light,
producing a graded hyperpolarisation in membrane potential:
- Rods are sensitive to low light levels (e.g. starlight)
- Cones are sensitive to high light levels (e.g. sunlight, indoor lighting)
There are no photoreceptors at the optic nerve head => this is the blind spot
Transmission and Transduction
4) Chemical receptors in taste and smell
Receptors for salty, sweet, sour, bitter and unami taste sensations.
Salty sensations => Na+ channels
Sour sensation => H+ channels (H+ influx closes H-gated K+ channels)
Other taste sensations => special G-protein coupled receptors
Many receptor cells are non-spiking
• Photoreceptors
• Taste receptors
• Hair cells furthermore they are unmyelinated
find more resources at oneclass.com
find more resources at oneclass.com
“ensory neurons are tuned – they represent or encode discrete stimulus qualities
Somatosensory system – different neurons are associated with conveying somatosensory
information from each finger, each toe. This helps localisation, or stimulus detection and
identification
Coding – single neuron rate code
Single neurons can encode information in their rate of action potentials
Neurons may be tuned for: - isual positio, orietatio, diretio, speed…
- auditor pith, itesit, loatio …
- touch location, pressure, frequency
Rate odig theories suggest that the preise tiig of atio potetials does’t atter –
only the number of spikes within a certain time window is important.
Stimulus properties are encoded by populations of neurons
1) Eah euro prefers a differet stiulus.
2) The responses of any individual neuron is highly variable
i.e. the response recorded to multiple repetitions of the same stimulus will vary.
=> a sigle euro’s firig rate gies a ureliale estiate of a stiulus propert.
By comparing the responses of multiple neurons (a population) the stimulus can be
estimated more accurately
Maps in sensory cortex
Cortex contains maps. Not necessarily mapping out spatial structure of the world – but of
important sensory qualities.
Topographic maps in cortex
1) somatosensory cortex contains a somatotopic map
Adjacent groups of neurons in S1 respond to touch of adjacent regions of the skin surface
Note that there are 3 major regions (face, upper limb, lower limb) and a few discontinuities
Note again that surface area is proportional to sensitivity. Where are we most sensitive –
finger tips and lips. These are the largest.
Topographic maps in cortex
2) auditory cortex contains a tonotopic map
In auditory cortex, adjacent neurons are optimally activated with the same frequency.
As you move across cortex, this preferred frequency systematically changes.
The tonotopic map is a map of sound frequencies, or pitches
Topographic maps in cortex
3) visual cortex contains a retinotopic map
Adjacent photoreceptors in the retina encode adjacent parts of the visual field
Similarly, adjacent neurons in primary visual cortex (V1) encode adjacent parts of the visual
field. Thus, V1 otais a retiotopi ap.
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
All sensory systems have a number of common anatomical and functional properties. Lecture outline: the difference between sensation versus perception, transduction- converting a physical stimulus into an electrical signal, e(cid:374)sor(cid:455) (cid:374)euro(cid:374)s are (cid:862)tu(cid:374)ed(cid:863) they represent or encode discrete stimulus qualities, maps in sensory cortex. The difference between sensation versus perception: transduction- converting a physical stimulus into an electrical signal, e(cid:374)sor(cid:455) (cid:374)euro(cid:374)s are (cid:862)tu(cid:374)ed(cid:863) they represent or encode discrete stimulus qualities, maps in sensory cortex. Sensation is the process of encoding of events and stimuli by the nervous system. Sensation depends on low level physical, biochemical and neural events. Binocular rivalry illustrates the difference between sensation and perception. The retina in each eyes encode the red(right) and green(left) grating. This visual information is passed to the primary visual cortex. You do not simultaneously perceive a red and green grating you only perceive one image at a time. Transduction- converting a physical stimulus into an electrical signal.
