5. Introduction to Sensory Physiology and Perception

Question 1

What is the sensory pathway?

Answer 1

1. external event stimulates sensory receptor
2. sensory receptor must be able to detect stimulus and covert into electrical potential (stimulation transduced into change in membrane potential- receptor potential)
3. if depol succeeds in getting afferent to threshold –> AP occurs and info propagated to CNS via primary afferent

• Stimulation, Transduction, Threshold, Propagation
• not always external stimulus, some internal ones too, detected by e.g. interoreceptors

Question 2

What happens when info gets to CNS?

Answer 2

Subconscious

• control of movement: proprioception and vestibular input to the motor pathways
• autonomic: olfactory input like nice smell, stimulates salivation and inc gastric motility
• behavioural responses: see or smell promotes feeding behaviour (feeling of hunger)

Arousal (sleep, wakefulness and attention)

• modulatory pathways inv w/ this; control sleep/wake cycle and chose out of the many stimuli ramming our sensory system which needs to attend to –> these pathways not only control sensory pathways but are affected by them too
• sleep and wakefulness: sensory input can wake a person from sleep and sensory deprivation can induce sleep
• focusing attention: can put your conc on one sensory thing which suppresses the awareness of others
• switching attention: a salient stimulus can recapture awareness away from the original one e.g. hearing our name

Sensation and perception (focus on today’s lecture)
- sensation is awareness that event has occurred but perception is the ability to process and understand the info (what, where it is, meaning, what should I do?)

Question 3

What is the pathway of fine touch/somatosensory receptors (SSRs)?

Answer 3

• some receptors are actually not separate from the axon that carries the signal, and it can instead be a brach of it
• fine aborisation (fine branching structure at end of a nerve fibre) in skin is where the sensitive membrane is found
• sensitive membrane is part of distal aborisation of an axon that runs up to dorsal root and turns up and heads to brain stem
• this axon is primary afferent of this particular sensory system (receptor is continuation of axon instead of separate)

Question 4

Why are fine touch/somatosensory receptors (SSR) easy to injure but resilient/easy to heal?

Answer 4

• if you scrape hand -> sensitive nerve ending wiped out but b/c the cell body is a long way away (receptor not separate cell but part of long primary afferent axon), the nerve endings regrow and once injury heals -> regain sensation
• can actually cut peripheral axon way back, as long as cell body unaffected it can heal and regrow and find way back to general area and reinnervate

Question 5

What happen when receptor distorts when stimulated? What is the problem with these type of axons?

Answer 5

on the distal aborisation, the receptive part has areas of sensitive membrane at the tips of end of branches that have proteins inserted that respond to the mechanical distortion of that membrane

1. membrane distorts (e.g pushing against skin) -> channels pulled open –> depolarisation (receptor potentials cause by pressing of sensitive endings)
2. they are graded electrotonic potentials -> if depol is big enough to bring main axon to threshold -> AP

problem with these types of axons is that they are primitive (achieved by only a few proteins in membrane). for most of our sensory system more complex receptors req e.g. visual, taste, hear and equilibrium -> for these

Question 6

how are separate receptor cells different to receptors attached to axons?

Answer 6

• synapse between r cell and axon
• separate cells produce receptor potentials -> rel glutamate into synapse -> binds to post synaptic afferent -> cause excitatory post synaptic potential -> may produce AP
• unlike touch (where axon grew after damage), sep receptors are delicate and irreplaceable so if damaged or die -> cannot be replaced (e.g. photorecptors destroyed by light damage, genetic mutation, vit A deficiency etc. and auditory hair cells damaged by noise trauma, gen mut and ototoxicty like chemo agents)
• advantage to this over the touch receptors though is that bc receptor and afferent is separated, afferent may still be intact (and if you can artificially gather info from env with pattern of stimulation -> restore sensation bc brain will reconstruct incoming afferent APs as if they came from a receptor e.g. cochlear implant

Question 7

3 types of threshold are?

Answer 7

Action potential threshold which is -50mV

• memb potiential at which APs are triggered
• this is a function of vgNa+ channel and can be treated as a constant

Activation threshold

• for low threshold receptor and takes little energy to get up to AP (sensitive), and for high threshold take a lot of energy to bring up to AP (r.g. nociceptor
• min stim strength that’ll depol a receptor up enough to generate APs
• sensory system use receptors with range of diff activation threshold
• so activation is a measure of sensitivity of the axon, how easy it is to depol

Perceptual threshold

• point at which individual can detect a stim
• min stim strength that will generate enough enough APs to be detected
• most easily tested in clinical env -> patient says ‘yes i feel that’ and is bought about by amalgamation of AP threshold, activation threshold and attention e.g. if patient attention is wandering, then perceptual threshold will shift (as modulatory pathways will be switching down activity in cortex -> dec likely to detect

Question 8

what is lateral inhibition and why is it useful?

Answer 8

• if 3 axons/afferent stimulated, and if stimulation is stronger in middle axon than others around it, the stronger excitation of that one will inhibit the excitation of the axons surrounding it via inhibitory interneurons
• if pressure same across all 3 and all 3 primary afferent stim the same, then secondary afferent and lateral inhibition dampen down firing rate of ALL afferent and produce a low level of response bc all 3 have the same activation -> produce equal inhibitions of their neighbours via interneurones
  Advantages
• spatial discrimination is enhanced by LI as it allows us to focus in at one point
• allows receptors to encode stimulus contrast over huge range without saturation
• the signal is stripped of non vital info
  by ensuring sensory system respond to change th/ adaptation (when after maintained stimulation, receptor sets itself back to ground level so can detect any new changes, avoids saturation) and LI it allows:
• highlight salient features
• allow greater sensitivity while avoiding saturation
• save the cost of unnecessary APs

Question 9

how can damage and disease affect receptors?

Answer 9

• will generally will increase perceptual threshold (making things less easy to sense) -> occurs when receptors are lost
• h/e there are times where damage and disease can result in lower perceptual threshold -> hypersensitivity