Sensory transduction and sensory receptors

Question 1

Describe the composition of the layers in the retina and their functions:

Answer 1

Outermost layer = photoreceptors (rods/cones) which mediate the sense of sight

Middle retinal layer = bipolar cells which receive signals from photoreceptors and transmit these to the RGC

Innermost layer = RGC axons form the optic nerve, via which they project to the brain

Question 2

Describe the chemical environment at rest in photoreceptor cells:

Answer 2

At rest (dark), glutamate is continuously released and stops when there is a stimulus

There is also a high [cGMP] which leads to the opening of non-specific cGMP gated ion channels

Leading to the free movement of Ca2+/Na+

Therefore DP the membrane = glutamate release

Question 3

Explain why the potential sits at an intermediate level:

Answer 3

So the cell can respond strongly to both directions

Question 4

Describe the signal transduction pathway in photoreceptor cells:

Answer 4

Rhodopsin in outer segment, absorbs a light photon causing a co-factor within the protein to change from the cis to trans form

Leads to a series of unstable intermediates

The last of which activates a G protein in the membrane called transducin

Transducin activates cGMP specific-PDE which hydrolyses cGMP to 5’ cGMP

Leads to decreased in intracellular [cGMP] = closing of cGMP-gated ion channels

Which leads to decrease in Na+ flow = HP as membrane becomes more negative

Due to the change in membrane potential Ca2+ influx decreases + decreased [Ca2+] due to closing of VGCC

Leads to less calcium-induced exocytosis of glutamate to bipolar cells

Glutamate excites postsynaptic bipolar and horizontal cells

Due to the decrease in glutamate one population of bipolar cells will be DP and another HP depending on receptor = metabotropic/ionotropic

Question 5

Describe the termination of the signal transduction pathway in photoreceptor cells:

Answer 5

Hydrolysis of GTP to GDP inactivates T in a timely manner = sped up by GAPs

Low intracellular Ca2+ = dissociation of GCAP-Ca2+ leading to GCAP restoring intracellular cGMP levels, which leads to the opening of cGMP-gated ion channel allowing movement of +ve ions = restoring dark current

Low intracellular Ca2+ = dissociation of GAP-Ca2+ = deactivation of transducin

Low intracellular levels lead to dissociation of RK-recoverin-Ca2+ = RK phosphorylates metarhodopsin II which reduces its binding affinity for transducin

Arrestin deactivates metarhosopsin II = restoration of dark current

Low intracellual Ca2+ levels makes calmodulin-Ca2+ complex within cGMP-gated ion channels sensitive at low cGMP levels = restoring dark current

Enzymatic reduction of retinal to retinol
Deactivation of PDE
Phosphorylation of opsin

Question 6

Describe how GAP stops the transformation of cGMP to GMP:

Answer 6

Interacts w/alpha subunit of transducin

Causes hydrolysis of GTP to GDP

Therefore halts the action of the PDE

Question 7

Compare the control of cGMP levels by cGMP PDE and guanylate cyclase:

Answer 7

cGMP activity is increased by light, leading to closure of cGMP-gated ion channels and decrease in Na+ due to decreased [cGMP]

Leading to hyperpolarisation, decrease in Ca2+ influx and therefore intracellular Ca2+

GC activity is increased by decreased intracellular Ca2+
Leads to increase in [cGMP] = opening of cGMP-gated ion channels
Leading to Na+ influx and DP
Leading to Ca2+ influx and therefore increase in intracellular Ca2+

Question 8

Describe the fate of all-trans retinal and arrestin:

Answer 8

All-trans retinal transported to pigment epithelial cells to be converted into all-trans retinol, precursor to 11-cis retinol

Removal of arrestin

Question 9

Compare the adaptation abilities of rods and cones after photopic illumination:

Answer 9

Cones adapt very rapidly as not completely bleached after period of photopic illumination as some PP still active so can still function

Initial phase is biochemical with rapid photopigment regeneration thereafter

Rods can do far better but need around ~20 mins to reach that sensitivity, PP completely bleached after photopic illumination and inactive PP are inhibitory

Slow recovery due to slow regeneration of rhodopsin

Question 10

Describe PP regeneration in cones:

Answer 10

All-trans retinal converted to all-trans retinol by all-trans retinol dehydrogenase

Transported to muller cells

Then converted into 11-cis retinol by all-trans retinol isomerase

Then either stored in MC as retinyl esters to transported back to cones

Where it is transformed into 11-cis retinal by 11-cis retinal dehydrogenase

Question 11

Explain how the PP regeneration pathway is an explanation of dark adaptation in cones:

Answer 11

Presence of 11-cis retinal dehydrogenase in cones

Not present in rods, only in retinal pigment epithelium

Question 12

Explain the difference in PP regeneration between cones and rods in terms of rate, supply etc:

Answer 12

Rate at which both use PP is similar, however regeneration much quicker in cones

Due to the much faster secondary visual cycle used by cones only, in muller cells

Slower regeneration in rods reduces amount of pigment burnt through to no purpose when rods saturated

Supply is the rate limiting step

Question 13

Compare the segments of rods and cones:

Answer 13

Rods = long outer segments with densely packed discs, capable of capturing each photon passing along length

Cones = outer segment much shorter and contain less PP

Question 14

Describe the number of rods needed for a visual effect:

Answer 14

5-14 rods needed to absorb each photon in retina

To produce a perceivable visual effect in humans

Question 15

Compare the sensitivity of rods and cones in terms of their responses:

Answer 15

Rods = sensitivity boosted by prolonged response, can follow a flickering right up to ~20Hz + suffer temporal blurring

Cones = responses terminated much more quickly, can follow a flickering light up to ~70Hz at the expense of sensitivity

Question 16

Compare the adaptation of rods and cones in terms of supply and responses:

Answer 16

Rods = Limited adaptation and restricted supply of 11-cis retinal
Responses saturate and become non-functional at light levels greater than twilight

Cones = Very efficient adaptation and a very rapid supply of 11-cis retinal
Can function without saturation over a huge range of light levels

Question 17

Define critical flicker frequency (CFF) and describe what it depends on:

Answer 17

Lowest frequency at which a flickering light is indistinguishable from a non-flickering light of the same mean luminance

Depends on termination

Question 18

What are tip links, describe their protein composition:

Answer 18

Extracellular filaments that connect stereocilia to each other

Or to the kinocilium in the inner hair cells of the inner ear

Made up of two cadherin molecules = protocadherin-15 and cadherin 23

Question 19

Briefly describe mechanotransduction:

Answer 19

Where cells convert mechanical stimulus into electrochemical activity

Occurs at base of tip links which contain spring-gated cation-selective channels

Allow K+/Ca2+ to enter hair cell from the endolymph that bathes its apical end

If hair cells deflected towards kinocilium = DP
If hair cells deflected away from kinocilium = HP

Question 20

Describe the fast reflux, the issue it creates and how that issue is solved:

Answer 20

Channels allow fast ion influx to create a rapid response - big and high permeability

However, if Na+ used then small cells will have a serious metabolic problem in pumping those ions back out

Solved by outsourcing ion pumping from stria vascularis

K+ passively corrected by K+ diffusing out via basal leak channels

So restoration of MP restores conc change as well

Question 21

Describe the advantages of using a cation in fast reflux:

Answer 21

Extremely rapid response (endolymph potential co-varies with high frequency hearing limit)

Outsourcing metabolically demanding activity

Question 22

Describe the function of myosin in terms of tip links:

Answer 22

Crawls along actin cytoskeleton

Controlling tip link tension

Hence sensitivity

Question 23

Describe the function of Ca2+ in adaptation in tips:

Answer 23

Ca2+ effects both speed and extent of adaptation in tip links of stereocilia

If [Ca2+ buffer] increased in cell, then Ca2+ is more rapidly bound once it enters = slows adaptation

If cell DP, then driving force for Ca2+ influx decreases, which reduces Ca2+ entry = slowing/abolishing adaptation

Increase in Ca2+ oppposes channel opening

Question 24

Compare fast and slow adaptation:

Answer 24

Slow - keeps tip link tension at middle of optimal working range in face of changes in structure/constant deflection

Fast - operates within timescale of the auditory response = speeding closure of channels

• Responsible for stereocilia tuning to specific frequencies at different locations on the BM
• At different locations kinetic of fast/slow appropriate to frequency response

Question 25

What is a thermoreceptor:

Answer 25

A sensory-end organ stimulated by heat or cold

Question 26

What is a nociceptor:

Answer 26

Neurons preferentially sensitive to noxious stimuli or stimuli that would become noxious if prolonged

Question 27

Explain the term polymodal in terms of thermo-sensitive nociceptors:

Answer 27

Respond to a range of damaging stimuli

As they have a range of different channels and receptors in their distal endings

Question 28

What are temperature sensitive afferent:

Answer 28

Membrane channels that change shape w/temperature

Open when temp reaches specific range

Question 29

What is TRP:

Answer 29

Transient receptor potential family of ion channels

Many are temperature transducers

Question 30

Describe the function of TRPV1 in terms of temperature response:

Answer 30

Change shape at 45 degrees celsius (temp at which heat = damaging)

Allowing cations to enter
Depolarising the cell
Signalling that damage is about to occur or already has

Question 31

What is a KO mouse:

Answer 31

A knockout mouse is a lab mouse in which an existing gene has been inactivated or “KOd” by replacing it or disrupting it with an artificial piece of DNA

Question 32

Describe the action of inflammatory mediators:

Answer 32

Increase the prob of opening and hence shift the activation curve towards lower temperatures

2 mechanisms:

• Activate PKA/PKC via a GPCR
• Activate tyrosine kinase w/growth factor receptors

Phosphorylate channels and change activation temp from 43 degrees celsius - downwards

Can shift activation below body temp (hence inflammatory pain w/out need for stimulation)

Question 33

Describe the response of TRPV1 to inflammation:

Answer 33

Injury evokes +ve FB and the reverse of adaptation, inflammation upregulates TRV1 expression

Upon tissue damage and inflammation = inflammatory mediators released

Increase sensitivity of nociceptors to noxious stimuli = increased sensitivity to painful stimuli (hyperalgesia)

Or pain sensation in response to non-painful stimuli (allodynia)

Most sensitising mediators activated PLC pathway

Phosphorylation of TRPV1 by protein kinase C = sensitisation of TRPV1

PIP2 cleavage by PLC-beta = disinhibition of TRPV1 and increased sensitivity of TRPV1 to noxious stimuli

Question 34

Describe the link between TRPV1 and neurogenic pain:

Answer 34

Upregulation and sensitisation of TRPV1 associated with neuropathy and if it becomes self-sustaining then may result in neurogenic pain

Question 35

Describe how capsaicin is used to protect plants:

Answer 35

Protect from being eaten

By mimicking one of the inflammatory mediators that stimulate TRPV1

Question 36

Describe 3 things that cause TRPV1 channels to open:

Answer 36

Temp greater than 43

Presence of inflammatory mediators or H+

When bound to botanical “warning” chemicals of the vanilloid family

Question 37

Describe desensitization of TRPV1:

Answer 37

Upon prolonged exposure to capsaicin
- Ca2+ influx/intracellular Ca2+ mediates this

Signalling pathways such as calmodulin, calcineurin, decrease of PIP2

Overall, desensitization of TRPV1 = paradoxical analgesic effect of capsaicin