Sensory transduction and sensory receptors

Question 1

Name key receptor types

Answer 1

-photoreceptors
-mechanoreceptors
-chemoreceptors
-thermoreceptors

Question 2

Describe photoreceptors

Answer 2

• Cone photoreceptors
• Resting membrane potential: -40mV

Question 3

Response to light is very rapid

Answer 3

-very rapid onset and offset
-flicker fusion freq is 60-70Hz

Question 4

What about the equally rapid termination?

Answer 4

Amplification explains the very rapid response onset

Question 5

Describe the different stages of photoreceptors

Answer 5

• Resting membrane potential
• initiation of the light response
• linking light response to membrane potential
  -terminating the response
  -resetting the system
  -control of cGMP levels

Question 6

Describe rods

Answer 6

• have long outer segments with densely stacked discs
  
  • that are thought capable of capturing every photon passing along their length
  • to generate a useable response even when photons are very sparse
• Rod sensitivity is boosted further by a prolonged response
  
  • so they can follow a flickering light only up to ~20Hz
  • and suffer temporal blurring
• rods have limited adaptation and a restricted supply of 11-cis retinal
  
  • so they saturate and become non-functional at light levels > twilight

Question 7

Describe cones

Answer 7

• outer segments are much shorter and contain less photopigment
  
  • so they cannot capture a useable amount of photons in scotopic conditions
  • cone responses are terminated very quickly
  • so they can follow a flickering light up to ~70Hz
  • at the expense of lower sensitivity
• Cones have very efficient adaptation and very rapid supply of 11-cis retinal
  
  • so they can function without saturation over a huge range of light levels

Question 8

Give examples of mechanoreceptors

Answer 8

-touch receptors
-hair cells
-baroreceptors
-proprioceptors

Question 9

What do mechanoreceptors respond to?

Answer 9

deformation of their membrane

Question 10

describe auditory inner hair cell responses

Answer 10

-40mV RMP

Question 11

what is response speed explained by?

Answer 11

-mechanical gating
-destruction of tip links causes hearing loss
-the proteins at the base of the tip links are linked to hearing loss

Question 12

how is transduction in hair cells gated?

Answer 12

-mechanically
-this reveals non-specific cation channels at the base of the tip links
-advantage: extremely rapid responses (<50us onset) and high sensitivity (responds to 1nm deflection)

Question 13

Receptor adaptation

Answer 13

-myosin crawls along actin, cytoskeleton, controlling tip
-link tension and hence sensitivity
-increase in Ca2+ opposes channel opening (boosting oscillation)

Question 14

which channel used to feel painfully hot?

Answer 14

TRPV1

Question 15

TRP channels and heat sensation

Answer 15

• TRP indicates the ‘transient receptor potential’ family of ion channels
• Many TRP channels are temperature transducers
• TRP proteins change shape at a particular range of temperatures
• allowing the cation channel to open
• depolarising the cell, and signalling that the tissue is within that temperature range

Question 16

TRPV1 channels and nociceptive heat

Answer 16

-responses will saturate
-intensity is encoded in response strength
-but pathway to perception is highly modified at all levels

Question 17

why do foods taste hot?

Answer 17

-plants produce substances intended to make them unappetising to mammals
-they bind to and activate TRPV1 channels and generate the sensation of heat
-possibly via binding sites for endogenous inflammatory mediators

Question 18

when do TRPV1 channels normally open?

Answer 18

temp >45C

Question 19

TRPV1 AT LOWER TEMP:

Answer 19

-Inflammatory mediators or H+ ions
-botanical ‘warning’ chemicals such as capsaicin
-when they open = feels painfully hot

Question 20

stimulus-response relationships

Answer 20

-greater sensitivity -> smaller dynamic range
-increasing intensity is encoded as increasing hyperpolarisation up to a point

Question 21

transduction mechanism

Answer 21

-excess +ve charge
-adv: outsourcing of metabolically demanding activity

Question 22

TRPV1 and inflammation

Answer 22

-injury evokes +ve feedback and the reverse of adaptation
-TRPV1 up regulation and sensation occurs in neuropathic conditions
-it can become self-sustaining leading to neurogenic pain

Question 23

why does inflammation feel hot?

Answer 23

-inflammation has shifted the dynamic range to lower temp

Question 24

photopigment regeneration

Answer 24

1. retinal pigment epithelium
2. mueller cell
   -11-cis retinal
   -all-trans retinol
   -11-cis retinol
   -only cones have the retinol to retinal converting enzyme

Question 25

light/dark adaptation

Answer 25

see pic

Question 26

Biochemical adaptation in cones

Answer 26

see pic