Sensory Transduction and Ion Channels

Question 1

What is the response of the cone receptors?

Answer 1

• Hyperpolarised by closing Na+ channels in a rapid response to flash (opposite effect when decrease in light)
• Fast response in cones can follow a rapid flicker and they are longer in rods
• Single photon is perceptible under optimal conditions

Question 2

What happens when a cone R is hit by light?

Answer 2

• Glutamate is released when the cell is depolarised (resting)
• Outer segment of cone R contains tightly packed layers of phospholipid membranes that hold the chromophore (light-sensitive)
• There is a -ve internal membrane potential due to K+ leaking from the cell and the resting membrane potential is -45mV
• The Na+ channels are open by default and when light hits, they close so cell hyperpolarises
• This prevents the release of glutamate

Question 3

What happens after light hits a cone R and it hits retinal?

Answer 3

• When light hits retinal, causes the 11th C to rupture and reforms to a more stable trans configuration
• This activates opsin which activates G protein
• The G proteins activate an enzyme that destroys cGMP
• The fall in cGMP intracellularly causes it to diffuse from its channels and the Na+ channels close

Question 4

How do you terminate the retinal response?

Answer 4

• Hydrolysis of GTP to GDP and inactivates T in a itme dependent manner
• Phosphorylation of opsins
• Arrestin binding becomes more likely with greater phosphorylation
• Enzymatic reduction of retinal to retinol after removal

Question 5

What are the difference between rods and cones

Answer 5

• Rods have long outer segments with densely stacked discs
• Cones outer segments are much shorter and contains fewer photopigment
• Cones can only detect relatively bright target spot and can adapt very rapidly after a period in phototopic illumination

Question 6

How are mechanoreceptors stimulated?

Answer 6

Mechanical deformation of the membrane causes leakage and depolarisation

Question 7

What are the different tpyes of mechanoreceptors?

Answer 7

Meissners corpuscle - gentle touch (more superficial, capsule allows more sensitivity)

Pacininian corpuscle - strong vibrations (deep capsule filters all but rapid change in pressure)

Muscle spindle - passive muscle strethc

Baroreceptor - small artery