C 15 - 7 & 9

Question 1

What is retinal?

Answer 1

Derived from vit a in pigmented layer, a pigment. It is then mounted on disc using opsin. Combination is rhodopsin.

Question 2

What is opsin?

Answer 2

Protein that positions the retinal.

Question 3

Events of phototransduction?

Answer 3

1. Retinal absorbs light and changes shape from 11-cis to all-trans. Visual pigment activates.
2. Visual pigment activates transducin (a g-protein)
3. Transducin activates phosphodiesterase/PDE.
4. PDE converts cGMP into GMP causing cGMP levels to fall.
5. As cGMP levels falls, cGMP-gated cation channels close, resulting in hyperpolarization.

(in the dark, cGMP binds to cation channels and holds them open allowing Na+ and Ca+ to enter)

Question 4

Two forms of retinol?

Answer 4

cis in the dark, trans in the light

Question 5

Which retinal ganglion cross over in the optic chiasma?

Answer 5

Ganglionic axons originating from the medial region of each retina. The lateral region ganglion axons do not cross over.

Question 6

What is the response of a rod/cone to light?

Answer 6

hyperpolarization of the receptor cell

Question 7

What happens to cGMP channels in light vs. in dark?

Answer 7

Dark: cGMP binds to cation channels holding them open, Na+ and Ca+ enter. Depolarizes to “dark potential” of -40mV.

Light: cGMP breaks down, cation channels close, Na+ and Ca+ stop entering, cell hyperpolarizes to about -70mV.

Question 8

Visual pathway to the brain

Answer 8

Photoreceptor –>bipolar cells –>ganglion cells –>
axons enter optic nerve –>
optic chiasma (some cross to opposite side)–> become optic Tracts –>
**Some axons project to superior colliculi (reflex movement)
**Some go to pretectal nucleus (pupilary reflex), **Some go to hypothalamus (sleep/wake) –> Remaining 75% go to synapse with lateral geniculate nuclei in thalamus –>
Occipital lobe/primary visual cortex.

Question 9

phototransduction: In the dark, signal transmission

Answer 9

1. cGMP-gated channels open, photoreceptor depolarizes (Na+ and Ca2+ enter (dark current), -40).
2. Voltage gated Ca2+ channels open in synaptic terminals and release inhibitory neurotransmitter glutamate.
3. Neurotransmitter is released continuously.
4. Neurotransmitter causes IPSPs in bipolar cell - hyperpolarizes to -70mV.
5. Hyperpolarization closes voltage-gated Ca2+ channels inhibiting neurotransmitter release from bi-polar cell.
6. No neurotransmitter reaches ganglion cell, no EPSP, no AP to optic nerve.

Question 10

phototransduction: in the light, signal transmission

Answer 10

Hyperpolarization occurs when light activates opsin and cause cis –> trans (bleaching/photopigment activation.)

1. light stimulation causes a drop in cGMP, cGMP-gated channels close, photoreceptor hyperpolarizes to -70
2. Voltage gated channels close in photoreceptor synaptic terminals.
3. No neurotransmitter/glutamate is released (No IPSP’s).
4. Bi-polar cell becomes depolarized.
5. Depolarization opens Ca2+ channels, bi-polar cell releases excitatory neurotransmitter.
6. EPSP occur in the ganglion cell, AP propogate.