L21 Phototransduction (Imported from Quizlet)

Rhodopsin

What is rod photopigment?

Three varieties of opsins - S, M and L

What are cone photopigments?

Melanopsin

What is retinal ganglion photopigment?

Hyperpolarised

Photoreceptors are ________________ by light

It becomes more negative

When light is switched on, what happens to the membrane potential?

It becomes more positive

When light is switched off, what happens to the membrane potential?

Open, dark current, depolarise

cGMP-gated non-selective cation channels are ______ in the dark allowing a Na+ influx known as the ______ ___________ to _______________ photoreceptors

Potassium channels that are always open, potassium moves out of the photoreceptor which prevents the photoreceptor from become too depolarised

What is present in the inner segment and what does it do?

Decreases, closing, hyperpolarising

Light _____________ cGMP levels, __________ the channels and preventing Na+ influx, _____________ photoreceptors

Cyclic guanosine monophosphate

What does cGMP stand for?

Na/K pump is found in the inner segment that removes Na in exchange for K

To maintain the Na gradient, what is found and what does it do?

5-7

_-_ photons can evoke a sensation of light in humans

Rhodopsin

______________ is activated by light

Transducin, transducin GTP

Rhodopsin stimulates the G-protein ____________, to become ____________ _____

Phosphodiesterase

The α subunit activates the enzyme _______________________ (PDE)

cGMP, Na+ channels

PDE reduces _________ levels, closing ___ _____________

Amplification, enzyme cascade

Signal ______________ occurs as this is an _____________ ____________

Retinal and opsin

Rhodopsin is made of 2 key components, what are they?

G-protein coupled receptor

Opsin is a member of what family?

Shape, conformational change, transducin

Retinal absorbs the photos and changes its _________, leading to a _______________ ________ in opsin - this allows opsin to activate ______________

A current equivalent to 1 million Na+ ions

Absorption of 1 photon can lead to the hydrolysis of ~1400cGMP molecules and suppresses what?

Guanylyl cyclase

In the dark cGMP is constitutively produced by the enzyme ____________ __________

Cone

This example response is from which: a rod or cone?

They become easily saturated

Rods cannot process bright light, why?

Bleached, low, hyperpolarisation

Rhodopsin is ___________, cGMP levels are so _______ that no additional _________________ can occur

In bright light

Cones are not saturated as easily, so when are they used?

Hyperpolarise greatly

Photoreceptors initially do what?

Depolarise, bright light

Photoreceptors gradually _______________ with continued ________ ________

Calcium

What does light adaptation require?

Ca2+ normally enters the cells and blocks guanylyl cyclase

This reduces cGMP production, so closes some ion channels

Describe light adaptation in the dark

Channels are shut so Ca2+ cannot enter cells

Block on guanylyl cyclase is relased

More cGMP produced = more channels open

Describe light adaptation in the light

Depolarises photoreceptors

The Na+ entry through cGMP-dependent non-selective cation channels (the dark current) does what in dark conditions?

Photopigments, breakdown

Photons activate ______________ leading to cGMP ______________

Adaptation to continued light stimulation

Calcium mediates what?

No, there are different types of bipolar cells and bipolar cells have complex receptive fields

Are bipolar cells just relay neurons that pass information straight from photoreceptors to retinal ganglion cells? Explain your reasoning

Glutamate

This is classified based on bipolar response to _____________

OFF bipolar cells

Which bipolar cell uses ionotropic glutamate receptors?

Positive ions come into the cell and depolarisation occurs

In the dark, what happens with OFF bipolar cells?

ON bipolar cells

Which bipolar cell uses metabotropic glutamate receptors (GPCRs)?

Releasing glutamate and binds to inhibitory metabotropic glutamate receptors, hyperpolarisation occurs

In the dark, what happens with ON bipolar cells?

Reduction in glutamate binding to them and more likely to get depolarisation

In the light, what happens with ON bipolar cells?

When specific areas of the retina are illuminated

When will retinal ganglion cells fire action potentials?

Centre-surround

Bipolar cells have what kind of organisation?

Direct contact with photoreceptors in the centre, indirect contact via horizontal cells to photoreceptors in the field surround

What does centre-surround organisation mean?

Centre-surround receptive fields

Bipolar cells have _______-_________ ______________ _______