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1016MSC - Sem 1.5 > WEEK 8 - Topic 17 - Phototransduction > Flashcards

WEEK 8 - Topic 17 - Phototransduction Flashcards

1
Q

What does retinal turn into when bound to opsin?

A

11-cis-retinal

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2
Q

What does retinal turn into when struck by light and broken off from opsin?

A

All-trans-retinal

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3
Q

Outline the light capturing part of phototransduction (1st step of phototransduction). Including the part where the visual pigment regenerates.

A
  1. Pigment synthesis: 11-cis-retinal, derived from vitamin A, is combined with opsin to form rhodopsin.
  2. Pigment bleaching: Retinal absorbs light and changes shape to its all-trans isomer (all-trans-retinal). Rhodopsin breaks down to retinal and opsin and this is known as bleaching of the pigment.
  3. Pigment regeneration: Enzymes slowly convert all-trans-retinal to its 11-cis isomer in the cells of the pigmented layer and this requires ATP. Rhodopsin is regenerated when 11-cis-retinal is rejoined to opsin.
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4
Q

How is 11-cis-retinal formed?

A
  • Derived from vitamin A

- Regenerated from all-trans-retinal

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5
Q

In the dark what form is retinal?

A

11-cis-retinal

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6
Q

What is the regeneration speed of rods and cones?

A

Rods: Slow (5 min)

Cones: Fast (90 s)

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7
Q

Bleaching of pigment occurs in which phase?

A

Light phase

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8
Q

Regeneration of pigment occurs in which phase?

A

Dark phase

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9
Q

What does bleaching of the pigment generally mean?

A

Rhodopsin breaks down

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10
Q

Why does it take a while for eyes to adjust when going from an area of bright light to a dark room?

A

Regeneration of rhodopsin in rods is slow (5 minutes)

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11
Q

How are intermediate colours perceived? Such as yellow and orange?

A

When there is activation of more than one type of cone (e.g. for orange, activation of both red and yellow cones but more red)

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12
Q

How do we see white colour?

A

When all three types of cones are activated equally

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13
Q

Outline phototransduction after the light is absorbed. (2nd step of phototransduction)

A
  1. Retinal absorbs light and it changes shape to its trans-form.
  2. Light-activated visual pigment activates G protein called transducin.
  3. Transducin activates phosphodiesterase (PDE).
  4. PDE converts cGMP into GMP, causing cGMP levels to fall.
  5. As cGMP levels fall, cGMP-gated cation channels close and cations stop entering the cell.
  6. Photoreceptor hyperpolarises to about -70 mV.
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14
Q

In the dark, are cGMP-gated cation channels opened or closed?

A

Open

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15
Q

In the dark, the cell is depolarised to…?

A

Cell is depolarised to its dark potential of about -40 mV.

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16
Q

What does EPSP stand for?

A

Excitatory postsynaptic potential

17
Q

What does IPSP stand for?

A

Inhibitory postsynaptic potential

18
Q

Which part of the visual pigment absorbs light?

A

Retinal

19
Q

Outline phototransduction in the light after light is captured

A
  1. Retinal absorbs light and changes shape to its trans-form.
  2. cGMP levels fall and hence cGMP-gated cation channels close (K+ channels stay open) and cations stop entering the cell. Photoreceptor hyperpolarises to about -70 mV.
  3. Voltage-gated Ca2+ channels close in synaptic terminals and neurotransmitter release is inhibited.
  4. Lack of IPSPs in bipolar cell results in depolarisation of bipolar cell.
  5. Depolarisation opens voltage-gated Ca2+ channels, causing neurotransmitter release.
  6. EPSPs occur in ganglion cell resulting in depolarisation of ganglion cell.
  7. Action potentials generated, propagating along the optic nerve.
20
Q

What cations enter the cell when the photoreceptor hyperpolarises?

A

Na+ and Ca2+

21
Q

What is IPSP?

A

A graded potential in a postsynaptic neuron that inhibits action potential generation; usually hyperpolarising.

Inhibitory postsynaptic potential

22
Q

What is EPSP?

A

Depolarising graded potential in a postsynaptic neuron.

Excitatory postsynaptic potential

23
Q

What is IPSP?

A

A graded potential in a postsynaptic neuron that inhibits action potential generation; usually hyperpolarising.

24
Q

What is EPSP?

A

Depolarising graded potential in a postsynaptic neuron.

1016MSC - Sem 1.5 (16 decks)

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