L23 The Visual System: PhototransductionX Flashcards

1
Q

What is the name of the rod photopigment?

A

Rhodopsin

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2
Q

What are the 3 varieties of opsins of the cone photopigments?

A

S,M and L.

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3
Q

What is the name of the retinal ganglion photopigment?

A

Melanopsin

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4
Q

What does scotopic mean and which photoreceptor is activated?

A

Scotopic means night vision and it is activated by rod photoreceptors.

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5
Q

Are photoreceptors hyper polarised (more negative) by light?

A

Yes, photoreceptors (both rods and cones) are hyperpolarized (become more negative) by light.

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6
Q

Do cone and rod cells absorb different wavelengths of light or the same wavelengths?

A
  • Rod cells absorb a broad range of wavelengths, but don’t differentiate between them.
  • Cone cells absorb different and more specific ranges of wave lengths, enabling color vision.
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7
Q

What does adaption mean with regards to the visual system?

A

Despite a continued stimulus (continued bright light), the response decreases (becomes less hyperpolarised, gradually depolarise)

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8
Q

What channels are present on the membrane of the rod cells?

A

cGMP-gated non-selective cation channels.

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9
Q

What happens to the cGMP- gated non-selective cation channels during dark conditions ?

A

cGMP-gated non-selective cation channels are open in the dark allowing a Na+ influx known as the dark current to depolarises photoreceptors

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10
Q

What happens to cGMP-gated non-selective cation channels in light conditions?

A

Light decreases cGMP levels, closing the channels and preventing Na+ influx, hyperpolarising photoreceptors

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11
Q

Explain the process of transduction when light is absorbed

A
  1. Rhodopsin is activated by light.
  2. Stimulates G protein, transducting to become transductin GTP.
  3. Alpha subunit actuvates the enzyme phosphodiesterase(PDE).
  4. PDE reduces cGMP levels closing Na+ channels.
  5. Signal maplification occurs as this is an enzyme cascade.
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12
Q

How sensitive is light to humans?

A

5 to 7 photons can evoke a sensation of light in humans.

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13
Q

What does simple amplification through an enzyme cascade mean?

A

Simple amplification - small input = large output
Enzyme cascade - each activated enzyme can process many substrate molecules

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14
Q

What are the saturation responses in bright light of rods and cones?

A
  1. Rods cannot process bright light as they easily become saturated.
  2. Cones are not staurated as easily, so are used in bright light.
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15
Q

What does Ca2+ do in dark conditions?

A

They normally enters cells and blocks guanylyl cyclase.
This reduces cGMP production, so closes some ion channels.

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16
Q

Why does light adaptation require calcium?