Physiology of the visual system Flashcards

1
Q

Describe the steps involved in perception of light

A

1- The retina contains a sheet of light-responsive photoreceptor cells
2- Each cell has a receptive field that generates an electrical response when stimulated by light
3- Signal is transmitted across the retina and terminates in the cortex

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2
Q

What cells do the visual signal pass through on its way to the geniculate nucleus?

A

Photoreceptors
Bipolar cells
Ganglion cells

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3
Q

Does light perception cause excitation or inhibition of a membrane potential?

A

It inhibits membrane potential leading to hyperpolarization

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4
Q

What voltage is the membrane potential at rest in the dark?

A

-40mV

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5
Q

In the dark are sodium channels open or closed?

A

Open
- This allows influx of sodium and the maintained depolarisation

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6
Q

What is the mechanism to change membrane potential when light is perceived?

A
  • Light stimulation of rhodopsin leads to g-protein activation
  • G-protein activates cGMP phosphodiesterase
  • PDE hydrolyses cGMP reducing its concentration and closes Na+ channel
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7
Q

Are rods or cones more numerous?

A

Rods

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8
Q

Where are cones most concentrated?

A

Fovea

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9
Q

What does convergence mean?

A

Convergence is the number of rods/cones sending their signal to a ganglion cell
- The lower the convergence the more sensitive the ganglion cell is to change in light

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10
Q

Is there higher convergence in the rod or cone system?

A

Rod system
- More rods per ganglion than cones

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11
Q

What are cones meant for?

A

Vision in daylight
- Responsible for colour vision

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12
Q

What are rods meant for?

A

Vision in dim light

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13
Q

What aspect of cones are responsible for colour vision?

A

Short-wave, Middle-wave and long-wave cones containing different opsins.

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14
Q

Is blue short or long wavelength?

A

Short

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15
Q

Is red short or long wavelength?

A

Long

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16
Q

What does lateral inhibition do?

A

emphasizes contrast

17
Q

What is the difference between the ON and OFF pathway?

A

The bipolar cell contains a inotropic receptor in the OFF pathway and a metabotropic receptor in the ON pathway

18
Q

How do bipolar cells react to light in the OFF pathway?

A

There is less receptors activated and therefore the bipolar cell is more negative and sends less signals to the ganglion cells

19
Q

How do bipolar cells react to light in the ON pathway?

A

Light stops inhibition of influx of Ca+ and inhibits outflowing K+, this means the bipolar cell is more positive and therefore sends more signals to stimulate the ganglion cells.

20
Q

How is the receptive field of a ganglion cell?

A

Center-surround organisation
- The center must be dark and light on the outer circle to fire most AP’s
- Looks like a doughnut

21
Q

What cells are responsible for center-surround organisation of receptive field?

A

Horizontal cell
Amacrine cell

22
Q

What are the 3 types of retinal ganglion?

A

M- (movement)
P- (form & colour)
nonM-nonP- (colour)

23
Q

What layers of the geniculate nucleus does info from the M-type ganglion cells go to?

A

1&2

24
Q

What layers of the geniculate nucleus does info from the P-type ganglion cells go to?

A

3,4,5,6

25
Q

What is the competition hypothesis?

A

During a critical period of development there is a mononuclear deprivation of one eye at the visual cortex and therefore one eye becomes dominant