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Medical Neuroscience > Lecture 28-Visual System I > Flashcards

Lecture 28-Visual System I Flashcards

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USMLE® Step 1 flashcards
1
Q

Which two parts of the eye control focusing power?

A

cornea and lens

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2
Q

Which part of the eye accounts for 2/3 of the focusing poers?

A

cornea

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3
Q

In addition to the cornea, which structure is involve in “fine tuning” the focus of an image based on distance from the eye?

A

lens

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4
Q

How is the curvature of the lens controlled?

A

contraction/relaxation of the ciliary muscle

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5
Q

What is the function of the ciliary muscle?

A

controls curvature of the lens (accommodation)

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6
Q

How do the ciliary muscle and lens adjust when looking at distant objects?

A
  • ciliary muscle relaxes

- lens flattens

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7
Q

How do the ciliary muscle and lens adjust when looking at near objects?

A
  • ciliary muscle contracts

- lens is rounded

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8
Q

When is the ciliary muscle at its largest diameter?

A

when looking at distant objects (flatten lens-tighten suspensory ligaments)

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9
Q

What happens as the lens looses elasticity with age?

A

-focusing power on nearby objects declines (presbyopia)

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10
Q

What is presbyopia?

A
  • happens as the lens looses elasticity w/age

- focus on nearby objects declines

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11
Q

What are the 2 types of photoreceptors?

A

Rod and Cones

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12
Q

Which photoreceptors are more receptive to lights?

A

rods

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13
Q

Which photoreceptors are more receptive to color?

A

cones

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14
Q

What is scotopic vision?

A

night vision

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15
Q

What would happen if you only functioned on “rod” vision?

A

you would only see in shades of gray

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16
Q

What is photopic vision?

A

day time vision

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17
Q

Which photoreceptors have a higher density in the peripheral retina?

A

rods

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18
Q

Which photoreceptors have a higher density in the fovea?

A

cones

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19
Q

Where in the retina are the cones more highly concentrated?

A

cones-FOVEA

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20
Q

What is the light absorbing pigment found in photoreceptors?

A

rhodopsin

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21
Q

What are the 2 parts of the rhodopsin protein?

A
  • 11-cis retinal (chromopore-Vit A derivative)

- opsin (protein part)

22
Q

Which part of the rhodopsin molecule is responsible for catching light?

A

11-cis retinal (chromopore)

23
Q

What happens to the 11-cis retinal part of the of the rhodopsin molecule in the presence of light?

A

isomerizes to all trans-retinal

24
Q

How does light affect the photoreceptor cells

A

causes isomerization of the chromopore (11-cis retinal) to all trans-retinal

25
Q

What happens to the all-trans retinal portion of the rhodopsin molecule after photoisomerization?

A

It detaches from the opsin protein

26
Q

What causes the G-protein transducin to be activated?

A

activated rhodopsin (after photoisomerization and separation of the 2 parts)

27
Q

What does the G-protein transducin activate? What does this cause?

A

Transducin activates phosphodiesterase (PDE) which hydrolyzes cGMP (decreased cGMP)

28
Q

How does the phosphodiesterase (PDE) affect cGMP levels?

A

decreases them

29
Q

What is the role of cGMP in darkness?

A

keeps ion channels in outer segment open

30
Q

What is the “dark current”?

A

constant flow of ions in the dark that keeps the photoreceptor more polarized and leads to a continuous release of glutamate

31
Q

What is the resting membrane potential during the dark current for a photoreceptor? Why is this so?

A

-35mV, more depolarized because cGMP keeps ion channels open and maintains a constant ion current

32
Q

What neurotransmitter is released at the synaptic ending of a photoreceptor?

A

glutamate

33
Q

What happens to the photoreceptor membrane potential in the presence of light?

A

hyperpolarizes

34
Q

What does the reduction in cGMP by the PDE cause in the photoreceptor?

A

-closure of membrane channels-> reduction in dark current -> hyper polarization -> decreases glutamate release

35
Q

Do photoreceptors generate action potentials?

A

No- they send receptor potentials based on the intensity of light

36
Q

What are the 2 advantages to having a phototransduction cascade?

A
  1. amplification

2. modulation

37
Q

How does the phototransduction cascade provide amplification?

A

1 rhodopsin activates 100s of transducins which activate 1000s of PDEs which significantly affect cGMP levels

38
Q

Why is modulation of the phototransduction cascade important? How is it controlled?

A
  • important for light and dark adaptation

- controlled by Calcium feedback mechanism

39
Q

Where do photoreceptors send info to be pre-processed before signals reach the brain?

A

bipolar cells

40
Q

What are the two types of bipolar cells?

A 
H= off-center bipolar cells
D= on-center bipolar cells
41
Q

What type of receptors do H-bipolar cells express? How do they respond when stimulated?

A
  • ionotropic GLU receptors

- hyperpolarize when stimulated

42
Q

What type of receptors do D-bipolar cells express? How do they respond when stimulated?

A
  • metabotropic GLU receptors

- depolarize when stimulated

43
Q

Where do bipolar cells synapse?

A

retinal ganglion cells

44
Q

What is contained within the optic nerve?

A

axons of many retinal ganglion cells

45
Q

Where do reinal ganglion cells project?

A

to the brain (lateral geniculate nucleus of the thalamus) via the optic nerves/tract

46
Q

In what plane would the horizontal cells be found?

A

Between the photoreceptors and bipolar cells

47
Q

What cells mediate lateral inhibition in the retina?

A

horizontal cells (and amacrine cells)

48
Q

What is the purpose of the lateral inhibition provided by the horizontal cells?

A

enhance contrast

49
Q

Which cells provide lateral inhibition at the bipolar retinal ganglion cell synapse?

A

amacrine cell

50
Q

What are the 3 forms of rhodopsin that cones contain?

A

blue (short-420nm)
green (medium-496nm)
red (long-560nm)

Medical Neuroscience (32 decks)

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