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Neuroscience > The visual system > Flashcards

The visual system Flashcards

1
Q

What is cranial nerve III?

A

Oculomotor nerve

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2
Q

What does cranial nerve III control?

A

Movement of the extraocular muscles

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3
Q

What is cranial nerve II?

A

Optic nerve

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4
Q

What is the sclera?

A

Tough outer protective wall of the eyeball

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5
Q

What does binocular vision allow?

A

Depth perception

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6
Q

What is the fovea?

A

Thinnest part of the retina with the highest visual acuity because it contains many cone cells

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7
Q

What is the optic disk?

A
  • Blind spot

- Origin of blood vessels and optic nerve which block vision

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8
Q

What is the macula?

A
  • Region of the retina for central vision, not blocked by blood vessels to improve vision quality
  • Has fovea in the centre
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9
Q

What are zonal fibres?

A
  • AKA suspensory ligaments

- Suspends the lens by attaching to ciliary muscles which enables stretching of the lens

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10
Q

What is aqueous humour?

A
  • The fluid between the cornea and the lens which provides nutrients to the cells of the cornea
  • Clear to allow refraction
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11
Q

Which fluid is located between the cornea and the lens?

A

Aqueous humor

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12
Q

What is vitreous humour?

A

Fluid inside the eyeball which maintains the shape and outward pressure

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13
Q

Which fluid fills the eyeball?

A

Vitreous humor

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14
Q

How much refraction happens at the cornea?

A

80% (most of it)

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15
Q

How much refraction happens at the lens?

A

20%

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16
Q

What is the refractive index?

A
  • Measure of the speed of light within a certain media

- Liquids have a higher refractive index than gases because liquids are more dense

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17
Q

What does a large difference in refractive index between two media mean?

A

More refraction

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18
Q

What is the degree of refraction determined by?

A
  • Difference in refractive index between the two media

- The angle at which light hits the interface i.e. the cornea (perpendicular means no refraction)

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19
Q

What is the focal distance?

A

Distance from the refractive surface (i.e. cornea) to convergence of parallel light rays (i.e. retina)

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20
Q

Are light rays parallel when coming from distant objects?

A

Yes

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21
Q

Are light rays parallel when coming from near objects?

A

No

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22
Q

How does the lens accommodate to focus light from distant objects?

A
  • The light rays are parallel so the cornea provides enough refraction do focus them on the retina
  • Ciliary muscle relaxes, suspensory ligaments contract so lens is flattened
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23
Q

How does the lens accommodate to focus light from near objects?

A
  • Non-parallel light rays require more refraction than the cornea can provide so the lens fattens
  • Ciliary muscle contracts, suspensory ligaments relax so lens is fat
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24
Q

What is emmetropia?

A

Normal sight

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25
Q

What is hyperopia?

A

Far-sightedness

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26
Q

What does far-sighted mean?

A

Can see far away but not close up

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27
Q

What happens in hyperopia?

A
  • The eyeball is too short so the point where the light rays converge would be behind the retina
  • More refraction needed from a convex lens in glasses
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28
Q

Which lens is needed for glasses in hyperopia?

A

Convex

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29
Q

What is myopia?

A

Short-sightedness

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30
Q

What does short-sighted mean?

A

Can see close up but not far away

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31
Q

What happens in myopia?

A
  • The eyeball is too long so light rays converge before they hit the retina
  • Refraction decreased by a concave lens in glasses which makes the rays more divergent
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32
Q

Which lens is needed for glasses in myopia?

A

Concave

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33
Q

What are ganglion cells?

A
  • The output from the retina

- The only cells which produce action potentials, other cells produce graded potentials

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34
Q

What are bipolar cells?

A

Connect the photoreceptors to the ganglion cells

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35
Q

What are photoreceptors?

A

Rods and cones

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36
Q

What are amacrine cells?

A

Modify energy transfer between bipolar cells and ganglion cells

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37
Q

What are horizontal cells?

A

Modify information transfer between photoreceptors and bipolar cells

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38
Q

What is the pigmented epithelium?

A

Epithelial layer underneath the photoreceptors which nourishes them

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39
Q

What is the duplicity theory?

A

Can’t have high sensitivity and high resolution in a single receptor

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40
Q

What do rod cells perceive?

A

Black and white

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41
Q

What do cone cells perceive?

A

Colour

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42
Q

What is the structure of rod cells? (3)

A
  • Greater number of disks
  • Higher concentration of photopigment
  • 1000 times more sensitive to light than cones
43
Q

What does scotopic mean?

44
Q

Which photoreceptors enable vision in low light?

45
Q

Which photoreceptors have low visual acuity?

46
Q

What is visual acuity?

A

Ability to distinguish shapes

47
Q

Which photoreceptors are more sensitive to light?

48
Q

What is the structure of cone cells?

A
  • Fewer disks

- Lower light sensitivity

49
Q

What does photopic mean?

A

Daylight conditions

50
Q

Which photoreceptors have high visual acuity?

51
Q

Which photoreceptors are more abundant?

52
Q

What does mesopic mean?

A

Intermediate light conditions

53
Q

Which photoreceptors are in the fovea?

A

Cones - no rod cells present in the fovea

54
Q

Where are rod cells located in the retina?

A

Around the periphery of the fovea

55
Q

What are the features of rod cells?

A
  • High sensitivity
  • Low resolution
  • Low visual acuity
56
Q

What are the features of cone cells?

A
  • Low sensitivity
  • High resolution
  • High visual acuity
57
Q

Why do rods have high sensitivity and low resolution?

A
  • Many rod cells converge onto one ganglion cell
  • Increases sensitivity because EPSPs summate which makes it easier to reach threshold for action potential in low light
  • Convergence means the brain can’t tell which rod cell is being stimulated so can’t tell where the light stimulus is coming from = low resolution
58
Q

Why do cones have low sensitivity and high resolution?

A
  • Each cone cell has its own ganglion cell so a higher intensity light stimulus is needed to reach threshold for action potential so lower sensitivity
  • One to one organisation means the brain can tell which cone cell has been stimulated so can tell exactly where the light is coming from = high resolution
59
Q

Where in the retina are there no photoreceptors?

A

Optic disk - blind spot

60
Q

Which photopigments do cones have?

A

Opsin photopigments (each cone contains 1 of 3 varieties)

61
Q

Which photopigment do rods contain?

62
Q

Where are the photopigments in the photoreceptors?

A

In the membranes of the membranous disks

63
Q

Which photopigment do retinal ganglion cells contain?

A

Melanopsin

64
Q

Which wavelength of light is rhodopsin (rods) most sensitive to?

65
Q

What are the 3 varieties of opsins?

A
  • S = short wavelength
  • M = medium wavelength
  • L = long wavelength
66
Q

Which wavelength of light are S cones most sensitive to?

67
Q

Which wavelength of light are M cones most sensitive to?

68
Q

Which wavelength of light are cones most sensitive to?

69
Q

Which colour do S cones detect?

70
Q

Which colour do M cones detect?

71
Q

Which colour do L cones detect?

72
Q

What does melanopsin detect?

A
  • Light/dark

- Important in circadian rhythms

73
Q

What is the resting membrane potential of a photoreceptor?

74
Q

What is the effect of light stimuli on photoreceptors?

A

HYPERpolarisation

75
Q

What is the dark current?

A

Influx of Na+ into photoreceptors through cGMP-gated cation channels in the dark, causing depolarisation

76
Q

When are photoreceptors at their resting potential?

A

In the dark

77
Q

What is happening in photoreceptors at rest?

A
  • cGMP in the cytoplasm binds to cation channels in the cell membrane of photoreceptors which allows influx of Na+
  • Causes depolarisation
  • K+ channels are open at rest to allow K+ out to prevent too much depolarisation
  • Na+/K+ ATPase maintains Na+ gradient
  • Ca2+ regulates cGMP levels
78
Q

What happens to photoreceptors in the light?

A
  • cGMP concentration decreases so channels shut = no more Na+ influx
  • K+ continues to leave via channels
  • Causes hyperpolarisation (more negative)
79
Q

How many photons evoke a sensation of light in humans?

80
Q

What does each photopigment contain? (2)

A
  • Opsin (varies with rhodopsin, S/M/L cones)

- Retinal (same in all)

81
Q

What is the opsin part of the photopigment equivalent to?

A

G protein coupled receptor

82
Q

How does light cause a reduction in cGMP in photoreceptors?

A
  • Light causes retinal to change conformation which causes change in opsin = transducin activated
  • Alpha subunit of transducin activates PDE
  • PDE cleaves cGMP into GMP
  • Reduction in cGMP = channels can’t open = no Na+ in = hyperpolarisation
83
Q

Which G protein is the opsin coupled to?

A

Transducin

84
Q

What is transducin?

A

G protein which is coupled to the opsin (receptor)

85
Q

What is PDE?

A

Phosphodiesterase

86
Q

What is the point of having such a complex mechanism for phototransduction?

A

Allows for signal amplification at every step

87
Q

What is saturation?

A

Peak hyperpolarisation response to a light source due to all the Na+ channels being shut - limiting factor

88
Q

Why are rods saturated more easily than cones?

A

Rhodopsin gets bleached in bright light

89
Q

What is adaptation?

A
  • Photoreceptors initially hyperpolarise in light

- Gradually depolarise with continued bright light

90
Q

Which enzyme makes cGMP?

A

Guanylyl cyclase

91
Q

What is the role of Ca2+ in photoreceptors in the dark?

A
  • Regulates cGMP levels
  • Enters through cGMP-gated cation channels
  • Ca2+ blocks guanylyl cyclase to reduce cGMP and prevent too much depolarisation
92
Q

What is the role of Ca2+ in adaptation?

A
  • In the light, cation channels shut = no Ca2+ influx
  • Ca2+ can’t block guanylyl cyclase so more cGMP is made
  • Channels start to open again so cations come in = gradual depolarisation = adaptation
93
Q

What are the 2 types of bipolar cells?

A

On and off

94
Q

Which neurotransmitter do photoreceptors release?

95
Q

When do photoreceptors release glutamate?

A

When depolarised i.e. in the dark

96
Q

What happens to the levels of glutamate being released by photoreceptors when it is light?

A

Reduced (cells are hyperpolarised)

97
Q

Are photoreceptors neurons?

98
Q

How do off bipolar cells respond to light?

A
  • Light causes photoreceptors to hyperpolarise so they release less glutamate
  • Causes off bipolar cells to hyperpolarise because ionotropic glutamate receptors aren’t being opened so less cations in
  • Switched off by light
99
Q

How do on bipolar cells respond to light?

A
  • Light causes photoreceptors to hyperpolarise so they release less glutamate
  • Causes on bipolar cells to depolarise because inhibitory metabotropic glutamate receptors are no longer being stimulated
  • Switched on by light
100
Q

What kind of glutamate receptors do off bipolar cells have?

A

Ionotropic

101
Q

What kind of glutamate receptors do on bipolar cells have?

A

Metabotropic

102
Q

What happens to on bipolar cells in the dark?

A
  • Glutamate released by photoreceptors, binds to inhibitory metabotropic glutamate receptors
  • Causes hyperpolarisation
103
Q

What is the centre-surround organisation of bipolar cells’ receptive fields?

A

Bipolar cells are directly connected to photoreceptors in the centre and indirectly connected to photoreceptors in the surround by horizontal cells

Neuroscience (18 decks)

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