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Neuroscience > Vision > Flashcards

Vision Flashcards

1
Q

What is the range of visible wavelengths?

A

The visible wavelengths in humans range from 400-700nm

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2
Q

What is the human range of light intensity

A

The human range of light intensity is 10-1010 photons

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3
Q

What is the greatest site of refraction?

A

Cornea

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4
Q

What is the focal distance?

A

The distance from where the refractive surface lies (cornea) and the point where parallel light rays converge is called the focal distance.

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5
Q

What are dioptres?

A

Reciprocal of focal distance (considering refractive index)

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6
Q

What is the refractive power of the cornea?

A

42 dioptres

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7
Q

Myopia

A

Shortsight: If the focal point lies before the retina - the individual is shortsighted

This can be caused by myopia - a too long eyeball

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8
Q

Hyperopia

A

Longsight: If the focal point is (theoretically) behind the retina the individual is longsighted

This can be caused by hypermetropia - a too short eyeball

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9
Q

Myopia corrective lens

A

This is corrected with a concave lens - extending the focal length

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10
Q

Hyperopia corrective lens

A

This can be corrected by a convex lens - shortening the focal length

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11
Q

Astigmatism

A

A visual astigmatism creates different focus in different planes of the cornea. Often due to irregular cornea shape.

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12
Q

How can you test for an astigmatism

A

Can be tested for using a hemicircle of radial lines

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13
Q

How many dioptres is the lens responsible for in accomodation?

A

A few

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14
Q

Mechanism of accommodation

A

Increase in power of lens caused by contraction of annular ciliary muscle (parasympathetically innervated), which reduces tension in radial zonular fibres, allowing lens to relax to a more convex state (especially on anterior surface).

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15
Q

Presbyopia

A

Failure of accommodation with age. Usually considered to be caused by lens material becoming stiffer and less elastic with age.

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16
Q

Effects of presbyopia

A

Longsight, fixed with a convex lens

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17
Q

Cataracts

A

Condition in which the lens becomes cloudy

Straightforward to treat by removing the lens surgically and replacing it with an artificial lens BUT cataracts still cause millions to be blind.

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18
Q

Glaucoma

A

Aqueous humor normally flows and circulates around the eye through trabecular network (at the drainage angle)

If this becomes blocked (canal of schlemm), the drainage of fluid is affected

Normal intraocular pressure (IOP), maintained by production of aqueous humour, is about 16.5mmHg, glaucoma, associated with elevated IOP (>21mm Hg).

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19
Q

Amblyopia

A

Lazy eye, reduced focus

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20
Q

Perimetry

A

Used to detect defects in peripheral vision

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21
Q

Where is the blind spot

A

The blind spot in each eye is 10-15deg horizontally on the temporal side of each eye

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22
Q

Cone wavelength and colour

A

Small, medium and long wavelengths associated with blue, green and red respectively.

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23
Q

How is colour determined?

A

Relative excitation of different cone types is the basis for colour vision - independent of intensity.

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24
Q

Deuteranomaly

A

Abnormal middle wavelength (green-absorbing) pigment (5%), (most common).

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25
Q

Protanomaly

A

Abnormal long wavelength pigment, red, protanopia (absence of that pigment)

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26
Q

Trichomat

A

Normal colour vision

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27
Q

Anomalous trichromat

A

One of the cones altered slightly, affects all colours slightly, examples (deuteranomaly and protanomaly)

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28
Q

Why is colour blindness mainly in males?

A

Genes responsible for the most common forms of colour blindness are on the X chromosome.

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29
Q

How is visual acuity tested?

A

Snellen chart

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30
Q

Normal visual acuity?

A

6/6 or 20/20

Letters with gaps about 1 min arc (1/60 degree) can just be read. But under ideal conditions, gaps of 0.5 min can be resolved.

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31
Q

Layers of cells in the retina

A 
Inner limiting membrane
Nerve fibre layer 
Ganglion cell layer 
Inner plexiform layer 
Inner nuclear layer 
Outer plexiform layer
Outer nuclear layer 
External limiting membrane
Inner segment / outer segment layer
Retinal pigment epithelium
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32
Q

Inner limiting membrane

A

BM elaborated with Muller cells

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33
Q

Nerve fibre layer

A

Axon of ganglion cell bodies

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34
Q

Ganglion cell layer

A

Nuclei of ganglion cells, axons of which become optic nerve fibres, some amacrine cells

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35
Q

Inner plexiform layer

A

Synapse between bipolar cell axons and dendrites of ganglion and amacrine cells

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36
Q

Inner nuclear layer

A

Contains the nuclei and surrounding cell bodies (perikarya) of the amacrine cells, bipolar cells, and horizontal cells (send information laterally between cells).

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37
Q

Outer plexiform layer

A

Projections of rods and cones ending in the rod spherule and cone pedicle, respectively.

These make synapses with dendrites of bipolar cells and horizontal cells.

In the macular region, this is known as the Fiber layer of Henle.

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38
Q

Outer nuclear layer

A

Cell bodies of rods and cones

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39
Q

External limiting membrane

A

Layer that separates the inner segment portions of the photoreceptors from cell nuclei

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40
Q

Inner segment/ outer segment layer

A

Inner segments and outer segments of rods and cones. The outer segments contain a highly specialized light-sensing apparatus.

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41
Q

Retinal pigment epithelium

A

Single layer of cuboidal epithelial cells.

This layer is closest to the choroid, and provides nourishment and supportive functions to the neural retina, The black pigment melanin in the pigment layer prevents light reflection throughout the globe of the eyeball; this is extremely important for clear vision

42
Q

Photoreceptor cell common structure

A

Outer segment located at distal surface of the neural retina

Inner segment, located more proximally

Cell body

Synaptic terminal

43
Q

Rod vs cone structure

A

Rods have a long cylindrical outer segment within which discs are stacked

Cones have shorter more tapered segment, continuous with the outer membrane.

44
Q

Rod vs cone function

A

Rods signal the absorption of a single photon (responsible for vision under dim illumination) but as light increases, the rod response becomes saturated and they no longer respond to changes in intensity.

Cones are much less sensitive to light but are solely responsible for vision in daylight. They have faster responses - dense packing of cones in the fovea determines our visual acuity.

Primates have three types of cone cells differentiated by the wavelength they can respond to (thus colour)

45
Q

Photoreceptor resting potential state in the dark

A

Na+ ions flow through non-selective cation channels into the photoreceptor.

This means the resting potential sits around -40mv

46
Q

What is the visual pigment in rod cells and its properties?

A

Rhodopsin (membrane embedded opsin part and light absorbing retinal part)

47
Q

Rhodopsin upon light exposure

A

Absorption of proton leads to conformational change - it becomes unstable and splits into opsin and retinal, which changes from cis-retinal to trans-retinal.

48
Q

What happens to trans-retinal

A

Trans-retinal is transported from rods to pigment epithelial cells where its reduced to trans-retinol (vitamin A)

Isomerisation of photopigment activates transducin (a G-protein)

Transducin activates a phosphodiesterase (PDE)

PDE reduces the level of cyclic GMP

49
Q

What does reduced cGMP cause?

A

Reduced cGMP causes sodium channels to close, and therefore the photoreceptor hyperpolarizes.

This change in the cell’s membrane potential causes voltage-gated calcium channels to close.

This leads to a decrease in the influx of calcium ions into the cell and thus the intracellular calcium ion concentration falls.

50
Q

What does decreased intracellular calcium lead to?

A

Less glutamate is released via calcium-induced exocytosis to the bipolar cell.

51
Q

What does reduction in glutamate release cause?

A

Reduction in the release of glutamate means one population of bipolar cells will be depolarized and a separate population of bipolar cells will be hyperpolarized, depending on the nature of receptors (ionotropic or metabotropic) in the postsynaptic terminal

52
Q

What happens when glutamate binds to an ionotropic receptor?

A

Bipolar cell will depolarize (and therefore will hyperpolarize with light as less glutamate is released).

53
Q

What happens when glutamate binds to a metabotropic receptor?

A

Hyperpolarization, so this bipolar cell will depolarize to light as less glutamate is released.

54
Q

Dark adaptation depends on

A

Regeneration of the visual pigment from opsin and 11-cis retinal.

Time required for dark adaptation and pigment regeneration is largely determined by the local concentration of 11-cis retinal and the rate at which it is delivered to the opsin in the bleached rods.

55
Q

Different dark adaptation rates in rods and cones

A

Rods are more sensitive to light and so take longer to fully adapt to the change in light (maximum sensitivity ~ two hours, 5-10 mins for some vision).

Cones take approximately 9–10 minutes to adapt to the dark.

56
Q

How are OFF ganglion cells generated?

A

When glutamate binds to an ionotropic receptor, the bipolar cell will depolarize (and therefore will hyperpolarize with light as less glutamate is released). THUS ROD HPYERPOLARIZATION LEADS TO BIPOLAR HYPERPOLARIZATION.

This hyperpolarization does not activate ganglion cells - thus these are OFF ganglion cells

57
Q

How are ON ganglion cells generated?

A

Binding of glutamate to a metabotropic receptor results in a hyperpolarization, so this bipolar cell will depolarize to light as less glutamate is released. THUS ROD HYPERPOLARIZATION LEADS TO BIPOLAR DEPOLARZATION

This depolarization activates ganglion cells - thus these are ON ganglion cells.

58
Q

Horizontal cells

A

Make antagonistic lateral connections between receptors and bipolar cells

Horizontal cells are depolarized by the release of glutamate from photoreceptors, which happens in the absence of light. Depolarization of a horizontal cell causes it to hyperpolarize nearby photoreceptors.

Though in the presence of light, the lower glutamate leads to horizontal hyperpolarization and leads to depolarizing of nearby photoreceptors.

Hence, bipolar cells have opposing input from surrounding receptors, and hence respond to local contrast rather than to light per se.

59
Q

Amacrine cells

A

Lateral antagonism between bipolar cells and ganglion cells - inhibition interneurons.

60
Q

Pupillary light reflex

A

Photosensitive retinal ganglion cells, convey information via the optic nerve.

Some axons connect to the pretectal nucleus of the upper midbrain instead of the cells of the lateral geniculate nucleus (which project to the primary visual cortex).

Axons synapse on (connect to) neurons in the Edinger-Westphal nucleus. Parasympathetic preganglionic neuronal axons in the oculomotor nerve synapse on ciliary ganglion neurons.

Short post-ganglionic ciliary nerves leave the ciliary ganglion to innervate the Iris sphincter muscle of the iris.

61
Q

Where does optic nerve project to?

A

Chiasm (partial decussation of visual fields), optic tract, LGN thalamus, optic radiation, V1

62
Q

Hemianopia

A

Loss of vision in half of visual field (optic tract lesion)

63
Q

The artery supplying the primary visual cortex

A

Calcarine branch of the posterior cerebral

64
Q

Dark current

A

Depolarisation caused by inner Na+ current

65
Q

Colour-opponent cells project

mostly to the

A

Parvocellular layers

66
Q

Cells which are insensitive to wavelength project to the

A

Magnocellular layers

67
Q

Unlike retinal or thalamic neurons, cortical neurons are often sensitive to the

A

Orientation/motion of lines and can be sensitive to Binocular disparity.

68
Q

What are P cells

A

Parvocellular cells, are neurons located within the parvocellular layers of the lateral geniculate nucleus (LGN) of the thalamus - respond to colour

69
Q

What are M cells

A

Magnocellular cells, magnocellular layer of the lateral geniculate nucleus of the thalamus, not colour, contrast detection instead

70
Q

Properties developed in V1

A

Orientation, binocular integration and disparity, colour contrast (though note colour starts via Parvocellular in thalamus but not CONTRAST)

71
Q

Parvocellular pathway projects to

A

Cytochrome oxidase blobs

72
Q

Magnocellular pathway projects to

A

Interblob region

73
Q

What colour contrasts do retinal ganglion cells respond to?

A

Red on, green off

Green on, red off

Yellow on (red and green), blue off

Blue on, yellow off

74
Q

Which LGN layers receive which eyes information?

A

1,4 and 6 receive the contralateral eye vision, 2,3 and 5 receive ipsilateral

75
Q

What is the opponent process theory, where does it occur?

A

Theory of colour vision generation (occurs in the retinal ganglion cells)

76
Q

Describe opponent process theory when centre colour is activated, use an example.

A

Red light shines

L cone hyperpolarises

ON ganglion with L cone in centre of receptive field

ON ganglion cell depolarized by red light (activated)

M cone in surround depolarises - depolarises horizontal cell - hyperpolarises OFF ganglion

77
Q

Describe how colour opponent theory works anatomically.

A

ON ganglion with L cone in centre of receptive field has mixture of L and M cones in the surround area

M cone synapses onto OFF ganglion cells via horizontal cell

(central ON area in which stimulation tends to excite neural responses and a surrounding OFF area in which stimulation tends to suppress neural responses by lateral inhibition)

78
Q

Describe opponent process theory when surround colour shone, use an example

A

Green light shines

L cone depolarises

ON ganglion hyperpolarises

M cone hyperpolarises horizontal cell hyperpolarises OFF ganglion depolarises

79
Q

How do visual fields decussate?

A

Fibres from the nasal hemiretina (vision from temporal area), cross the midline to proceed to the contralateral hemisphere.

As temporal hemiretina of one eye sees the same half of the visual field as the nasal hemiretina of the other, partial decussation ensures all the information from one hemifield is processed in the cortex of contralateral hemisphere.

80
Q

How is the visual cortex split into R/L eye/vision?

A

L cortex deals with right visual field

R cortex deals with left visual field

81
Q

Homonymous hemiopia

A

Half visual field missing (same half) lesion in contralateral tract

82
Q

Bitemporal hemianopia

A

Lesion at chiasm (temporal visual field missing)

83
Q

How is colour processing more complex at the cortex?

A

‘Double-opponent’ cell is excited by green and inhibited by red in its receptive field centre, and excited by red and inhibited by green in its receptive field surround.

84
Q

Do single opponent retinal ganglion or parvocellular LGN cells respond selectively to colour contrast?

A

No, only the cortex

85
Q

Colour contrasts in the cortex?

A

RG YB BW

86
Q

Thick stripes

A

Contain neurons selective for direction of movement and binocular disparity as well as cells responsive to illusionary contours and disparity cues.

87
Q

Pale stripes

A

Contain orientation selective neurons

88
Q

Thin stripes

A

Hold cells specified for colour

89
Q

What does V2 contain?

A

Thick, thin and pale stripes

90
Q

Ventral pathway

A

Identifying what the object is

91
Q

Dorsal pathway

A

Movement and identifying location

92
Q

What sort of ion channels are modulated during visual transduction?

A

Non specific Na+ channels, Ca2+ channels

93
Q

What neurotransmitter is released by photoreceptors?

A

Glutamate

94
Q

Which cells generate the centre-surround properties of visual receptive fields?

A

Ganglion cells

95
Q

Which midbrain structure do you associated with visual localisation?

A

Superior colliculus

96
Q

Which is the major thalamic relay for visual information to the cerebral cortex?

A

Optic radiation

97
Q

Which cells have their axons within the optic nerve?

A

Ganglion cells

98
Q

Which part of the human retina has axons that cross to the contralateral side of the
brain in the optic chiasm?

A

Nasal

99
Q

Lesions of right temporal lobe (meyer’s Loop) of the optic radiation on one side produces

A

A loss of the upper, outer quadrant of vision on the same side in both eyes, known as homonymous superior quadrantanopia or superior quadrantic hemianopia.

100
Q

The effect of a lesion of the left optic tract

A

Right homonymous hemianopia

Neuroscience (19 decks)

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