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MBBS2/Physiology~HEAD & NECK > Visual System > Flashcards

Visual System Flashcards

1
Q

what are the layer of the eyeball

A

fibrous layer
vascular layer
retina

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2
Q

what is the fibrous layer comprised of

A

-sclera
-cornea

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3
Q

what is the vascular layer comprised of

A

-iris
-ciliary body
-choroid

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4
Q

what is the retina comprised of

A

-pigmented layer
-neural layer

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5
Q

what does the iris contain

A

-blood vessels
-pigment cells (melanocytes)
-2 layers of smooth muscle

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6
Q

what does contraction of smooth muscle in the iris do

A

changes the diameter of the pupil

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7
Q

what is the pupil

A

its the central opening of the iris that is black in colour

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8
Q

what does the pupil do

A

it regulates the amount of light entering the eye

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8
Q

what does the pupil do

A

it regulates the amount of light entering the eye

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9
Q

what happens to the pupil during close vision and bright light

A

pupils constrict

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10
Q

what happens to the pupil during during distant vision and dim light

A

pupils dilate

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11
Q

what is the cornea

A

its a thin transparent epithelium that is devoid of blood vessels and covers the iris

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12
Q

what is the choroid like

A

its vascular accounting for 85% of total blood flow in the eye and it has a relatively low oxygen content

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13
Q

what does the choroid do

A

-supplies the outer retina with nutrients
-maintains the temperature and volume of the eye

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14
Q

what is the choroid covered with

A

sclera

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15
Q

the choroid is attached to what

A

the outermost layer of the retina

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16
Q

what is the sclera

A

its the white of the eye

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17
Q

what does the sclera do

A

-provides structural support
-provides attachment of extrinsic muscles

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18
Q

what does the sclera fuse with posteriorly

A

the sheath of the optic nerve

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19
Q

what is scleral/ conjunctival icterus

A

its yellowing of the sclera due to build up of bilirubin

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20
Q

what secrets aqueous humor into the anterior chamber

A

ciliary epithelium

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21
Q

what does aqueous humor do

A

-it keeps the anterior chamber of the eye pressurized at 20mmHg
-it maintains eye shape and relative positions of the components
-it provides oxygen, nutrients and metabolic waste removal to the lens and the cornea
-for refraction

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22
Q

what drains the anterior chamber

A

the canal of schlemm ( scleral venous sinus)

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23
Q

where does the canal of schlemm empty into

A

extraocular veins

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24
Q

how often is the aqueous humor replaced

A

every 90mins

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25
Q

what is a glaucoma and what causes it

A

its a disease that damages the optic nerve and can cause vision loss and blindness.

its caused by fluid build up in the anterior chamber because of clogging of the canal of schlemm resulting in poor drainage of the aqueous humor

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26
Q

what does vitreous humor do

A

-maintains intraocular pressure
- refraction
-phagocytosis

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27
Q

what is the lens

A

its an onion like structure with closely packed columnar cells arranged in concentric shells and encased by a thin and transparent capsule

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28
Q

cells of the lens have a high conc of what

A

alpha crystallins

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29
Q

what do alpha crystallins do

A

they increase the lens density and enhance its focusing

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30
Q

whats a disease of the lens

A

cataracts

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31
Q

the ciliary muscle has smooth muscle oriented in what 3 directions

A

-longitudinal
-radial
-circular

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32
Q

ciliary muscle has the greatest density of what

A

mitochondria

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33
Q

whats the innervation of the ciliary muscle

A

parasympathetic innervation (ACh) through oculomotor nerve

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34
Q

how is the ciliary muscle attached to the lens

A

through inelastic zonular fibers

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35
Q

what does the ciliary muscle do

A

provides accommodation

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36
Q

what are tears

A

they are plasma ultrafiltrate produced from the lacrimal gland

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37
Q

what do tears do

A

-bathe the cornea in a thin layer 10 micrometers thick
- reduce friction
-remove debris
-allow oxygen to diffuse from air to corneal cells
-have antibodies and lysozymes for infection
-their superficial oil layer slows evaporation for good optical surface

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38
Q

what is the flow of tears

A

lacrimal gland> lacrimal duct> superior/ inferior lacrimal canal> lacrimal sac> nasolacrimal duct> nasal cavity

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39
Q

do newborns shed tears

A

no, because their lacrimal glands are still developing but by 2 weeks they produce a few tears to keep their eyes moist

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40
Q

when are full tears produced by newborns

A

2 months

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41
Q

what does the optic disc/ blind spot lack

A

photoreceptors (so any image that falls on this region wont be seen)

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42
Q

what comes together in the optic disc/blind spot

A

optic nerves

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43
Q

why dont we notice the blind spot in every day to day observation of the world

A

because the brain fills the blind spot with an extrapolation of the image surrounding the blind spot

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44
Q

what does the retina consist of

A

-single cell thick pigmented epithelium
-photoreceptors
-neural layers

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45
Q

what does the pigmented layer of the retina contain

A

it contains melanin

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46
Q

what does the melanin in the pigmented layer of the retina do

A

absorbs stray light rays

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47
Q

what does the pigmented layer of the retina do

A

it provides essential nutrition and waste removal for the photoreceptors

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48
Q

why is visual acuity reduced in people with albinism

A

this is because of light scattering, due to no/little melanin in the pigmented layer to absorb the excess light and the light is reflected onto the retina degrading the visual image

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49
Q

how many rods and cones are in the photoreceptor layer of the retina

A

-120 million rods
-6 million cones

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50
Q

what is the center of the posterior portion of the retina

A

macula lutea

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51
Q

whats the fovea centralis

A

its the depression in the center of the macula lutea

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52
Q

what does the fovea centralis form

A

the center of the visual axis

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53
Q

what is the neuronal layer of the retina

A

its a forward extension of the brain

54
Q

what cells does the neuronal layer of the retina consist of

A

-ganglion
-bipolar
-horizontal
-amacrine

55
Q

what do bipolar cells in the neuronal layer of the retina do

A

they take the electrical info from the photoreceptors cells and pass it along to other retinal cells

56
Q

what do horizontal cells in the neuronal layer of the retina do

A

-they receive input from photoreceptors and project neurites laterally to influence surrounding bipolar cells and photoreceptors
-they also help integrate and regulate the input from multiple photoreceptors cells, increasing the visual acuity

57
Q

what do amacrine cells of the neuronal layer of the retina do

A

they receive input from bipolar cells and project laterally to influence surrounding ganglion cells, bipolar cells and other amacrine cells

58
Q

what do ganglion cells of neuronal layer of the retina do

A

they contribute axons to the optic nerve

59
Q

what is an inverted retina

A

its where the photoreceptors face away from the light causing the incoming light to travel through the front of the retina to reach the photoreceptors

60
Q

what does an inverted retina allow for

A

-absorption of excess light by the pigment epithelium, preventing reflection that may distort the image
-photoreceptors to be in close contact with the choroid which supplies the photoreceptors with blood and provides a cooling system for the retina as the light is converted largely into heat

61
Q

what are the only light sensitive cells in the retina and what is the one exception

A

rods and cones

some types of ganglion cells are an exception

62
Q

whats the only source of output from the retina

A

ganglion cells

63
Q

what are the only retinal neurons that fire action potentials

A

ganglion cells

64
Q

what do the other retinal cells do that don’t fire action potentials

A

they depolarize or hyperpolarize and release a neurotransmitter (glutamate) ,but they dont fire action potentials

65
Q

what does the eye have for image formation

A

-lens system
-variable aperture system i.e. the pupil
-retina that corresponds to the film

66
Q

what are the 4 refractive interfaces of the lens system

A

-interface between air and the anterior surface of the cornea
-interface between the posterior surface of the cornea and the aqueous humor
-interface between the aqueous humor and the anterior of the lens
-interface between the posterior surface of te lens and the vitreous humor

67
Q

what process can increase the refractive power of the lens

A

accommodation/focusing

68
Q

what happens to a persons lens as they become older

A

it gets larger, thicker and less elastic

69
Q

what is the condition where the lens becomes non-accommodating

A

presbyopia

70
Q

as a persons lens change due to old age what will they need

A

bifocal glasses, with the upper segment focused for far seeing and the lower segment for near seeing

71
Q

what do you call normal vision

A

emmetropia

72
Q

what are some errors of refraction

A

hyperopia- is farsightedness and its usually due to the eyeball being too short or a lens system that is too weak, causing near objects to look blurry

myopia- is nearsightedness and its due to the eyeball being too long or the lens system having too much refractive power, causing far away objects to look blurry

astigmatism- is due to too great a curvature of the cornea in one plane of the eye, which can cause far away and nearby objects to look blurry or distorted

73
Q

what are the corrections for errors of refraction

A

-concave for myopia
-convex for hyperopia
-cylindrical for astigmatism

74
Q

what does the pupil do

A

its continuously adjusting for different ambient light levels

75
Q

what does the pupillary light reflex do

A

constricts the pupil in response to light

76
Q

the pupillary light reflex involves connections between what

A

the retina and neurons in the brain stem that control the sphincter muscles that constrict the pupils

77
Q

the pupillary light reflex is what

A

consensual i.e. if you shine a light into only one eye, it will cause the constriction of both pupils

78
Q

what does the lack of consensual pupillary light reflex show

A

a sign of serious neurological disorder involving the brain stem

79
Q

whats visual acuity

A

its the ability of the eye to distinguish 2 points near each other

80
Q

what does visual acuity depend on

A

-spacing of the photoreceptors in the retina
-precision of the eyes refraction

81
Q

how is distance across the retina described

A

in terms of degrees of visual angle

82
Q

what does a photoreceptor comprise of

A

-synaptic terminals
-inner segment
-outer segement

83
Q

what is the outer segment of the photoreceptors

A

its a highly modified cilium

84
Q

the outer segment of the photoreceptors is the site of what

A

signal transduction

85
Q

disk membranes in rods and cones contain what photopigment

A

R-rhodopsin
C-color pigments i.e. iodopsin

86
Q

what do rods and cones do on exposure to light

A

they decompose and excite the nerve fibers leading from the eye

87
Q

what makes rods more sensitive to light than cones

A

a higher photopigment conc and greater number of disks

88
Q

what vision are rods used for

A

scotopic vision (black and white vision in low light conditions)

89
Q

what does loss of rod function result in

A

night blindness

90
Q

where do rods predominate

A

in the extrafoveal retinal portions

91
Q

what is rhodopsin made up of

A

a purple pigment which consists of the protein opsin that is bound to the yellow photosensitive pigment called retinal

92
Q

where is retinal derived from

A

vitamin A

93
Q

how is rhodopsin incorporated into disks

A

its first synthesized from inner segments, then it moves across through the stalk and into the outer segments in vesicles

94
Q

what happens when retinal absorbs photons

A

it changes from 11-cis retinal to all-trans retinal and causes conformational changes in opsin leading to the activation of transducin a G protein

95
Q

what vision do cones provide

A

photopic vision for colour in bright light

96
Q

what pigment do cons contain

A

iodopsin

97
Q

what does iodopsin consist of

A

retinal combined with a photopigment opsin protein

98
Q

what do you call someone one who sees with normal colour vision

A

trichromats

99
Q

what are the 3 types of cones and what are they sensitive to

A

S cones- sensitive to blue light
M cones- sensitive to green light
L cones- sensitive to red light

100
Q

what do cones do

A

provide colour vision and great visual acuity

101
Q

what do people with colour blindness lack

A

one or more of the cones

102
Q

red-green colour blindness is common in who

A

males

103
Q

men acquire abnormal red-green vision if they inherit what

A

a defective X chromosome from mother

104
Q

women acquire abnormal red-green vision if they inherit what

A

abnormal X chromosomes from both parents

105
Q

whats transduction

A

its a process by which an environment stimulus causes an electrical response in a sensory receptor cell

106
Q

describe phototransduction in dark current (in darkness)

A

-in the dark the rod isnt excited and the receptor potential is approx. -30mv (depolarized)
-the membrane is depolarized because guanylyl cyclase catalyses synthesis of cGMP from GTP, and the cGMP causes gated cation channels to be open causing an influx of Na+
-this then results in glutamate release when the receptors are depolarized

107
Q

what causes a reduction in glutamate release in phototransduction

A

when receptors are hyperpolarized

108
Q

what does glutamate cause in phototransduction in the dark

A

inhibitory post synaptic potential (IPSP) in bipolar cells of the retina

109
Q

describe phototransduction in the light ( bleaching)

A

-when exposed to light there is conformation change of retinal from 11-cis retinal to all-trans retinal and release of opsin
-opsin then activates transducin which in turn activates cGMP PDE (phosphodiesterase)
-PDE catalyses the conversion of cGMP to GMP
-this results in closure of the Na+ channels as cGMP in no longer there and hyperpolarization of the cell occurs (-70mv)
-glutamate release is turned off and this excites the bipolar cell of the retina

110
Q

what happens when you have been in bright light for a long time (light adaption)

A

large portions of the photochemicals in both rods and cones will have been reduced to retinal and opsin during the bleaching reaction hence resulting in a reduction of the photosensitive chemicals remaining in the rods and cones, resulting in the reduction of sensitivity of the eye to light

111
Q

what happens during dark adaptation

A

-there is a gradual increase in the photoreceptor sensitivity due to rhodopsin regeneration in the dark

112
Q

neurotransmitter molecules released by rods and cones induce what

A

graded potentials in bipolar and horizontal cells

113
Q

what happens when bipolar and amacrine cells transmit excitatory signals to the ganglion cells

A

the ganglion cells become depolarized and an action potential is sent

114
Q

where do the right and left optic nerves come together

A

at the optic chiasm

115
Q

what happens to fibers from the nasal half of each retina

A

they cross over to the opposite side in the optic chiasm

116
Q

where do the optic tracts go

A

they go and synapse in the lateral geniculate body of the thalamus, then the fibers continue to the visual cortex in the occipital lobe where visual images are perceived

117
Q

What is legal blindness

A

Acuity no greater than 20/200 with best correlation

118
Q

What happens when you look at a bright red circle for a while and then you look at a white background

A

-You see a blue/green afterimage and this is because the prolonged viewing of the red stimulus causes adaptation in the red selective cones causing them to stop firing and the ganglion cells connected to the red cones decrease firing
-but the blue/green cones are not fatigued so the ganglion cells receiving green light send signals through the red-green channel and the brain will only receive a green signal

119
Q

How can the red green channel signal white

A

It must receive signals from both the red and green cones so that they can cancel each other

120
Q

Genes that encode for the blue pigment are found where

A

Chromosome 7

121
Q

What is red green colour blindness

A

It’s a difficulty in distinguishing red and green

122
Q

What’s blue-yellow colour blindness

A

It’s difficulty distinguishing blue and green and yellow and ref

123
Q

what do you call the condition where you are red-green weak

A

Protanopia

124
Q

What do you call the condition where you are blue-yellow weak

A

Tritanopia

125
Q

What do you call it when you can’t see any colour

A

Monochromacy

126
Q

How many rods synapse with a single bipolar cell

A

Between 6-600

127
Q

What effect do the multiple rods synapsing with a single bipolar cell do

A

Increases light sensitivity of rod vision but the image may be perceived as slightly blurry

128
Q

How many cones will often synapse with a single bipolar cell

A

1 cone

129
Q

What effect does a single cone synapsing with a single bipolar cell do

A

Causes sharper vision

130
Q

What do horizontal cells transmit

A

Inhibitory signals in areas lateral to excited rods and cones

131
Q

What do the inhibitory signals transmitted by the horizontal cells in areas which are lateral to the excited rods and cones do

A
  • it enhances visual contrast between areas in the retina that are strongly stimulated and adjacent areas with weak stimulation
  • allows for differentiation of colour
132
Q

Where do other axons from the optic chasm pass to

A

They go to the suprachiasmatic nuclei of the hypothalamus where they form connections that synchronize a variety of endocrine and other circadian rhythms associated with the light-dark cycle

MBBS2/Physiology~HEAD & NECK (3 decks)

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