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Gelatt 6th Edition > Physiology of the Eye > Flashcards

Physiology of the Eye Flashcards

1
Q

Eyelids closure is mediated by what nerve……

A

efferent fibres of the facial nerve (VII)

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2
Q

what muscles close the eyelids?

A

orbicularis oculi muscles

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3
Q

which nerve and muscle are responsible for lifting the upper eyelid?

A

oculomotor (CN III), levator palpebral superioris

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4
Q

Give the stimulus, receptors, afferent pathway, interneuron, efferent pathway, effectors and response for the CORNEAL REFLEX

A 
Stimulus - corneal touch
Receptors - somesthetic
Afferent - Trigeminal nerve (ophthalmic)
Interneuron - subcortical
Efferent - facial nerve (VI)
Effectors - orbicularis oculi muscles
Response - blink
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5
Q

Give the stimulus, receptors, afferent pathway, interneuron, efferent pathway, effectors and response for the PALPEBRAL REFLEX

A 
Stimulus - eyelid touch
Receptors - somesthetic
Afferent - Trigeminal nerve (ophthalmic & maxillary)
Interneuron - subcortical
Efferent - facial nerve (VI)
Effectors - orbicularis oculi muscles
Response - blink
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6
Q

Give the stimulus, receptors, afferent pathway, interneuron, efferent pathway, effectors and response for the MENACE RESPONSE

A

Stimulus - menacing gesture
Receptors - Photoreceptors
Afferent - Optic nerve
Interneuron - cortical, cerrebellum
Efferent - facial nerve (VI), abducens (VI), glossopharyngeal IX
Effectors - orbicularis oculi muscles, retractor bulbi muscle
Response - blink, retract globe

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7
Q

Give the stimulus, receptors, afferent pathway, interneuron, efferent pathway, effectors and response for the DAZZLE REFLEX

A 
Stimulus - bright light
Receptors - Photoreceptors
Afferent - Optic nerve
Interneuron - subcortical
Efferent - facial nerve (VI)
Effectors - orbicularis oculi muscle
Response - blink
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8
Q

when do puppies and kittens open their eyelids?

A

between 10 and 15 days

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9
Q

what type of muscle makes up the feline third eyelid?

A

smooth muscle (9 of them)

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10
Q

the drawing in of the feline third eyelid into the medial canthus is innervated by what nerves

A

postganglionic adrenergic sympathetic nerve fibres

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11
Q

what type of muscle controls the avian third eyelid?

A

skeletal

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12
Q

what species have a harderian gland?

A

pigs, rodents, rabbits, some ruminants and some birds

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13
Q

what are the functions of the pre-corneal tear film?

A 
smoothes out minor irregularities
removes foreign matter from cornea and conjunctiva
lubrication
provides nutrients to avascular cornea
controls local bacterial flora
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14
Q

what is the estimated tear volume of the horse and the approximate tear volume turnover rate?

A

234 microL

7 minutes

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15
Q

what are the layers of the pre-corneal tear film?

A

Fat/oil layer
Aqueous layer
Mucin layer

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16
Q

What is the origin, composition (brief) and function of the fat/oil layer?

A

Origin - meibomian glands
Composition - meibum lipid mixture
Function - stabilisation of the tear film, prevents evaporation and overflow, provides a smooth optical surface, barrier against foreign particles, antimicrobial activity, seals lid margins during prolonged closure, prevents maceration of the of lid skin by tears

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17
Q

What is the origin, composition (brief) and function of the aqueous layer?

A

Origin - lacrimal gland (66%), TEL gland (33%), harderian gland (in certain spp), accessory lacrimal glands in conjunctiva
Composition - 98% water, 2% solids (proteins)
Functions - lubricaiton, provide nutrients, antimicrobial, removes foreign matter

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18
Q

What is the origin, composition (brief) and function of the mucin layer?

A

origin - conjunctival goblet cells, stratified squamous cells of corneal and conjunctival epithelium
composition - tear mucins/secreted mucins (glycoproteins), glycocalyx/membrane-spanning mucins (polysaccharides)
functions -
secreted mucins - lubrication, makes corneal surface more hydrophilic to permit spreading, stabilisation of PTF
membrane-spanning mucins (gloycocalyx) - promotion of water retention, dense barrier to pathogens, signal transduction, direct interaction with the actin cytoskeleton

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19
Q

are tears slightly acidic or alkaline?

A

alkaline

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20
Q

name some non-specific antimicrobial substances found in the pre-corneal tear film

A

lysozyme, lactoferrin, alpha-lysine, complement

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21
Q

name some specific anti-microbials found in the PTF

A

immunoglobulins A, G and M

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22
Q

what is lysozyme?

A

antibacterial enzyme that hydrolyzes bacterial cell walls

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23
Q

which companion animal species does not produce lysozyme?

A

cat

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24
Q

which immunoglobulin is present in the highest concentrations in the PTF?

A

IgA

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25
Q

What is the mechanism of action for lactoferrin?

A

reversibly binds iron that would be available for bacterial metabolism and growth

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26
Q

what is the mechanism of action for IgA?

A

coats bacterial and viral microorganisms leading to agglutination, neutralization and lysis

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27
Q

Which nerve provides parasympathetic and sympathetic innervation to the lacrimal gland?

A

lacrimal nerve (branch of trigeminal)

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28
Q

How is corneal transparency achieved?

A 
lattice-like organisation of stromal collagen fibrils
transparency of cells within cornea 
relative dehydration
hypocellularity
unmyelinated nerve fibres
nonkeratinized epithelium
absence of blood vessels and pigment
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29
Q

what proportion of the cornea is made up of the stroma?

A

90%

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30
Q

what is the brief composition of the corneal stroma?

A 
water
collagens
glycosaminoglycans
glycoproteins
cellular and nerve components
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31
Q

what type of collagen is most abundant in the cornea?

A

type 1

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32
Q

what is the main function of the glycosaminoglycans in the corneal stroma?

A

maintaining regular spacing between fibrils

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33
Q

what happens to the quiescent keratocytes in the cornea upon corneal wounding?

A

they transform into activated fibroblasts and myofibroblasts

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34
Q

which corneal layers are responsible for maintaing the cornea in a relatively dehydrated state?

A

epithelium and endothelium

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35
Q

what allows nutrients from the aqueous humour to enter the cornea?

A

leakiness of the endothelium

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36
Q

where do the corneal epithelium and anterior stroma obtain oxygen from?

A

pre-corneal tear film

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37
Q

where do the endothelium and posterior stroma obtain oxygen from?

A

aqueous humour

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38
Q

which layer of the cornea has the highest need for glucose?

A

endothelium

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39
Q

what conformation of dogs has the lowest corneal sensitivity?

A

brachycephalic

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40
Q

what are of the cornea has the greatest sensitivity?

A

central

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41
Q

which nerve provides sensation to the cornea?

A

trigeminal (ophthalmic branch)

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42
Q

why are superficial corneal ulcers more painful than deep ulcers?

A

the anterior cornea is more densely innervated compared to the posterior cornea

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43
Q

what type of nociceptor is most abundant in the cornea?

A

polymodal nociceptor

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44
Q

apart from providing sensation, how else do corneal nerves maintain corneal epithelial health?

A

secrete neurotransmitters and neuropeptides that are critical to corneal epithelial proliferation and function

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45
Q

neuropeptides CGRP and substance P also play a role in what process in the cornea?

A

neurogenic inflammation

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46
Q

which nerve innervates the constrictor muscle of the pupil?

A

oculomotor (III)

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47
Q

what innervates the dilator muscle of the pupil?

A

sympathetic nerves

48
Q

what is the primary type of nerve fibre in the feline iris sphincter muscle?

A

beta-adrenergic nerve fibres

49
Q

why does a darker iris colour prolong the onset and duration of many drugs?

A

melanin can bind to drugs

50
Q

why don’t birds usually have a consensual PLR? (but why might one be stimulated?)

A

total decussation of nerve fibres at the optic chiasm

a strong beam of light can pass through the thin orbital tissues/bones to stimulate the other retina

51
Q

what type of muscle are the avian iris sphincter and dilator muscles?

A

mainly striated

52
Q

What drugs can cause mydriasis in birds?

A

some neuromuscular blocking agents

53
Q

what is responsible for lymphatic drainage from the eye?

A

the uveal tract

54
Q

in most species, how is blood flow provided to the ciliary body?

A

iridal major arterial circle, branches of the anterior ciliary arteries and long posterior ciliary arteries

55
Q

what are the two primary ocular barriers?

A

blood-aqueous barrier

blood-retinal barrier

56
Q

the blood aqueous barrier relies on tight junctions in which 3 areas?

A

nonpigmented ciliary body epithelium, non-fenestrated iris capillaries and posterior iris epithelium

57
Q

what are the epithelial and endothelial portions of the blood-retinal barrier?

A

epithelial - retinal pigmented epithelium

endothelial - nonfenestrated retinal capillary endothelium

58
Q

where is the most permeable point of the blood retinal barrier?

A

optic nerve head

59
Q

what structure produce aqueous humour?

A

ciliary body processes

60
Q

what are the two outflow pathways for aqueous humour?

A

corneoscleral trabecular outflow

uveoscleral outflow

61
Q

what structures does the aqueous humour provide nutrients and waste removal for?

A

cornea and lens

62
Q

what are the functions of the ciliary body?

A 
production of aqueous humour
generation of IOP
influencing conventional AH outflow
provide blood and nerve supplies for anterior segment
accommodation
formation of the blood aqueous barrier
entry nonconventional AH outflow
Drug detoxification
63
Q

what are the 2 parts of the ciliary epithelium?

A

outer pigmented epithelium

inner nonpigmented epithelium

64
Q

what are the 3 basic mechanisms for aqueous humour production?

A

diffusion, ultrafiltration and active secretion by the NPE

65
Q

what 2 enzymes are critical for AH production?

A

Na-K-ATPase, carbonic anhydrase

66
Q

what reaction is catalysed by carbonic hydrase?

A

CO2 + H2O HCO-3 + H+

67
Q

what is another name for the Tyndall effect?

A

aqueous flare

68
Q

what is the most abundant cation in aqueous humour?

A

sodium

69
Q

what is the range in proportion of AH that drains via conventional outflow?

A

~50-95%

70
Q

what are the 3 portions of the trabecular meshwork?

A

uveal (innermost)
corneoscleral (middle)
juxtacanalicular zone (outermost)

71
Q

what two factors are critical to the pathogenesis of POAG in humans (and likely dogs with the ADAMTS10 mutation)?

A

excessive deposition of ECM, concomitant stiffening of the trabecular meshwork

72
Q

which outflow pathway is dependant on IOP?

A

conventional pathway

73
Q

in diurnal species, when is the IOP typically highest?

A

in the morning

74
Q

what comprises 80-90% of the lens’ water-soluble proteins?

A

crystallins

75
Q

where are the soluble proteins concentrated in the lens?

A

cortex

76
Q

where are the insoluble proteins concentrated in the lens?

A

nucleus

77
Q

which proteins are considered the structural proteins of the lens?

A

soluble proteins

78
Q

what happens to water soluble proteins in the lens with aging?

A

coalesce to form high-molecular-weight aggregates and become less water soluble

79
Q

in the cataractous lens, what happens to the levels of insoluble proteins?

A

increases

80
Q

what are the 2 classifications of len crystallin proteins?

A

classical (alpha and beta/gamma)

Taxon-specific

81
Q

how is oxidative stress from constant exposure to light and oxidants reduced in the lens?

A

scavenging antioxidants (glutathione, carnitine and ascorbic acid)

82
Q

what is the primary energy source for the lens? and where does it come from?

A

glucose

aqueous humour

83
Q

by what process is energy derived and where is it mainly used in the lens?

A

anaerobic glycolysis

active cation transport and protein synethesis

84
Q

what is the major end product of glucose metabolism in the lens and what happens to it?

A

lactic acid

diffuses into the AH

85
Q

what happens to glucose metabolism in the lens in diabetic dogs?

A

aldose reductase is activated as an alternate route for glucose metabolism

86
Q

what is the result of the altered glucose metabolism in the lens with diabetic patients?

A

accumulation of sorbitol which causes swelling associated with the increased osmotic pressure
a cataract forms

87
Q

what makes up >98% of the vitreous?

A

water

88
Q

what provides the framework and plasticity of the vitreous?

A

collagen

89
Q

what occupies the space between collagen fibers in the vitreous?

A

hyaluronic acid

90
Q

what are the cells in the vitreous called?

A

hyalocytes

91
Q

what is the function of hyalocytes in the vitreous?

A

ECM synthesis
vitreous cavity immunology regulation
modulation of inflammation

92
Q

what is a common complication of progressive liquefaction of the vitreous?

A

separation of the posterior vitreous cortex from the retinal inner limiting membrane - risk factor for rhegmatogenous retinal detachment

93
Q

how does the vitreous protect the lens and retina?

A

absorbs retinal and lenticular waste products (lactic acid and free radicals)

94
Q

what causes vitreous liquefaction?

A

decrease in hyaluronic acid

95
Q

name the 4 rectus extraocular muscles in domestic species

A

dorsal (superior), ventral (inferior), medial (nasal), lateral (temporal)

96
Q

which rectus EOMs does the oculomotor nerve innervate?

A

dorsal, ventral, medial

97
Q

What innervates the lateral rectus muscle?

A

abducens nerve

98
Q

what are the two oblique muscles of the eye and their innervation?

A

ventral oblique - oculomotor n

dorsal oblique - trochlear n

99
Q

what species don’t have a retractor bulbi muscle?

A

primates and birds

100
Q

List the specific features of the avian eyelids/TEL (7)

A

Lower eyelid is very important-larger and more mobile (Pg 150) (opposite to dog, Pg149)
TEL is nearly transparent
TEL movement is under direct muscular control, no passive components as in other common domestic sp
TEL movement replaces lids for blinking, moves across ocular surface frequently (15-20x/minute, as compared to 3-5x/minute in the normal, unrestrained dog)
No cilia; just filoplumes, feathers within eyebrow region
Superficial tear gland (inferotemporal), and deeper Harder’s gland in some sp (posterior to sclera, behind TEL)
Musculature-quadratus m and pyrimidalis m

101
Q

True or false: The outer lipid layer of the PTF is 0.1um thick and is produced by holocrine sebaceous glands.

A

True

102
Q

True or false: The middle aqueous layer of the PTF is 7um thick (80% of the total thickness) and is evaluated clinically by the Schirmer tear test.

A

FALSE (60% total thickness)

103
Q

true or false: The deep mucin layer of the PTF is 0.02-0.05um thick and improves tear film stability by increasing the surface tension

A

False - ~1um thick and decreases surface tension

104
Q

True or false: Potassium levels in the tears are higher than that of plasma (in humans, bovine and rabbit) due to facilitated diffusion.

A

False: higher due to active transport, also higher in horses

105
Q

True or false: Basal tear production is approximately 25% of the total tear production.

A

False: it’s 50%

106
Q

How do tears enter the nasolacrimal system?

A

Blinking forces tears along palpebral margin to move medially, into lacrimal pool
Tears flow into canaliculi by capillary action when facial muscles relax
Assisted by normal respiration
Blinking closes lacrimal sac, which acts as a pump
Muller’s muscle
Pseudoperistaltic action along nasolacrimal duct (in man)
Autoregulation ? (in man)

107
Q

Discuss the glucose metabolism of the cornea. Compare and contrast to lens metabolism.

A

Epithelium- main (85%) pathway is glycolysis, forming pyruvic acid which can then enter either lactate acid pathway (anaerobic, Embden-Meyerhof) or TCAC path; lactic acid (via lactate dehydrogenase); minor pathways include pentose phosphate shunt (NADPH etc) and TCAC (Kreb’s cycle), aerobic, forming ATP/CO2; O2 from tear film

Stroma- low metabolic needs, O2 from aqueous humour

Endothelium- v high metabolic needs (uses 5x more glucose than epithelium), O2 from aqueous humour; main pathway is anaerobic to lactic acid (like epithelium)

NB when lids closed, increased anaerobic metabolism, increased lactate, increased corneal oedema

Compare and contrast to lens metabolism
 Similar to cornea
 80% glycolysis, forming pyruvic acid, then lactic acid (anaerobic, Embden-Meyerhof)
 10% pentose phosphate shunt
 3% Kreb’s cycle (TCAC), aerobic, forming ATP (20% of lens energy and CO2)
Additional pathway
 Sorbitol path including aldose reductase, fructose kinase (diabetic cataracts)
 NB galactosemic cataracts in marsupials-Slatter/Gelatt

108
Q

true or false: Choroidal blood supply arises largely from the short posterior ciliary arteries and has a strong autoregulatory mechanism.

A

False: little or no autoregulation

109
Q

True or false: Choroidal proteins are important in the formation of a low osmotic pressure gradient to help maintain retinal attachment

A

False: high osmotic gradient

110
Q

True or false: Choroidal and retinal vasculature both have extremely good auto-regulation

A

False: retina only

111
Q

true or false: The blood flow to the optic nerve head in the cat has autoregulation over an IOP range of 6 to 30mmHg

A

False: those figures refer to most efficient range in humans, wider range in the cat of 30-75mmHg.

112
Q

in the normal canine lens what proportion is made up of water and what proportion of proteins? What proportion of the proteins are soluble?

A

65% water and 35% proteins

85% soluble proteins

113
Q

Ascorbic acid concentration is lower in the lens than the aqueous humour in which species?

A

dog, rabbit, guinea pig

114
Q

With aging what happens to the alpha/beta light crystallins and the gamma/beta heavy crystallins?

A

alpha and beta/light increase

beta/heavy and gamma decrease

115
Q

In the lens, reduced glutathione makes up what percentage of the total glutathione?

A

98%

Gelatt 6th Edition (7 decks)

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