Physiology of the Eye Flashcards
Eyelids closure is mediated by what nerve……
efferent fibres of the facial nerve (VII)
what muscles close the eyelids?
orbicularis oculi muscles
which nerve and muscle are responsible for lifting the upper eyelid?
oculomotor (CN III), levator palpebral superioris
Give the stimulus, receptors, afferent pathway, interneuron, efferent pathway, effectors and response for the CORNEAL REFLEX
Stimulus - corneal touch Receptors - somesthetic Afferent - Trigeminal nerve (ophthalmic) Interneuron - subcortical Efferent - facial nerve (VI) Effectors - orbicularis oculi muscles Response - blink
Give the stimulus, receptors, afferent pathway, interneuron, efferent pathway, effectors and response for the PALPEBRAL REFLEX
Stimulus - eyelid touch Receptors - somesthetic Afferent - Trigeminal nerve (ophthalmic & maxillary) Interneuron - subcortical Efferent - facial nerve (VI) Effectors - orbicularis oculi muscles Response - blink
Give the stimulus, receptors, afferent pathway, interneuron, efferent pathway, effectors and response for the MENACE RESPONSE
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
Give the stimulus, receptors, afferent pathway, interneuron, efferent pathway, effectors and response for the DAZZLE REFLEX
Stimulus - bright light Receptors - Photoreceptors Afferent - Optic nerve Interneuron - subcortical Efferent - facial nerve (VI) Effectors - orbicularis oculi muscle Response - blink
when do puppies and kittens open their eyelids?
between 10 and 15 days
what type of muscle makes up the feline third eyelid?
smooth muscle (9 of them)
the drawing in of the feline third eyelid into the medial canthus is innervated by what nerves
postganglionic adrenergic sympathetic nerve fibres
what type of muscle controls the avian third eyelid?
skeletal
what species have a harderian gland?
pigs, rodents, rabbits, some ruminants and some birds
what are the functions of the pre-corneal tear film?
smoothes out minor irregularities removes foreign matter from cornea and conjunctiva lubrication provides nutrients to avascular cornea controls local bacterial flora
what is the estimated tear volume of the horse and the approximate tear volume turnover rate?
234 microL
7 minutes
what are the layers of the pre-corneal tear film?
Fat/oil layer
Aqueous layer
Mucin layer
What is the origin, composition (brief) and function of the fat/oil layer?
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
What is the origin, composition (brief) and function of the aqueous layer?
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
What is the origin, composition (brief) and function of the mucin layer?
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
are tears slightly acidic or alkaline?
alkaline
name some non-specific antimicrobial substances found in the pre-corneal tear film
lysozyme, lactoferrin, alpha-lysine, complement
name some specific anti-microbials found in the PTF
immunoglobulins A, G and M
what is lysozyme?
antibacterial enzyme that hydrolyzes bacterial cell walls
which companion animal species does not produce lysozyme?
cat
which immunoglobulin is present in the highest concentrations in the PTF?
IgA
What is the mechanism of action for lactoferrin?
reversibly binds iron that would be available for bacterial metabolism and growth
what is the mechanism of action for IgA?
coats bacterial and viral microorganisms leading to agglutination, neutralization and lysis
Which nerve provides parasympathetic and sympathetic innervation to the lacrimal gland?
lacrimal nerve (branch of trigeminal)
How is corneal transparency achieved?
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
what proportion of the cornea is made up of the stroma?
90%
what is the brief composition of the corneal stroma?
water collagens glycosaminoglycans glycoproteins cellular and nerve components
what type of collagen is most abundant in the cornea?
type 1
what is the main function of the glycosaminoglycans in the corneal stroma?
maintaining regular spacing between fibrils
what happens to the quiescent keratocytes in the cornea upon corneal wounding?
they transform into activated fibroblasts and myofibroblasts
which corneal layers are responsible for maintaing the cornea in a relatively dehydrated state?
epithelium and endothelium
what allows nutrients from the aqueous humour to enter the cornea?
leakiness of the endothelium
where do the corneal epithelium and anterior stroma obtain oxygen from?
pre-corneal tear film
where do the endothelium and posterior stroma obtain oxygen from?
aqueous humour
which layer of the cornea has the highest need for glucose?
endothelium
what conformation of dogs has the lowest corneal sensitivity?
brachycephalic
what are of the cornea has the greatest sensitivity?
central
which nerve provides sensation to the cornea?
trigeminal (ophthalmic branch)
why are superficial corneal ulcers more painful than deep ulcers?
the anterior cornea is more densely innervated compared to the posterior cornea
what type of nociceptor is most abundant in the cornea?
polymodal nociceptor
apart from providing sensation, how else do corneal nerves maintain corneal epithelial health?
secrete neurotransmitters and neuropeptides that are critical to corneal epithelial proliferation and function
neuropeptides CGRP and substance P also play a role in what process in the cornea?
neurogenic inflammation
which nerve innervates the constrictor muscle of the pupil?
oculomotor (III)
what innervates the dilator muscle of the pupil?
sympathetic nerves
what is the primary type of nerve fibre in the feline iris sphincter muscle?
beta-adrenergic nerve fibres
why does a darker iris colour prolong the onset and duration of many drugs?
melanin can bind to drugs
why don’t birds usually have a consensual PLR? (but why might one be stimulated?)
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
what type of muscle are the avian iris sphincter and dilator muscles?
mainly striated
What drugs can cause mydriasis in birds?
some neuromuscular blocking agents
what is responsible for lymphatic drainage from the eye?
the uveal tract
in most species, how is blood flow provided to the ciliary body?
iridal major arterial circle, branches of the anterior ciliary arteries and long posterior ciliary arteries
what are the two primary ocular barriers?
blood-aqueous barrier
blood-retinal barrier
the blood aqueous barrier relies on tight junctions in which 3 areas?
nonpigmented ciliary body epithelium, non-fenestrated iris capillaries and posterior iris epithelium
what are the epithelial and endothelial portions of the blood-retinal barrier?
epithelial - retinal pigmented epithelium
endothelial - nonfenestrated retinal capillary endothelium
where is the most permeable point of the blood retinal barrier?
optic nerve head
what structure produce aqueous humour?
ciliary body processes
what are the two outflow pathways for aqueous humour?
corneoscleral trabecular outflow
uveoscleral outflow
what structures does the aqueous humour provide nutrients and waste removal for?
cornea and lens
what are the functions of the ciliary body?
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
what are the 2 parts of the ciliary epithelium?
outer pigmented epithelium
inner nonpigmented epithelium
what are the 3 basic mechanisms for aqueous humour production?
diffusion, ultrafiltration and active secretion by the NPE
what 2 enzymes are critical for AH production?
Na-K-ATPase, carbonic anhydrase
what reaction is catalysed by carbonic hydrase?
CO2 + H2O HCO-3 + H+
what is another name for the Tyndall effect?
aqueous flare
what is the most abundant cation in aqueous humour?
sodium
what is the range in proportion of AH that drains via conventional outflow?
~50-95%
what are the 3 portions of the trabecular meshwork?
uveal (innermost)
corneoscleral (middle)
juxtacanalicular zone (outermost)
what two factors are critical to the pathogenesis of POAG in humans (and likely dogs with the ADAMTS10 mutation)?
excessive deposition of ECM, concomitant stiffening of the trabecular meshwork
which outflow pathway is dependant on IOP?
conventional pathway
in diurnal species, when is the IOP typically highest?
in the morning
what comprises 80-90% of the lens’ water-soluble proteins?
crystallins
where are the soluble proteins concentrated in the lens?
cortex
where are the insoluble proteins concentrated in the lens?
nucleus
which proteins are considered the structural proteins of the lens?
soluble proteins
what happens to water soluble proteins in the lens with aging?
coalesce to form high-molecular-weight aggregates and become less water soluble
in the cataractous lens, what happens to the levels of insoluble proteins?
increases
what are the 2 classifications of len crystallin proteins?
classical (alpha and beta/gamma)
Taxon-specific
how is oxidative stress from constant exposure to light and oxidants reduced in the lens?
scavenging antioxidants (glutathione, carnitine and ascorbic acid)
what is the primary energy source for the lens? and where does it come from?
glucose
aqueous humour
by what process is energy derived and where is it mainly used in the lens?
anaerobic glycolysis
active cation transport and protein synethesis
what is the major end product of glucose metabolism in the lens and what happens to it?
lactic acid
diffuses into the AH
what happens to glucose metabolism in the lens in diabetic dogs?
aldose reductase is activated as an alternate route for glucose metabolism
what is the result of the altered glucose metabolism in the lens with diabetic patients?
accumulation of sorbitol which causes swelling associated with the increased osmotic pressure
a cataract forms
what makes up >98% of the vitreous?
water
what provides the framework and plasticity of the vitreous?
collagen
what occupies the space between collagen fibers in the vitreous?
hyaluronic acid
what are the cells in the vitreous called?
hyalocytes
what is the function of hyalocytes in the vitreous?
ECM synthesis
vitreous cavity immunology regulation
modulation of inflammation
what is a common complication of progressive liquefaction of the vitreous?
separation of the posterior vitreous cortex from the retinal inner limiting membrane - risk factor for rhegmatogenous retinal detachment
how does the vitreous protect the lens and retina?
absorbs retinal and lenticular waste products (lactic acid and free radicals)
what causes vitreous liquefaction?
decrease in hyaluronic acid
name the 4 rectus extraocular muscles in domestic species
dorsal (superior), ventral (inferior), medial (nasal), lateral (temporal)
which rectus EOMs does the oculomotor nerve innervate?
dorsal, ventral, medial
What innervates the lateral rectus muscle?
abducens nerve
what are the two oblique muscles of the eye and their innervation?
ventral oblique - oculomotor n
dorsal oblique - trochlear n
what species don’t have a retractor bulbi muscle?
primates and birds
List the specific features of the avian eyelids/TEL (7)
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
True or false: The outer lipid layer of the PTF is 0.1um thick and is produced by holocrine sebaceous glands.
True
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.
FALSE (60% total thickness)
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
False - ~1um thick and decreases surface tension
True or false: Potassium levels in the tears are higher than that of plasma (in humans, bovine and rabbit) due to facilitated diffusion.
False: higher due to active transport, also higher in horses
True or false: Basal tear production is approximately 25% of the total tear production.
False: it’s 50%
How do tears enter the nasolacrimal system?
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)
Discuss the glucose metabolism of the cornea. Compare and contrast to lens metabolism.
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
true or false: Choroidal blood supply arises largely from the short posterior ciliary arteries and has a strong autoregulatory mechanism.
False: little or no autoregulation
True or false: Choroidal proteins are important in the formation of a low osmotic pressure gradient to help maintain retinal attachment
False: high osmotic gradient
True or false: Choroidal and retinal vasculature both have extremely good auto-regulation
False: retina only
true or false: The blood flow to the optic nerve head in the cat has autoregulation over an IOP range of 6 to 30mmHg
False: those figures refer to most efficient range in humans, wider range in the cat of 30-75mmHg.
in the normal canine lens what proportion is made up of water and what proportion of proteins? What proportion of the proteins are soluble?
65% water and 35% proteins
85% soluble proteins
Ascorbic acid concentration is lower in the lens than the aqueous humour in which species?
dog, rabbit, guinea pig
With aging what happens to the alpha/beta light crystallins and the gamma/beta heavy crystallins?
alpha and beta/light increase
beta/heavy and gamma decrease
In the lens, reduced glutathione makes up what percentage of the total glutathione?
98%
