CA of the Eye 2

Question 1

The globe structure

Answer 1

1. 3 layers
   a) Fibrous outer layer - supports eyeball shape - scelera and cornea
2. Uvea vascular layer - provides nutrition to structures of the eye and acts to alter light transmission - iris, ciliary body and choroid
3. Neural inner layer = retina
   - at front only firbous layer, if all 3 light wouldn’t get in

Question 2

How is they eye divided internally?

Answer 2

into anterior and posterior segments by the position of the lens, with the anterior segment being further divided into anterior and posterior chambers by the iris.
AC - between iris and cornea
PC - between iris and back of lense

Question 3

What is the Cornea

Answer 3

1. is the clear curved rostral surface of the eyeball which transmits and refracts light (should be transparent
2. Elliptical in shape, exaggerated in ungulates to maximise horizontal field of vision
3. D+C 0.7mm thick which isn’t consistent along whole cornea
4. Stroma is the main body, consists of carefully arranged collagen fibrils
5. A vascular

Question 4

What is the corneal sclera limbus?

Answer 4

The junction between the cornea and sclera

1. can be pigmented

Question 5

Sclera vasculature

Answer 5

1. blood vessels which terminate at the limbus

Question 6

Cornea layers

Answer 6

1. Outer anterior epithelium. Base called bowman’s capsule and pain receptors located here
2. Stroma = main body
3. Descemet’s membrane, produced by endothelium. Becomes thicker as ages
4. Endothelium

Question 7

Endothelium of cornea importance

Answer 7

vital role in maintaining corneal clarity by actively pumping water out of the stroma. Damage to the endothelium causes corneal oedema (water can’t be pumped out) which disrupts the stromal lamellae and prevents light transmission and can lead to blisters (bullae) and ulcers forming under the anterior epithelium.

Question 8

Cornea innervation

Answer 8

CN 5
heavily innervated with sensory fibres, with pain receptors being most prevalent in the anterior epithelium and pressure receptors in the stroma. Damage is painful, esp anterior more than deeper. CNV

Question 9

neovascularisation

Answer 9

Blood vessels grow in from the limbus to promote healing cornea injury.
Can be superficial or deep.
Once cornea / deeper structure fixed V remains but no blood

Question 10

Sclera

Answer 10

1. starts at limbus, round whole eye
2. white part
3. forms the posterior fibrous tunic of the eye, enclosing the posterior globe until it meets the cornea at the corneoscleral limbus
4. There is a perforated zone posteriorly where the axons of optic nerve exits the eyeball – the lamina cribrosa
5. varies in thickness between ages and species and at different points of the globe.
   young = Thin- appears blue as the pigmented choroid is visible through the white tissue of the sclera.
   old animals- ivory colouration
   - jaundice (hyperbilirubinaemia) the sclera can take on a yellow colour.

Question 11

If animal presents with red eye how to know if BV deep or superficial?

Answer 11

1. Use adrenoreceptor agonist to differentiate
2. superficial BV will constrict quickly
3. Common conjunctiva e.g. allergic reactions
4. deeper episclera BV are straighter (episcleritis)

Question 12

The UVea

Answer 12

1. Vascular, middle layer of the globe
2. Posterior uvea = choroid
3. Anterior uvea = iris and ciliary body
4. Highly vascularised and pigmented (apart from albino)

Question 13

The iris

Answer 13

1. PArt of anterior uvea
   Pigmented
2. most rostral portion uvea
3. the centre of this ring is the pupil
4. is a sphincter, capable of constricting or dilating to vary the amount of light entering the posterior segment of the eye and altering the depth of focus
5. It varies in shape between species, with dogs and pigs having circular pupils, cats having pupils which are vertical when constricted and ungulates having horizontal oval-shaped pupils.
6. has a anterior border layer, middle stroma (muscles) and posterior epithelium, pigmented

Question 14

Constrictor muscle of iris

Answer 14

1. under parasympathetic innervation, CN 3
2. Located in the stroma of the pupillary zone
3. autonomic in most species BUT non mammalian they’re striated = can change shape of pupil under conscious control. Makes drugs chosed for dilation different mammalaian vs non
4. Shape of pupil is dependedn on constrictor muscle - cat slit, dog round

Question 15

Dilator msucle of iris

Answer 15

1. under sympathetic innervation
2. autonomic in most species BUT non mammalian they’re striated = can change shape of pupil under conscious control. Makes drugs chosed for dilation different mammalaian vs non

Question 16

Iris attached to…

Answer 16

Ciliary body which produces aqueous humour - liquid filling anterior segment of eye
Also suspends lense with zonular fibres
Fibres attached to folds of ciliary body called ciliary processes

Question 17

Non mamaliam species zonular fibres

Answer 17

Don’t have

ciliary processes attach directly to lense

Question 18

Ciliary body

Answer 18

1. consists of the ciliary processes, the ciliary body muscles and forms part of the iridocorneal angle.
2. produces aqueous humour and involved in its drainage
3. Aq humour goes into posterior chamber of anterior segment, flows through pupil and drained at root of ciliary body between iris and cornea
4. Pigmented and non pig epithelian layer - forms barrier between aqueous humour and blood supply

Question 19

Iridocorneal angle

Answer 19

1. v important
2. Angle between cornea nad iris
3. pectinate ligaments, underneath meshwork that drains aq humour
4. any disease of lesion that affects ICA affects drainage
5. inc intraoccular pressure
6. can lead to glaucoma

Question 20

Choroid

Answer 20

1. posterior uvea, layer between sclera and retina
2. highly vascularied, pigmented,
3. made up of 4 layers

Question 21

Layers of the choroid

Answer 21

• The suprachoroidea forms the transition between sclera and choroid and is elastic and heavily pigmented
• The large vessel layer contains a vascular plexus mainly consisting of veins which coalesce to form the vortex veins and which communicate with other venous plexi. This layer serves to cool the eye, protecting it from the heat produced from light absorption.
• The medium vessel layer contains vessels and the tapetum lucidum, which is fibrous in herbivores and cellular in carnivores. The tapetum reflects light that has passed through the retina, increasing the night-vision capabilities of the animal although at the expense of visual acuity. The vessels in this layer link with the choriocapillaris and these linking vessels can be visible on ophthalmoscopic examination as the “stars of Winslow”.
• The choriocapillaris forms the innermost layer of vessels and consists of fenestrated capillaries which supply the retina. Inside the choriocapillaris is a basement membrane and some connective tissue that separates the choroid from the retinal pigment epithelium (RPE).

Question 22

The Aqueous humour

Answer 22

1. Anterior segement of the eye
2. It is an ultrafiltrate of plasma produced by the ciliary processes of the ciliary body and drains at iridoconeal angle
3. Provides nutritional needs of the lens and cornea
4. Constant production and drainage of aq humour is necessary to maintain stable INTRAOCULAR PRESSURE (IOP). If production exceeeds = glaucoma

Question 23

What makes up the tear film? State what , role and where produced

Answer 23

1. Lipid portion part – meibomium gland- reduces evaportation and creates barrier at lid margin
2. Mucous/ Mucin layer – from conjunctival goblet cells, mixed with and stabilises aqueous layer
3. Aqueous layer portion (not humour) from lacrimal gland and 3rd eyelid – lubrication, protection ad nutrition of corneal, limbal and conjunctival epithelium

Question 24

Aqueous Humour, location and produced by

Answer 24

Role- provide nutritional support to lens and cornea
Location: anterior segment
Produced by: ciliary processes of the ciliary body which is highly folded to inc SA.
Produced by 3 processes: diffusion (osmotic pressure gradients), ultrafiltration (hydrostatic pressure differences) and active secretion (main.

Question 25

Aqueous humour drainage and importance

Answer 25

drains at the iridocorneal angle

2. The constant equal production and drainage of aqueous humor is necessary to maintain an appropriate and stable intraocular pressure (IOP)
3. if production exceeds drainage the IOP will rise – a serious condition called glaucoma. Stable IOP is necessary to maintain the position and health of the intraocular structures and confers some of the rigidity of the globe

Question 26

More detail on how Aq humour is produced

Answer 26

produced by three processes – diffusion, ultrafiltration and active secretion

1. Diffusion: solutes down their concentration gradients into the aqueous
2. Ultrafiltration - differences in hydrostatic pressure in the ciliary body capillaries and the IOP
3. AT - to active transport of Na+ by the non-pigmented ciliary epithelium into the aqueous humour which brings water across too. Active secretion is the most important factor in aqueous humour production, as differences in osmolarity between aqueous and plasma are small.

Question 27

Aqueous humour and bicarbonate

Answer 27

HCO3- enters the aqueous via the carbonic anhydrase reaction which occurs in the epithelium of the ciliary body: CO2 + H2O H2CO3 HCO3- + H+

To maintain electrochemical balance

The presence of HCO3- affects Na+ movement, which in turn influences water transport. This pathway provides a target for glaucoma therapy, as inhibition of carbonic anhydrase will reduce water movement into the aqueous.

Question 28

More detail on aq humour drainage

Answer 28

1. AQH circulates from posterior chamber to anterior chamber (in anterior segment)
2. Majority drains through pectinate ligament and ciliary cleft of the iridocorneal angle and then via the aqueous plexus to scleral venous circulation
3. alternative pathway: uveoscleral outflow - AQH bypasses AQ plexus and drains straight from cilary cleft. About 15% aq drainage in dog, 3 in cat 30% horse

Question 29

name 2 that don’t possess a retractor bulbi muscle

Answer 29

Birds and snakes

Question 30

What is dysfunction of one or more extraocular muscles that affect eyeball position known as?

Answer 30

STRABISMUS

Question 31

What causes ventrolateral strabismus?

Answer 31

Damage to the CNIII - Occulomotor - innervates all other extraoccular muscles of the eye! Damage/paralysis causes ventrolateral strabismus

Question 32

What causes extorsional strabismus

Answer 32

CNIV - Trochlear - innervates dorsal oblique m. (rotates dorsal part of globe medially and ventrally).

Question 33

What causes medial strabismus?

Answer 33

CNVI - Abducens - innervates laterial rectus m. (rotates globe laterally) and retractor bulbi m. (retracts globe into orbit). Damage/paralysis results in medial strabismis