Structure and function of the eye

Question 1

what muscle closes the eyelids?

Answer 1

orbicularis oculi

Question 2

what muscle opens the eyelids

Answer 2

levator palpebrum superioris

Question 3

what gland produces the lipid layer of the tear film?

Answer 3

meibomian glands

Question 4

what glands produce the aqueous portion of the tear film?

Answer 4

lacrimal gland

third eyelid gland

Question 5

what bones form the canine orbit?

Answer 5

frontal bone, maxilla, zygomatic arch, orbital wing of presphenoid, lacrimal bone

Question 6

what nerve and vessel enter the orbit through the optic canal?

Answer 6

optic nerve, internal ophthalmic artery

Question 7

what nerves leave the orbit through the orbital fissure?

Answer 7

oculomotor (III), trochlear (IV), abducens (VI), ophthalmic branch of maxillary nerve,

Question 8

what structure lies in the floor of the orbit?

Answer 8

zygomatic salivary gland

Question 9

is the corneal epithelium hydrophobic or hydrophilic?

Answer 9

hydrophobic

Question 10

name three reasons why the cornea is transparent

Answer 10

relatively dehydrated
collagen fibrils same diameter
collagen fibrils same distance apart

Question 11

what is the normal volume of the tear lake?

Answer 11

5-8ul

Question 12

what is the volume of a drop from an eye dropper bottle?

Answer 12

25-40ul

Question 13

give 3 functions of the iris

Answer 13

regulates amount of light entering the eye
lymphoid aggregation with immunological functions
communication

Question 14

what are the 3 layers of the iris?

Answer 14

anterior border layer
stroma with sphincter muscle
posterior epithelial layer

Question 15

what is the main lymphoid organ for the eye?

Answer 15

spleen

Question 16

name a critical soluble factor in the generation of anterior chamber acquired immune deviation

Answer 16

transforming growth factor beta

Question 17

what nerve provides the parasympathetic innervation of the iris sphincter muscle?

Answer 17

oculomotor n (CN III)

Question 18

how do age related cataracts form?

Answer 18

oxidative degentation from exposure to light - forms disulphide bridges between crystallins which aggregates the molecules

Question 19

what is the major protein of the lens

Answer 19

alpha crystalline

Question 20

what substances help to prevent photo-oxidation in the lens?

Answer 20

alpha crystalline
heat shock proteins
dietary anti-oxidants
glutathione
enzymes

Question 21

how is aqueous humour formed?

Answer 21

produced by ciliary body non pigmented epithelium
carbon dioxide is combined with water to form H+ and bicarbonate ions (catalysed by carbonic anhydrase)
bicarbonate ions move into the posterior chamber bringing Na+ and water with is.
active transport of sodium ions
ultrafiltration of chloride ions

Question 22

what forms the conventional drainage route for aqueous humour?

Answer 22

trabecular meshwork in the iridocorneal angle

Question 23

are dogs and cats dichromats or trichromats?

Answer 23

dichromats

Question 24

which extraocular muscles do not have their motor function innervated by the oculomotor nerve?

Answer 24

dorsal oblique (trochlear n, IV)
lateral rectus muscle (abducens n, VI)
retractor bulbi (abducen n, VI)

Question 25

name the extraocular muscles (7)

Answer 25

``` dorsal rectus dorsal oblique lateral rectus medial rectus ventral rectus ventral oblique retractor bulbi ```

Question 26

``` corneal collagen fibrils have a diameter of around: 3-5nm 20-25nm 200-205nm 500-505nm ```

Answer 26

20-25

Question 27

``` the meibomian gland is a modified: salivary gland serous gland apocrine gland sebaceous gland ```

Answer 27

sebaceous gland

Question 28

``` the lens protein alpha crystallin is a modified hormone modified enzyme chaperone molecule facilitator molecule ```

Answer 28

chaperone molecule

Question 29

myocillin is also known as: trabecular meshwork adaptable protein glaucoma associated adaptable inducer protein glaucoma associated inducible protein trabecular meshwork inducible glucocorticoid response protein

Answer 29

trabecular meshwork inducible glucocorticoid response protein

Question 30

the lacrimal gland is innervated by sympathetic fibres from the trigeminal nerve from meckels cave parasympathetic fibres from the nucleus of the facial nerve parasympathetic fibres of the trigeminal nerve from the gasserian ganglion sympathetic fibres from the vago-sympathetic trunk

Answer 30

parasympathetic fibres from the nucleus of the facial nerve

Question 31

``` the vitreous humour is composed of dermatan sulphate and type IV collagen hyaluronic acid and type IV collagen Dermatan sulphate and type II collagen hyaluronic acid and type II collagen ```

Answer 31

hyaluronic acid and type II collagen

Question 32

rhodopsin is called "visual purple" because...

Answer 32

it predominantly absorbs light of green-blue wavelengths

Question 33

the optic nerve is composed of general somatic afferent fibres running through the orbital fissure special somatic afferent fibres running through the orbital fissure special somatic afferent fibres running through the optic canal general somatic afferent fibres running through the optic canal

Answer 33

special somatic afferent fibres running through the optic canal

Question 34

describe the structure of the lens

Answer 34

Biconvex with equator; Y shaped suture lines Surrounded by a capsule, thicker anteriorly with an epithelium, and thinner posteriorly Central nucleus, formed in utero Surrounded by the cortex Lens fibre cells produced at the equatorial region and laid down throughout life

Question 35

how does the lens maintain it's transparency?

Answer 35

The lens fibre cells are elongated and are arranged in a parallel, closely interdigitating structure. The cells have no nuclei and are composed largely of protein (crystallins). The crystallins attract water molecules which hold the proteins in a liquid crystal formation; this allows anti-oxidants and enzymes to move through the lens.

Question 36

Briefly outline how this transparency is lost in age-related cataract

Answer 36

Age related cataract occurs due to photo-oxidation. Oxidation of thiol groups in the crystalline molecules results in aggregation of these proteins, which reduces transparency

Question 37

What tissues or cell types are present in the canine iris?

Answer 37

The anterior border layer contains melanocytes and fibroblasts The stroma is loosely arranged collagen with melanocytes, other chromphores and fibroblasts Within the stroma there are blood vessels, muscles (circumferential sphincter and axial dilator), nerves (sympathetic and parasympathetic) and lymphoid tissue Posterior epithelial layer

Question 38

List three conditions of the iris, each of which primarily affects a single tissue or cell type. For each condition you choose, give the cell type affected

Answer 38

``` Iris melanoma - melanocyte Horners syndrome - sympathetic nerves Lymphoma - lymphoid tissue Iritis - lymphoid tissue Iridal atrophy in older animals - muscle fibres ```

Question 39

How would the different cells within the retina be affected by progressive retinal atrophy and by glaucoma?

Answer 39

In PRA there is a decrease in the photoreceptors and their cell bodies in the ONL In glaucoma there is death of the ganglion cells but not generally a change in the photoreceptor layer

Question 40

What are the different layers of the cornea?

Answer 40

Epithelium – a stratified squamous structure Epithelial basement membrane Stroma – with collagen fibres, proteoglycans and water Descemet’s membrane – the basement membrane of the endothelium Endothelium

Question 41

Briefly describe how the layers of the cornea may be involved in a corneal ulcer?

Answer 41

Epithelium is eroded Basement membrane dystrophy/loss can cause non-healing ulcer Stroma – can be eroded to different degrees by ulcer – may be destroyed in a melting ulcer Descemet’s membrane – can bulge out if the ulcer gets that deep as it is elastic in nature Endothelium – its action in pumping water out of the cornea is critical in maintaining transparency but damage to this layer is seen in penetrating ulceration

Question 42

Describe the biochemical events occurring in a diabetic cataract

Answer 42

Excess glucose in the lens enters the polyol pathways where it is then metabolised to sorbitol which has an osmotic effect drawing water into the lens. Reactive oxygen species also influence the lens and advanced glycation end products are also formed which can result in lens opacification.

Question 43

give 3 antimicrobial molecules found in the tear film

Answer 43

immunoglobulin, lysozyme, lactoferrin