Structure and function of the eye Flashcards
what muscle closes the eyelids?
orbicularis oculi
what muscle opens the eyelids
levator palpebrum superioris
what gland produces the lipid layer of the tear film?
meibomian glands
what glands produce the aqueous portion of the tear film?
lacrimal gland
third eyelid gland
what bones form the canine orbit?
frontal bone, maxilla, zygomatic arch, orbital wing of presphenoid, lacrimal bone
what nerve and vessel enter the orbit through the optic canal?
optic nerve, internal ophthalmic artery
what nerves leave the orbit through the orbital fissure?
oculomotor (III), trochlear (IV), abducens (VI), ophthalmic branch of maxillary nerve,
what structure lies in the floor of the orbit?
zygomatic salivary gland
is the corneal epithelium hydrophobic or hydrophilic?
hydrophobic
name three reasons why the cornea is transparent
relatively dehydrated
collagen fibrils same diameter
collagen fibrils same distance apart
what is the normal volume of the tear lake?
5-8ul
what is the volume of a drop from an eye dropper bottle?
25-40ul
give 3 functions of the iris
regulates amount of light entering the eye
lymphoid aggregation with immunological functions
communication
what are the 3 layers of the iris?
anterior border layer
stroma with sphincter muscle
posterior epithelial layer
what is the main lymphoid organ for the eye?
spleen
name a critical soluble factor in the generation of anterior chamber acquired immune deviation
transforming growth factor beta
what nerve provides the parasympathetic innervation of the iris sphincter muscle?
oculomotor n (CN III)
how do age related cataracts form?
oxidative degentation from exposure to light - forms disulphide bridges between crystallins which aggregates the molecules
what is the major protein of the lens
alpha crystalline
what substances help to prevent photo-oxidation in the lens?
alpha crystalline heat shock proteins dietary anti-oxidants glutathione enzymes
how is aqueous humour formed?
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
what forms the conventional drainage route for aqueous humour?
trabecular meshwork in the iridocorneal angle
are dogs and cats dichromats or trichromats?
dichromats
which extraocular muscles do not have their motor function innervated by the oculomotor nerve?
dorsal oblique (trochlear n, IV) lateral rectus muscle (abducens n, VI) retractor bulbi (abducen n, VI)
name the extraocular muscles (7)
dorsal rectus dorsal oblique lateral rectus medial rectus ventral rectus ventral oblique retractor bulbi
corneal collagen fibrils have a diameter of around: 3-5nm 20-25nm 200-205nm 500-505nm
20-25
the meibomian gland is a modified: salivary gland serous gland apocrine gland sebaceous gland
sebaceous gland
the lens protein alpha crystallin is a modified hormone modified enzyme chaperone molecule facilitator molecule
chaperone molecule
myocillin is also known as:
trabecular meshwork adaptable protein
glaucoma associated adaptable inducer protein
glaucoma associated inducible protein
trabecular meshwork inducible glucocorticoid response protein
trabecular meshwork inducible glucocorticoid response protein
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
parasympathetic fibres from the nucleus of the facial nerve
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
hyaluronic acid and type II collagen
rhodopsin is called “visual purple” because…
it predominantly absorbs light of green-blue wavelengths
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
special somatic afferent fibres running through the optic canal
describe the structure of the lens
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
how does the lens maintain it’s transparency?
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.
Briefly outline how this transparency is lost in age-related cataract
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
What tissues or cell types are present in the canine iris?
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
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
Iris melanoma - melanocyte Horners syndrome - sympathetic nerves Lymphoma - lymphoid tissue Iritis - lymphoid tissue Iridal atrophy in older animals - muscle fibres
How would the different cells within the retina be affected by progressive retinal atrophy and by glaucoma?
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
What are the different layers of the cornea?
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
Briefly describe how the layers of the cornea may be involved in a corneal ulcer?
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
Describe the biochemical events occurring in a diabetic cataract
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.
give 3 antimicrobial molecules found in the tear film
immunoglobulin, lysozyme, lactoferrin
