Eye Flashcards
In the dog, which vessels pass through foramina to gain access to the orbit?
Ophthalmic artery and vein
Orbital vein
Cranial nerves 2, 3, 4, 6 (optic, oculomotor, trochlear, abducens)
Ophthalmic branch of cranial nerve 5
In the horse, a portion of the orbit is contributed to by the zygomatic process of which bone?
Temporal
What are the three layers of the orbital fascia?
Outer=periorbita
Fascia bulbi
Fascial sheaths of the extra-ocular muscles
Which species do not possess a retractor bulbi muscle?
Birds and snakes
What are the functions of the eyelids?
Close to exclude light
Protect the eyeball
Spread the tear film across the eye and into the lacrimal punctae
Help to remove foreign material from the eye
At what age do cats and dogs open their eyes?
What is the eyelid opening called?
10-14 days
Palpebral fissure
The cilia on the upper eyelid (eyelashes) are associated with which glands?
Modified sweat glands (glands of Moll) Sebaceous glands (glands of Zeis)
What causes a stye to form?
Infection of sebaceous or sweat glands in the upper eye lid
In the dog, the lateral palpebral ligament is absent, but what is its function replaced by?
Retractor anguli oculi muscle
Closure of the eyelids is due to what?
Contraction of the orbicularis oculi muscle, which is innervated by the facial nerve (CN VII)
Sensory innervation to the eyelids is via which nerve?
This branches into what?
Trigeminal (CN V)
Branches into ophthalmic branch (innervates most of upper lid and medial part of lower lid) and maxillary branch (innervates lower lid and joins with ophthalmic branch to supply lateral portion of upper lid)
What is the nictitating membrane?
Where does it sit?
Third eyelid
A reflection of conjunctiva containing a T-shaped cartilage support.
Sits ventromedially in the conjunctival sac, and the free edge is snug against the eyeball
What are the three portions of the tear film?
Lipid layer
Mucin layer
Aqueous layer
The lacrimal nerve is a branch of which nerve?
Ophthalmic branch of CNV
Provides parasympathetic and sympathetic innervation
What are the three layers of the globe and what do they do?
Fibrous layer- supports the eyeball shape
Uvea- provides nutrition to the structures of the eye and acts to alter light transmission
Neural layer- retina
What is the cornea?
How thick is it in dogs and cats?
The clear, curved rostral surface of the eyeball which transmits and refracts light.
0.7mm thick
Which 2 factors give the cornea its clarity?
Lack of corneal blood vessels
Arrangement of lamellae
What makes up the anterior and posterior uvea?
Anterior uvea= iris and ciliary body
Posterior uvea= choroid
Describe the pathway of aqueous humour
It is produced in the posterior chamber, flows through the pupil into the anterior chamber and drains at the iridocorneal angle
What are the 4 layers of the choroid?
Suprachoroidea-transition between sclera and choroid
Large vessel layer-cools the eye
Medium vessel layer-contains tapetum lucidum
Choriocapillaris-consists of fenestrated capillaries which supply the retina
What is aqueous humour produced by?
Ciliary processes of the ciliary body
What is the normal range for intra-ocular pressure in a dog?
15-25mmHg
How is a glaucoma formed?
When production of aqueous humour exceeds drainage and intra-ocular pressure rises
What are ‘floaters’?
Clumps of hyaluronic acid, occurs in old age when the lamellar arrangement of fibrils in the vitreous humour breaks down and the vitreous becomes more liquid.
How does a cataract form?
Changes inside the lens disrupt normal metabolism and lead to deposition of lens protein or accumulation of vacuoles and disruption of lens fibres.
This affects transparency and size of lens.
What are the 3 inner layers of the retina?
Nerve fibre layer
Ganglion cell layer
Inner plexiform layer
Where are cones most numerous?
Area centralis of the retina
Which area of the retina has the most ganglion cells?
Visual streak
Give 3 functions of the retinal pigment epithelium
Phagocytoses spent photoreceptor segments
Vitamin A metabolism
Moves metabolites into and out of the retina
How are cats able to actively protrude their third eyelids?
They possess some striated muscle in their third eyelids
What does each layer of the tear film do?
Lipid layer, mucin layer, aqueous layer
Lipid layer: produced from meibomian glands; reduces evaporation and creates a barrier at the lid margin
Mucin layer: produced by conjunctival goblet cells; stabilises the tear film
Aqueous layer: produced by lacrimal glands and 3rd eyelid gland; provides lubrication, protection and nutrition to the epithelium it covers
What suspends the lens?
Zonular fibres
Where is the main site of drainage of aqueous humour?
Iridocorneal angle
What are the 4 major layers of the choroid?
Suprachoroidea-transition between sclera and choroid, elastic, pigmented
Large vessel layer-contains vascular plexus, cools the eye
Medium vessel layer-contains vessels and tapetum lucidum
Choriocapillaris-consists of fenestrated capillaries which supply the retina
Aqueous humour is produced by which 3 processes?
Diffusion (of solutes down conc. gradient into the aqueous) Ultrafiltration (due to differences in hydrostatic pressure in ciliary body capillaries, and intra-ocular pressure) Active secretion (of Na, which brings water across too)
What is the function of the lens?
To focus light on the retina
Which structures are visible in the fundus?
Optic disc, retina, retinal pigment epithelium (RPE), choroid and sometimes the sclera
Where is the tapetum lucidum located?
Medium vessel layer of the choroid
In which ways can drugs be administered to penetrate the globe?
Topically
Crossing the cornea, conjunctiva or sclera
Systemically by crossing the blood:ocular barrier
After topical administration of an ocular drug, what are the 3 ways it can be distributed?
Drained by lacrimal drainage system
Penetrates into the eye via cornea/sclera
Enters systemic circulation through conjunctival and nasal vessels
How much of a topically-applied drug actually reaches the anterior chamber?
Between 1 and 10%
How can intra ocular bioavailability of a drug be improved?
Increasing retention of the drug in the Palpebral fissure
By increasing its ability to penetrate the cornea
What are the layers of the cornea that a drug has to penetrate?
Epithelium (tight junctions) (can get through epithelium by moving between cells (paracellular, water-soluble) or moving through cells (transcellular, lipid-soluble))
Bowman’s layer
Stroma (facilities diffusion of water-soluble drugs but not lipid-soluble)
Descemet’s membrane
Endothelium (gap junctions, both lipid and water-soluble drugs can pass through)
After a drug has penetrated the cornea, where does it go?
Aqueous humour -> iris -> ciliary body
Is the ionised form of a drug water or lipid soluble?
Ionised form= water-soluble
Nonionised form= lipid-soluble
What does the blood:ocular barrier consist of?
BAB (blood:aqueous barrier) and BRB (blood:retinal barrier)
Why would we want to dilate the pupil?
To facilitate better fundic examination
How do we dilate the pupil?
Use either:
Sympathomimetic to contract the dilator muscle
Parasympatholytic to relax the Iris sphincter muscle eg Atropine
What kind of stimulation causes pupils to constrict and dilate?
Sympathetic -> pupils dilate
Parasympathetic -> pupils constrict
How can you investigate Horner’s syndrome?
Dogs with Horner’s syndrome present with constricted pupils.
Give a sympathomimetic dug (α1-selective adrenergic drug eg phenylephrine) to dilate the pupils.
Which drug can you use as a tear substitute?
Pilocarpine, increases tear production, parasympathomimetic
How do we treat glaucoma?
By increasing outflow of aqueous humour, reducing production or both
What types of drugs can we use to treat glaucoma?
Osmotic diuretics (draws water out of aqueous) Carbonic anhydrase inhibitors (produce bicarbonate ions which draw water out of the aqueous, eg Brinzolamide) Beta blockers (reduce aqueous formation, b1 antagonists eg betaxolol) Parasympathomimetics (constrict the pupil which widens the drainage angle and so increases aqueous outflow eg pilocarpine)
What is the function of:
Atropine
Pilocarpine
Atropine: dilates pupil
Pilocarpine: constricts pupil
What are the functions of rods and cones?
Rods-sensitive to low light levels but not colour. Black and white night vision.
Cones-sensitive to bright light/day vision. Provide colour vision.
Where does the optic nerve leave the eyeball?
Slightly ventral to the posterior pole
The axons of which cells form the optic nerve?
Ganglion cells
Which layer of the retina is supported by the choroid?
Nervous layer (receptor cells, rods and cones)
What is the retinal pigment epithelium attached to?
Choroid
Which layers of the eye contribute to the iris?
Choroid, retina, cornea
What fills the anterior and posterior chambers?
Aqueous humour
What prevents backflow of aqueous humour from the anterior to posterior chamber?
The valve-like action of the iris
Why does a corneal ulcer take a long time to heal?
Superficial ulcers heal rapidly because the outermost layer of the cornea is an epithelium (epithelial cells have a huge capacity for regeneration).
However, a deep ulcer heals slowly because the collagen fibres in the main body of the cornea don’t have their own blood supply (important for delivering WBCs to produce scar tissue).
Sometimes a conjunctival flap is required to provide a blood supply
