1030 - Module G2 : The Eye

Nictitating Membrane

'Third eyelid' . Located medially between eyelids and eyeball. Reinforced by cartilage. Moves across to protect eye.

Sclera

'White of the eye'. Caudal part of fibrous coat consisting of fibrous tissue. Gives shape and protects inner structures

Steps of Image Formation & Transmission

1. Light > corner/ant chamber > pupil > lens 2. Lens focuses light rays on retina (Accommodation) 3. Light > Retina. Stimulates photoreceptors 4. Photoreceptors generate action potentials > optic nerve > optic disc > retina 5. Impulses transmitted optic nerve > optic chiasma (generate binocular vision) > visual cortex (cerebrum) > Image formed

4 Tests to Assess Vision

1. Observe animal negotiating obstacles 2. Menace Response 3. Direct Pupillary Response 4. Indirect (consensual) Pupillary Response

Iris

@ front of the eye. Colour portion. Muscular diaphragm. Muscles to constrict and dilate pupil

Optic Disc

Ares on retina where axons from ganglion neurons leave the eye as optic nerve. Has no rods or cones. "Blind Spot"

Indirect (consensual) Pupillary Response

Asses vision in globe, transmission to brian, vision in brain, Cn III function to iris in eye NOT being tested. Shine light into one eye and observe the other Normal Response = constriction of pupil of eye that light has NOT been shone into.

Menace Response

Assesses vision in globe, transmission to brain, vision in brain & motor innervation to eyelids. Flick fingers towards one eye Normal response = blink both eyes

Direct Pupillary Response

Assesses vision in globe, transmission to brain, vision in brain, & Cn III function to iris in eye being tested. Shine light into 1 eye Normal response = constriction of pupil in eye being tested

Extraocular Eye Muscles

Attach to the sclera. Skeletal muscles.

Vitreous Chamber

Between the lens and retina

Facial Nerve Paralysis

Caused by damage to the facial nerve. Becomes problematic if the orbicularis oculi muscle is affected & animal can't shut its eye, and lacrimal gland losses its facial innervation leading to serious drying of eye

Interior of the Eye

Cavity of the eye, divided by the lens and iris into 3 chambers

Pupil

Central opening of the iris that lets light into the eye.

Uvea

Choroid, iris, ciliary body. Mostly smooth muscle & vessels. Supply nutrition to eyeball. Controls shape of lens and size of pupil.

Iris

Coloured part of the eye. Increases/decreases pupil size.

Levator Palpebrae Superioris Muscle

Does not attach to the eye, but to the upper eyelid. Elevates the upper eyelid & is innervated by Cn III

Movement of eye & structures controlled by which nerves

Eyelids : Cn VII / Facial Pupil : Cn III / Oculomotor Globe Movement : Cn IV / Trochlear & Cn VI / Abducens

Ciliary Processes

Folds on the inner surface of ciliary body that anchor zonular fibres.

Pupil

Gets smaller/larger as iris changes. Central opening in the middle of iris. More light = smaller pupil. Less light = larger pupil

Glaucoma

Group of eye diseases characterized by an increase in intra-ocular pressure. Causes physiological changes in the optic disc and typical defects in the field of vision.

Cherry Eye

Hypertrophy and prolapse of the gland of the 3rd eyelid. Common in young dogs

Pinkeye

Infectious bovine keratitis. Common condition in cattle leading to economic loss. Caused by moraxella bovis

Conjunctivitis

Inflammation of the conjunctiva

Keratitis

Inflammation of the cornea

Retina

Inner coat of eye, functioning in image formation. Contains 3 layers of neurons

Ganglion Neurons

Inner layer. Pass the impulse through their axons to the optic nerve

Retina

Innermost, lines the back of the eye. Contains rods and cones. Visual image is formed here, converted to a nerve impulse and sent to the brain. Nerve fibers converge on the optic disc.

Bipolar Neurons

Intermediate layer. Receives impulses from rods & cones, passes them to neuronal ganglia

Vitreous Body

Jelly-like substance. Fills vitreous chamber, maintains shape of eye and holds retina in place

Limbus

Junction between sclera and cornea

Photoreceptors

Light sensitive, 1st layer. Contains dendrites called cones (color/sharpness) and rods (night vision/shapes)

Palpebral Conjunctiva

Lines inner surface of eyelid

Cataract

Loss of lens transparency. Most common cause is aging

Vascular Tunic (Uvea)

Middle layer of the eyeball. consists mainly of blood vessels and smooth muscles that supply nutrition to the eyeball & control shape of lens & size of pupil.

Extrinsic Extraocular Eye Muscles

Move the eyeball and located behind it. 4 Rectus (straight) muscles: dorsal, ventral, medial, lateral 2 Oblique Muscles: Dorsal, ventral

Lacrimal Puncta

Openings into lacrimal canals on upper and lower eyelids near medial angles

Fibrous Tunic

Outer coat of eyeball. Divided into the sclera and cornea

3 Layers of Neurons in the Retina

Photoreceptors Bipolar Neurons Ganglion Neurons

Cones

Photoreceptors in the retina. Colour/sharpness

Rods

Photoreceptors in the retina. Night vision/shapes

Lateral/Medial Canthi

Point where eyelids meet.

Choroid

Posterior part of vascular tunic. Thin, dark, highly vascular membrane inside sclera. Supplies the retina & absorbs light not reflected out of the eyeball.

Upper/Lower Eyelids

Protect surface. Cilia = eyelashes

Bulbar Conjunctiva

Reflection of the palpebral conjunctiva onto the eyeball

Tapetum Lucidium

Reflective area of the choroid

Enucleation

Removal of the eyeball

The Inner Nervous Layer

Retina, Rods, Cones

The Outer Fibrous Layer

Sclera, cornea, limbus

Lens

Soft & transparent. Elastic & biconvex. Helps focus clear image.

Anterior Chamber

Space between cornea and iris

Posterior Chamber

Space between iris and lens

Orbicularis Oculi Muscle

Surrounds eye and functions to close it. Innervated by palpebral branch of facial nerve

Zonular Fibres

Suspensory ligaments attach lens to ciliary body, hold lens in position and allow shape of lens to change

Zonular Fibers

Suspensory ligaments that aid the lens moving.

Ectropion

The eversion of the margins of the eyelids

Entropion

The inversion of the margins of the eyelids

Layers of the Globes

The outer fibrous layer The middle vascular layer The inner nervous layer

Ciliary Body

Thickest portion of the vascular tunic between the choroid and iris. Consists of ciliary muscle and ciliary processes.

Conjunctiva

Thin mucous membrane lining. Transparent conjunctival sac.

Choroid

Thin, dark, very vascular. Inside sclera. Absorbs light not reflected out of the eyeball

Cornea

Transparent anterior part, lets light into the eyeball

Cornea

Transparent anterior portion that lets in light. Does not contain blood vessels.

Lens

Transparent, biconvex body of the eye. Many layers

Lacrimal Caruncle

Triangular prominence in medial angle

The Middle Vascular Layer

Uvea, choroid, tapetum lucidum, ciliary body, zonular fibers, iris, pupil

Aqueous Humor

Watery fluid that fills posterior and anterior chambers. Secreted by ciliary processes into post chamber, flows through the pupil into ant chamber and drains @ periphery into scleral venous plexus which leads to veins of the eye. Maintains intraocular pressure & acts as a nutrient and waste transport medium

Sclera

White of the eye. Gives shape and protects inner structures

Intrinsic Extraocular Eye Muscles

Work with the size of pupil and shape of lens. Pupillary sphincter, pupillary dilator, ciliary muscles

Ciliary Body

thickest portion of the middle layer. Has ciliary muscle and ciliary processes