Block 2

Orbit

Eye socket, Lined w/ fascia
Complete bony rim in herbivores; Incomplete bony rim in carnivores

Globe

Eyeball

Adnexa

Everything except globe

l◦ Orbit/orbital contents

◦ Extraocular muscles

◦ Eyelids

◦ Conjunctiva

◦ Third eyelid/nictitating membrane

◦ Lacrimal system

Anterior Segment of Eye

◦ Cornea

◦ Anterior chamber (space between the cornea and iris/lens)

◦ Iris

◦ Iridocorneal angle

◦ Ciliary body

◦ Lens

Posterior Segment

◦ Vitreous humor

◦ Retina

◦ Choroid

◦ Optic Nerve

◦ Posterior sclera

Orbital Contents

◦ Conjunctiva

◦ Nictitans

◦ Fat

◦ Extraocular muscles

◦ Muscles of mastication

◦ Blood vessels

◦ Glands, Zygomatic salivary gland in floor of orbit

◦ Cranial nerves 2-8

◦ Autonomic nerves

Extraocular Muscles

◦ Move globe in orbit

◦ Muscle cone anchored in back of orbit

◦ Rectus and oblique muscles, retractor bulbi

Rectus muscles

Extraocular (rotate up, down, left, right)

◦ Dorsal

◦ Ventral

◦ Lateral

◦ Medial

Oblique muscles

Extraocular, rotate globe

◦ Dorsal/superior

◦ Ventral/interior

Retractor bulbi

pulls globe back in orbit

What do the eyelids include?

◦Skin - epidermis and hair follicles

◦Mucocutaneous junction - where skin meets mucus membrane

◦Palpebral (relating to/near the eye) Conjunctiva - mucus membrane, lines inside of eyelid

◦Meibomian Glands - sebaceous

◦Muscles - open/close eyelid

Meibomian Glands

◦ Row of sebaceous glands that lie along the eyelid margin

◦ Ducts open at the mucocutaneous junction

◦ Secrete the lipid layer of the tear film

Tarsus

Tissue (collagen) sheet that encompasses the structures of the eyelid margin

Gives eyelid margin rigidity

surface for muscle attachment

Levator palpebrae superioris and associated nerve

Elevates upper eyelid

CN 3

Orbicularis oculi and associated nerve

Surrounds margins and closes upper and lower eyelids

CN 7

What nerves innervate the eyelids?

◦ Cranial nerve 5 – sensory

◦ Cranial nerves 3 and 7 – motor

◦ Autonomic – Sympathetic nerve

Mueller’s Muscle

Smooth muscle that provides “tone” to the tarsus

Innervated by Sympathetic n.

Medial & Lateral Canthal Ligaments

Ligamentous bands

Anchor medial and lateral canthus (where eyelids meet) to periosteum of the orbital rim

Conjunctiva

◦ Allows smooth gliding of nictitans & eyelids over globe

◦ Mucus layer of tear film produced by goblet cells

◦ Immune protection for ocular surface

◦ Lymphoid tissue, immunoglobulins

◦ Corneal repair sx (mobile blood supply)

Fornix

cul-de-sac formed by reflections of conjunctiva at transition from lid to globe

Nictitating Membrane

Nictitans cover palpebral and bulbar surfaces of the nictitating membrane

◦Triangular piece of tissue in the medial fornix/canthus

◦ Moves dorsolaterally as globe retracted

◦ Spreads tear film

◦ Protects globe

◦ Removes particulate matter from the surface

Where are goblet cells located what product do they produce?

between epithelial cells, secrete mucus

What makes up the nictitating membrane?

◦ Stroma=fibrous CT

◦ T-shaped cartilage- structural support

◦ Gland of the nictitans

◦ Covered in goblet cell rich conjunctiva

Lacrimal Outflow Apparatus

Produce, distribute, and drain the pre-corneal tear film to maintain health of the ocular surface

Pre-corneal tear film (PTF)

Fluid layer which covers and maintains the health of the cornea and conjunctiva

What are the components of the aqueous tear film and what glands produce each component?

◦ Lipid layer: outer/superficial, produced by Meibomian gland

◦ Aqueous layer: middle/thickest, produced by lacrimal glands

◦ Mucin layer: innermost, produced by goblet cells

Lacrimal Glands

◦ Orbital lacrimal gland, in dorsolateral orbit

◦ Gland of the nictitans, at base of nictitans/vertical cartilage

Lacrimal Outflow Apparatus

Drains the tears from the eyelids to the nose

◦ Ocular puncta in eyelids/conjunctiva

◦ Canaliculi in eyelids/conjunctiva

◦ Lacrimal sac in medial canthus

◦ Nasolacrimal duct (NLD)

◦ Nasal puncta

Palpebral

Conjunctiva that lines eyelids

Bulbar

Conjunctive that lines exposed surfaces of the globe

What type of epithelium covers the cornea and conjunctiva?

Nonkeratinized stratified squamous

List the parts of the anterior segment

Cornea

Anterior sclera

Anterior chamber (between cornea and lens/iris)

Posterior chamber (between the iris and lens)

Iris

Lens

Iridocorneal angle

Ciliary body

List parts of the posterior segment

Vitreous humor

Retina

Choroid

Optic nerve

Posterior sclera

List the 3 tunics of the globe and their components

Fibrous - cornea, sclera

vascular - iris, cilliary body, choroid

nervous - retina, optic nerve

Name the functions of the cornea

Transmit light

Refract light

Protect internal contents

Name the 4 layers of the cornea and identify their location and function

Epithelium – stabilize tear film, provide protection; most of the cornea

Stroma – mostly water, no blood vessels, normal arrangement of collagen fibrils, hydrophilic, maintain deturgescence (dehydration) to remain clear

Descemet’s membrane – basement membrane

Endothelium – inner layer of cornea, pump fluid out of cornea, barrier to stroma

How fast can the corneal epithelium turnover/heal a defect?

7 days

Describe the Corneal scleral junction

Limbus, where cornea and sclera meet

Non-pigmented straight corneal epithelium transitions to pigmented and undulating bulbar conjunctiva with substantia propria

How does the collagen and epithelium differ between the sclera and cornea.

Sclera – irregularly arranged densely packed collagen fibers, opaque

Describe the 3 parts of the vascular tunic (uvea)

Iris, regulates amount of light to posterior portions, blood aqueous barrier

Ciliary body, accommodation (fine focusing) of the lens

Choroid, Provides vascular support of the retina

Name the muscles that constrict and dilate the pupil.

Sphincter and dilator

What species have a highly developed ciliary body?

Primates, raptors

Species with highly developed retina

describe outflow pathway of aqueous humor

produced by nonpigmented epithelium (NPE) in posterior chamber

flows through pupil

drains out through iridocorneal angle (ICA)

Describe the components of the iridocorneal angle and its function.

Junction of iris base to inside of cornea

Site of drainage of aqueous humor through trabecular meshwork

Describe the ciliary body and its parts

Pars plicata – ciliary processes

Pars plana – flat portion where retina inserts

Smooth muscle – 3 directions, longitudinal, circumferential, and radial

Accommodation

Zonules of lens insert between ciliary processes

Vascular and connective tissue core covered by PE and NPE

Label the main anatomic parts of the lens

Lens capsule

Anterior lens epithelial cells

Equatorial lens epithelial cells

Equatorial fibers

Anterior/posterior cortex

Nucleus

Describe the function of the lens

Fine tune focus and send light to retina

Describe where and how lens fibers are formed

Epithelial cells at equator divide and form lens fibers

Lens fiber elongate – moved internally by subsequent lens fiber

Central lens fibers become compressed to form fetal and adult nucleus

Where do lens fibers meet in the lens?

Anterior suture – meet at axial lens anteriorly; right side up Y

Posterior suture - meet at axial lens posteriorly; upside down Y

Lens capsule anatomy

Basement membrane

Thick anterior capsule (produces basement membrane)

Equator = transition

Posterior capsule remains thin

Describe why nuclear sclerosis occurs.

Lens continues to grow and add fibers throughout life

Increased central density leads to NS

Describe the composition of the vitreous humor and its general function.

Semifluid, mostly water

Optically transparent

Collagen fiber matrix

Support function to retina and lens

Describe the function of the choroid

Outer portion of the retina is supplied by choroid in dog, cat, and cow

All of the retina in horse, rabbit, guinea pig, etc.

Describe the location and function of the tapetum

Part of choroid in dorsal retina

Triangular shape

Reflective to increase vision in dim light

Most have EXCEPT birds, pigs, camelids, rabbits

Describe the general functions of the retinal photoreceptors and ganglion cells

Photoreceptors convert light into signals

Ganglion cells transmit visual information from retina in action potentials to different regions of the brain

Rods

night vision and motion

Cones

day vision, color, acuity

Retinal pigmented Epithelium (RPE)

Outermost layer of retina, over choroid

Non-pigmented over the tapetum

Describe the composition of the optic nerve and its general function.

Axons and retinal ganglion cells

Axon exits eye at optic disc

Transmits image to brain

Lamina cribosa

collagen supporting axons in cranial nerve

Describe the different vascular patterns and optic nerve shapes

Dog and cat – holangiotic, entire retina vascularized

• Cat – yellow tapetum, unmyelinated optic disk (y shape vasculature)

• Dog – variation in everything, but vascular anastomosis

Rabbit – merangiotic, horizontal vasculature (3 and 9 meridians)

Horse – paurangiotic, poor vascularized retina

Bird – anangiotic, nonvascularized retina

CN 3

Oculomotor nerve

Parasympathetic fibers to pupil

Innervates levator palpebrae superioris

Provides motor fibers to all EOM except (LR₆SO₄)³

• Lateral rectus

• Retractor bulbi

• Superior/dorsal Oblique

CN 5

Trigeminal nerve

Ophthalmic branch

• Sensory fibers

• Afferent arm of oculocardiac reflex

Maxillary branch

• Sensory to skid of lower eyelids

Mandibular branch

• Motor to muscles of mastication

CN 7

Motor to muscles of facial expressions

Closure of eyelids (orbicularis oculi)

Carries Parasympathetic fibers to lacrimal gland (Efferent arm)

CN 8

Vestibular nerve

Afferent component of ocular position

Fixed on object when head turns or eye track smoothly when head moves

CN 10

Vagus nerve

Efferent arm of oculocardiac reflex

What are the afferent and efferent arms of the cranial nerves and what are their functions?

Afferent

CN5 - arm of oculocardiac reflex

CN 8 – component of ocular position

Efferent

CN 7 – arm of lacrimal gland

CN 10- arm of oculocardiac reflex

What is the oculocardiac reflex?

Intraoperative handling of the eye results in bradycardia

Explain the sympathetic nervous system related to the eye

dilator muscle of pupil

Mueller’s muscle, elevate superior eyelid

Smooth muscle, globe position forward and blood vessels (vasoconstriction)

Explain the parasympathetic nervous system related to the eye

pupillary fibers – pupil constriction

lacrimal gland fibers – efferent arm

Palpebral/blink reflex

tap eyelids, medial and lateral

Corneal reflex

touch cotton swab to cornea

Pupillary light reflex

Stimulate an eye and examine both eyes for response

Direct PLR, stimulated pupil constricts

Consensual PLR, contralateral pupil constricts

Dazzle

Indicates light perception, not vision

What tests are involved for assessing vision?

Menace response, cottonball, maze, visual placing

Optical Vision Pathway

Light, cornea, lens, retina

Photochemical vision pathway

Photoreceptors, bipolar cells, retinal ganglion cells

Neurologic transmission/processing vision pathway

Axons, optic nerve, optic chiasm, optic tract

Lateral geniculate nucleus, optic radiations, visual cortex

What pathways is the optic nerve involved in?

Vision pathway and pupillary light reflex (PLR) pathway

Explain optic nerve decussation

Axon crossing of ON at optic chiasm

Greater decussation in prey species with greater monocular/lateral vision

Explain the visual fields

Medial (binocular) visual field = lateral retina & ON fibers

Lateral (monocular) visual field = medial retina & ON fibers

Afferent and efferent components of the pupillary light reflex (PLR) pathway.

Afferent - retina and ON to chiasm to optic tract, leaves before LGN, pretectal nucleus and central decussation

Efferent - Parasympathetic fibers of EWN travel w/ CN 3 to iris sphincter

Where do the pupillary motor fibers of the optic nerve leave the vision pathway?

Before the lateral geniculate nucleus (LGN)

Components of a general eye examination

o   Ocular size and position

o   Ocular clarity

o   Pupil size

Describe the formation and functions of CSF

Produced by choroid plexus epithelial cells at constant rate

Shock absorber to protect brain and spinal cord

Transports nutrients, hormones, and metabolites to CNS

Pleocytosis

Increased numbers of cells in CSF

What types of diseases are more likely to have abnormal CSF?

Acute diseases like meningitis rather than chronic like atrophy

List indications for CSF analysis

Abnormal neuro exam

Neck or limb pain

Fever of undetermined origin

Diagnosis of infectious disease

contraindications for CSF analysis

rabies suspect

anesthesia risk

post trauma or high intracranial pressure

Describe proper collection and handling of CSF samples

Sterile technique and spinal needle w/stylet

Process within 30min or store 4C for up to 24hr

Describe the characteristics of normal CSF and changes that occur with disease

Colorless and clear

low protein concentration in mg/dL, most is albumin

most common abnormality, sensitive but nonspecific

describe cells in CSF

counted w hemocytometer

no RBC

80-90% large mononuclear cells and lymphocytes

components of the cortex in the pyramidal motor system

Corona radiata

Internal capsule

Crus cerebri - midbrain

Longitudinal and transverse fibers of pons -met

Pyramids - myel

components of the tracts in the pyramidal motor system

Cortico nuclear

Cortico spinal (lateral and ventral)

corticopontocerebellar

components of the cortex in the extrapyramidal motor system

Tel, corpus striatum

Di, many

Mes, red nucleus

Met, pontine reticular formation

Myel, medullary reticular formation

components of the tracts in the extrapyramidal motor system

Rubrospinal

Pontine reticulospinal

Medullary reticulospinal

Olivocerebellar

Describe classic signs that develop with loss of normal UMN function

Hyperreflexia

Hypertonus

Spastic paralysis

Slow atrophy

Describe GSE and CN

Targets skeletal muscle

Located in spinal cord, ventral horn, alpha and gamma

CN: 3, 4, 6, 12

Describe SVE and CN

Targets branchial arch

CN: 5, 7, 9, 10, 11

Describe GVE and nerves

Two neuron system w synapse at ganglion

Targets cardiac, smooth muscle, and glands

CN: 3, 7, 9, 10