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In this section, students will be introduced to those anatomical and physiological concepts that are important to the study of ocular disease. In general, comments on pathophysiology will be covered in lectures dealing with each ocular structure. It will be very important to review the following notes prior to those lectures.

# Orbit

  • Cavity that contains eye; cone shaped, with apex posteriorly

# Osteology

  • Horse, cow: complete bony orbit

  • Dog, cat: bony orbit is incomplete dorsolaterally; orbital ligament spans from zygomatic process of frontal bone to frontal process of zygomatic arch

  • Optic foramen: optic nerve, internal ophthalmic artery

  • Orbital fissure (orbitorotundum in cow): CNs 3,4, &6, ophthalmic br. of CN 5, superior ophthalmic vein.

# Vasculature

  • External ethmoidal artery and external ophthalmic artery (branch of maxillary artery) Superior and inferior ophthalmic veins → orbital plexus → cavernous sinus Angular vein medially

# Innervation

Sensory
CN 2 and ophthalmic branch of CN 5

Motor
To extraocular muscles and levator palpebrae

  • CN 3: dorsal rectus, medial rectus, ventral rectus, ventral oblique, levator palpebrae

  • CN 4: superior oblique

  • CN 6: lateral rectus, retractor bulbi

Sympathetic
Ill-defined orbital musculature (sometimes termed “muscularis orbitalis”), responsible for maintaining orbital tone.

# Extraocular muscles

  • Originate around orbital fissure; rectus muscles insert approx. 5-7 mm posterior to the limbus, retractor bulbi inserts just behind equator of globe.

  • Dorsal, medial, ventral and lateral recti turn globe in their respective directions.

  • Retractor bulbi pulls globe posteriorly

  • Dorsal oblique causes intorsion (i.e., 12 o’clock position of globe moves medially) via the trochlea, ventral oblique causes extorsion (i.e., 12 o’clock position moves laterally)

# Periorbital fat

  • Occupies dead space in and around the muscle cone.

# Periorbita

  • Periosteum lining the orbital cavity.

# Tenon’s capsule

  • Reflection of periorbita over muscles and globe.

# Retrobulbar area

  • Term used to describe area behind posterior to the globe.

# Globe

Three approximately concentric spheres

  1. Fibrous tunic - cornea anteriorly, sclera posteriorly

  2. Vascular tunic - iris anteriorly, ciliary body in middle, choroid posteriorly

  3. Neural tunic - retina

Anterior segment vs. posterior segment
Anterior segment defined as lens and all structures anterior to it; posterior segment is defined as all structures posterior to the lens.

Fluid chambers of the eye

  • Anterior chamber - between cornea and iris; contains aqueous humor

  • Posterior chamber - between ciliary body and iris; also contains aqueous humor

  • Vitreous chamber - behind lens; contains vitreous humor and constitutes 80% of eye volume.

# Adnexa

Overview * Defined as orbit and contents, eyelids, lacrimal/nasolacrimal structures, conjunctiva, nictitans; the following discussion covers all of the above except orbit (covered previously).

# Eyelids

# Gross anatomy

  • Palpebral fissure - opening between upper and lower lids.

  • Medial and lateral canthi - medial and lateral corners of palpebral fissure.

# Histologic anatomy

  • Muscles

    • Levator - raises lid; innervated by CN 3; acts in conjunction with dorsal rectus.

    • Muller’s muscle - sympathetically controlled muscle that provides some tone to upper lid and helps raise upper lid during flight or fight response.

    • Orbicularis oculi - closes palpebral fissure; innervated by CN 7.

    • Lateral angular retractor - also called lateral palpebral (canthal) ligament; maintains almond shape to palpebral fissure; important landmark during enucleation.

    • Medial palpebral (or canthal) ligament - Also helps maintain almond shape to fissure.

  • Glands

    • Tarsal (aka Meibomian) - Row of glands just under the conjunctiva approx. 4 mm posterior to the lid margin. Duct goes from gland to pore on eyelid margin. Phospholipid secretion that contributes to the tear film.

    • Zeis - open into ciliary follicles; sebaceous secretion.

    • Moll - open onto margin near cilia; modified sweat gland.

  • Tarsal plate (also called “tarsus”): Fibrous tissue that gives lid form and support; relatively poorly developed in domestic animals.

  • Nasolacrimal puncta: Upper and lower openings in the conjunctiva near the medial canthus; function to drain tears away from the eye.

  • Cilia: “Eyelashes” - located on anterior surface of upper lid near the margin in most species; cats lack cilia.

# Physiology

  • Upper lid more mobile, closes from lateral to medial to spread tears and direct them into the nasolacrimal puncta.

  • Limits entry of light into the eye.

  • Protection of globe via palpebral reflex and menace response.

# Lacrimal/Nasolacrimal system

# Pre-corneal tear film - 3 layers

  1. Lipid layer: Most anterior of the 3 layers of the tear film Prevents evaporation of aqueous portion of tear film. Produced by the tarsal (Meibomian) glands

  2. Aqueous layer: Middle layer, comprises majority of tear film. Produced by main lacrimal gland (50%) and nictitating membrane gland (50%)

  3. Mucoid layer: Inner layer, acts as surfactant to help spread tear film across cornea. Produced by conjunctival goblet cells.

# Tear production

  • Main lacrimal gland - on dorsolateral surface of globe.

  • Gland of the nictitans - on ventroposterior aspect of nictitans.

# Tear drainage

  • Upper and lower nasolacrimal puncta → upper and lower canaliculi → both canaliculi join at the nasolacrimal sac → nasolacrimal duct → nasal cavity.

# Functions of tears

  • Corneal nourishment

  • Medium for sweeping debris from corneal surface

  • Lubricate corneal surface to facilitate movement of lids and nictitans Refractive surface

# Conjunctiva

# Overview/ Gross anatomy

  • Mucous covering the inner of the lids and the anterior portion of the globe.

  • Bulbar conjunctiva - portion covering globe.

  • Palpebral conjunctiva - portion covering inner surface of the lids

  • Fornix (or cul-de-sac) - portion where bulbar and palpebral portions come together.

# Histologic anatomy

  • Stratified squamous epithelium with microvilli, coated with mucopolysaccharides to help stabilize tear film.

  • Goblet cells - located primarily in palpebral conjunctiva and fornix, secrete mucin layer of tear film. Conjunctiva associated lymphoid tissue (CALT)

# Functions of conjunctiva

  • Prevents corneal desiccation via secretions of goblet cells and blinking.

  • Prevents microbe invasion via CALT and Ab secretion (the conjunctiva of most domestic species normally contains high numbers of potential pathogens).

# Nictitating Membrane (aka third eyelid)

# Overview/ Gross anatomy

  • Located in ventromedial conjunctival sac.

  • Sweeps across cornea in a dorsolateral direction; movement is passive, occurring when the animal retracts the globe into to orbital cavity.

  • Avian - located dorsomedially; movement is active and in a ventrolateral direction

# Histologic anatomy

  • T-shaped cartilage gives support to NM.

  • NM gland on ventroposterior aspect of nictitans, surrounds shaft of T-cartilage.

  • Anterior and posterior surfaces (also called palpebral and bulbar surfaces, respectively) of NM covered with conjunctiva.

  • Numerous lymphoid follicles located on bulbar surface.

  • Muscle not found in NM of most species; cat has some smooth muscle, the function of which is not clear.

# Functions of NM

  • Secretion of tears.

  • Protection of globe.

# Cornea and Sclera

# Cornea

# Overview/ Gross anatomy

  • Comprises anterior 1/5 of fibrous tunic

  • Approximately 0.5-0.6 mm thick

  • Joins sclera in transitional zone called “limbus”

# Histologic anatomy

# Epithelium

  • Anterior most layer of cornea

  • Constantly replenishing tissue (turnover approx. 7 days)

    Divided into 3 layers

    1. Basal cells: Caudal layer; single cell thickness; attached to basement membrane via hemidesmosomes. Only epithelial layer to undergo mitosis

    2. Wing cells: Middle transitional layer between basal cells and squamous cells. 2-3 cells thick in dog

    3. Squamous cells: Anterior layer, 2-3 cells thick in dog.

  • Microplicae and microvilli on surface help stabilize tear film

  • Tight junctions between squamous cells limit permeability Nonkeratinized

# Bowman’s layer

  • Condensation of anterior stroma just beneath basal epithelium

  • Present in primates

  • Absent in most domestic species (equivocal in cows)

# Stroma

  • 90% of corneal thickness

  • Collagen and GAGs; low cellularity

  • Collagen in lamellae that span corneal diameter

  • Keratocytes - cells that are responsible for maintenance of collagen, can transform into fibroblasts to form scar tissue

# Descemet’s membrane

  • Basement membrane of endothelium

# Endothelium

  • Single layer of hexagonal cells Posterior-most layer

  • Important in maintaining state of relative dehydration of cornea

# Physiology

# Corneal transparency

  • Devoid of blood vessels (except at limbus)

  • Nonpigmented (except at limbus)

  • Nonkeratinized surface epithelium

  • Size and organization of stromal collagen fibrils (bundles spaced at regular intervals)

  • Relative state of dehydration (NaKATPase pump in endothelium pumps water out of stroma into anterior chamber).

# Sclera

# Overview/ Gross anatomy Posterior 4/5 of fibrous tunic

  • Composition similar to cornea, but not transparent

# Histologic anatomy

# Intrascleral venous plexus

  • Anterior stromal veins

  • Collect aqueous from iridocorneal angle, transfers to systemic circulation Circumferentially oriented in dogs, cats; 4-5 mm posterior to limbus Radially oriented in horses

# Lamina cribrosa

  • Circular area of sclera at posterior pole

  • Contains small perforations through which optic nerve axon bundles exit eye.

# Uveal Tract

# Overview

  • Anterior uvea: iris and ciliary body

  • Posterior uvea: choroid

  • Very vascular, darkly pigmented

# Iris

# Gross anatomy

  • Ciliary zone - peripheral half of iris

  • Pupillary zone - central half

  • Collarette - line of demarcation between pupillary and ciliary zones

  • Iris rests against anterior surface of lens

# Histologic anatomy - divided into 4 layers:

  1. Anterior border layer

  2. Stroma: Loose collagenous fibers. Contains pupillary sphincter muscle.

  3. Dilator muscle layer: Myoepithelial; not as strong as sphincter muscle

  4. Posterior epithelium: Sometimes extends axially into pupil as “pupillary ruff”

# Pupil

  • Hole in center of iris

  • Round in dog, vertical slit in cat, horizontal oval in horse, cow

  • Corpora nigrum (or granula iridica): spherical protuberances of anterior and posterior epithelial layers that extend into the pupil; present in ungulates.

# Vascular supply

  • Long posterior ciliary arteries: Enter iris at 3 and 9 o’clock positions, each then divides into dorsal and ventral branches

  • Major arterial circle: Formed by dorsal and ventral branches of l.p.c.a., located in peripheral iris

  • Radial arteries: From major arterial circle toward pupil

# Innervation

  • Sensory: Long posterior ciliary nerves (ultimately from ophthalmic br. of CN5)

  • Pupillary sphincter: Parasympathetically innervated

  • Dilator: Sympathetically innervated

# Physiology

  • Interendothelial tight junctions of iridal vasculature prevent macromolecules from entering aqueous humor (therefore contributes to “blood aqueous barrier”)

  • Iris muscles controlled by autonomic nervous system

  • Pupil size controls amount of light entering eye

# Ciliary body

# Gross anatomy

  • Circumferential structure (i.e., doughnut shaped) posterior to iris Pars plana - posterior half of doughnut; flat

  • Pars plicata - anterior half of doughnut; thrown into 75-100 folds called ciliary processes

  • Zonules extend from processes and valleys between them to lens equator

# Histologic anatomy - divided into 3 layers:

  • Stroma: Adjoins sclera

    • Three muscles: longitudinal, radial and circumferential; longitudinal inserts anteriorly on trabecular meshwork of iridocorneal angle

  • Pigmented epithelium: Overlays stroma. Continuous with dilator muscle layer of iris

  • Nonpigmented epithelium Overlays pigmented epithelium: Continuous with posterior epithelium of iris Secretes aqueous humor

# Vascular supply

  • Long posterior ciliary arteries: Branches sent posteriorly to ciliary body just before entering iris

  • Anterior ciliary arteries: Travel anteriorly with rectus muscles, pierce sclera just posterior to limbus

# Innervation

  • Sensory: Long posterior ciliary nerves

  • Ciliary muscles: Parasympathetic innervation via short posterior ciliary nerves

  • Ciliary epithelium: Contains sympathetic innervation, the function of which is unknown

# Physiology

  • Accommodation: Thickening of lens in anterior-posterior dimension for focusing on near objects Accomplished via contraction of longitudinal ciliary muscle

  • Trabecular meshwork: Iridocorneal angle structure through which aqueous drains. Contains small pores which enlarge when longitudinal ciliary muscle contracts

  • Production of aqueous humor:

    • Diffusion

    • Active transport

      • Most important process in aqueous production

      • NaKATPase located between NPE cells; Na+ pumped into posterior chamber, water follows

      • Carbonic anhydrase - enzyme in NPE; necessary for aqueous production

    • Ultrafiltration

      • Movement of fluid in accordance with hydrostatic forces

  • Blood-aqueous barrier

    • Interendothelial junctions of iris vasculature Interepithelial junctions of NPE

    • Restricts entry of macromolecules (proteins, especially fibrinogenic proteins) into aqueous humor

# Iridocorneal angle

# Overview

  • Area where iris meets cornea Aqueous humor drainage

# Structures comprising the iridocorneal angle

  • Pectinate ligaments

    • Collagenous beams spanning from peripheral anterior iris to corneal endothelium Large pores

  • Trabecular meshwork

    • Smaller pores, offers some resistance to aqueous outflow

  • Ciliary cleft

    • Depression in anterior ciliary body where trabecular meshwork is located

  • Intrascleral venous plexus

    • Shunts aqueous from angle to systemic circulation

# Aqueous Humor Dynamics

# Why have aqueous?

  • Nourishment of avascular anterior segment structures

# Aqueous flow

  • NPE of ciliary body → posterior chamber → pupil → anterior chamber → between pectinate ligaments → through trabecular meshwork → into intrascleral venous plexus → into systemic circulation

# Aqueous production

  • See “Ciliary Body - Physiology”

# Aqueous drainage

  • Facility of outflow

    • Ease with which aqueous humor escapes

  • Conventional outflow

    • Outflow via trabecular meshwork and intrascleral venous plexus described above

  • Unconventional (aka uveoscleral) outflow

    • Some fluid will bypass meshwork to be absorbed directly into anterior ciliary body → supraciliary space → systemic circulation. Minor pathway in dog, cat, major pathway in horse

# Establishment and maintenance of intraocular pressure

  • Outflow resistance

    • Primarily at trabecular meshwork and intrascleral venous plexus

  • Maintaining “normal” IOP

    • Balance between production and drainage of aqueous humor

# Choroid

# Gross anatomy

  • Posterior 2/3 of uvea

  • Anterior choroidal margin meets posterior aspect of ciliary body stroma Very vascular, darkly pigmented

# Histologic anatomy - divided into 3 layers:

  • Suprachoroidea

    • Potential space between choroidal stroma and sclera Long posterior ciliary arteries and nerves reside here Site of unconventional aqueous humor outflow

  • Stroma

    • Large vessel layer (posterior)

    • Intermediate vessel layer (anterior)

      • Tapetum: Highly reflective layer to aid nocturnal vision. Triangular on frontal view, located dorsally.

  • Choriocapillaris

    • Single layer of capillaries between anterior stroma and retinal pigmented epithelium

# Physiology

  • Choriocapillaris supplies nutrition to outer half of retina

# Lens

# Overview

  • Accommodation in near vision (relatively poorly developed in domestic species)

# Gross anatomy

  • Anterior and posterior poles

  • Equator

  • Ciliary zonules (suspensory ligaments) from equator to ciliary body

# Histologic anatomy

# Capsules

  • Anterior and posterior; basement membrane of epithelium

  • Posterior epithelium only present in utero, so posterior capsule very thin; anterior capsule thickens throughout life

# Epithelium

  • Postnatally only present anteriorly and at equator Equatorial epithelial cells elongate to form lens fibers

# Cortex Outermost fibers

  • Y-sutures; visible lines where lens fibers come together

# Nucleus Innermost fibers

  • Subdivided into embryonal, fetal and adult nuclei

# “Geographic anatomy”

  • Axial (aka polar) vs. equatorial

  • Terms used to describe location of lens opacities

# Physiology

# Protein content

  • Highest concentration of protein in body (approx. 35%)

# Na+ pump

  • Found in lens fiber membranes and epithelium Maintains low intralenticular water content

# Glucose metabolism - glucose readily enters lens

  • Glycolysis

    • Responsible for 80% of glucose metabolism

    • Initial enzyme: hexokinase; saturated at glucose concentration of 200 mg/dl

  • TCA cycle

    • 5%

  • Pentose monophosphate shunt

    • 15%

  • Sorbitol pathway

    • Usually negligible, utilization increases when hexokinase saturated

    • End product is fructose, which can’t escape lens

# Glutathione and sulfhydryl proteins

  • Maintained in reduced form - prevents abnormal cross linking of proteins and subsequent opacification

# Production of lens fibers

  • Equatorial epithelial cells

  • Fibers meet in Y-sutures

# Accommodation

  • Contraction of ciliary body muscles → loosening of ciliary zonules → lens assumes more spherical shape (i.e., thickens in anterior-posterior dimension) → brings near objects into focus.

# Maintenance of transparency

  • High concentration of crystalline proteins

  • Few intracellular organelles

  • Very little interfibrillar water

  • Fibers densely packed

  • Avascular

  • Nonpigmented

# Vitreous

# Overview/ Gross anatomy

  • Occupies 75-80% of volume of eye

  • Transparent, avascular, low cellularity Gel-like consistency

# Histologic anatomy

  • Cloquet’s canal: Remnant following regression of hyaloid artery (H. artery regresses within a few weeks of birth in most species)

  • Primary vitreous: Immediately surrounding Cloquet’s canal Secondary vitreous: Majority of vitreous body

  • Tertiary vitreous: Synonymous with ciliary zonules

# Physiology

  • 98% water suspended in hyaluronic acid and collagen matrix

  • Often liquefies in response to posterior segment disease

# Retina

# Overview

  • Purpose: Transforms electromagnetic radiation (i.e., visible light) into an electrochemical signal that can be interpreted by the brain

# Gross anatomy

Ora Ciliaris Retinae (ora serrata in primates)
Junction of anterior retina and posterior ciliary body

Neurosensory retina

  • Innermost 90% of retina

  • Peripherally, continues anteriorly as nonpigmented ciliary body epithelium

Retinal pigmented epithelium

  • Peripherally, continues anteriorly as pigmented ciliary body epithelium

Area centralis
Central visual area

# Histologic anatomy

# Classically divided into 10 layers (see Fig 15-1, p. 415 in Severin ).

  1. Retinal pigmented epithelium

  2. Photoreceptors

  3. External limiting membrane

  4. Outer nuclear layer

  5. Outer plexiform layer

  6. Inner nuclear layer (includes bipolar cells)

  7. Inner plexiform layer

  8. Ganglion cell layer

  9. Nerve fiber layer

  10. Internal limiting membrane

# Three neuron chain in neurosensory retina transduces photic signal into electrochemical signal and sends to brain

  1. Photoreceptors

    • Rods - very sensitive; dim light vision

    • Cones - bright light vision, color vision, most acute vision

    • Rod outer segment: Thousands of membranous discs stacked like pancakes; rhodopsin embedded in membranes

    • Rods out number cones 100:1 in peripheral retina, 10:1 in area centralis

  2. Bipolar cell

    • Transfers potential generated in photoreceptor to ganglion cell

  3. Ganglion cell

    • Axons of ganglion cells make up optic nerve

    • Transfers potential from bipolar cells to midbrain (for pupillary light responses) and visual cortex

# RPE

  • Single cell layer between neurosensory retina and choriocapillaris

  • Important supporting role for photoreceptors

  • Nonpigmented dorsally to expose tapetum

# Müeller cell

  • Retinal “support” cell that spans from RPE to internal limiting membrane

# Vascular supply

Inner half of retina
Retinal vessels located in nerve fiber layer

Outer half of retina
Choriocapillaris

Short posterior ciliary arteries
9-12 arteries that pierce the sclera around the optic nerve to become retinal vessels

# Various vascular patterns seen in domestic animals:

  • Holangiotic - all quadrants, from peripapillary to periphery, supplied with retinal vessels (dog, cat)

  • Merangiotic - only selected sectors of retina supplied with vessels (rabbit)

  • Paurangiotic - vessels in all quadrants, but only in vicinity of optic disc (horse)

  • Anangiotic - no retinal vessels (avian)

# Physiology of neurosensory retina (will use rod as example)

Similarity to other sensory systems
Receptor cell, 3 neuron chain to brain

Electrical state of PR cell in dark

  • Relatively depolarized (approximately - 30 mV, vs. - 70 mV found in most excitable cells) due to outer segment permeability to Na+

  • Inner segment NaKATPase pump removes Na+ from cell

  • Inhibitory neurotransmitter between photoreceptor and bipolar cell continuously released in dark

# Transduction

  • Rhodopsin

    • Photopigment composed of the protein opsin and 11-cis retinal

    • Photon causes configurational change that results in closure of outer segment Na+

    • channels and, therefore, hyperpolarization

  • Hyperpolarization

    • Causes cessation of release of inhibitory neurotransmitter Bipolar cell therefore “switched on”

# PR-bipolar interaction

  • Removal of inhibitory neurotransmitter creates typical depolarizing potential in bipolar cell

# Bipolar-ganglion cell interaction

  • Typical depolarization

# Optic Nerve

  • Collection of axons of ganglion cells

  • Lamina cribrosa - Scleral structure through which axons exit the globe

  • Optic disc (aka papilla)

# Location in fundus

  • Just ventral to geometric axis in most species

  • Dog: may be in tapetal area, in non-tapetal area, or at tapetal/non-tapetal junction, depending on size of tapetum

# Myelination

  • Dog: myelination of ganglion cell axons extends for 1-2 mm into retina (i.e., disc is myelinated)

  • Other species: disc not myelinated.

# Physiologic cup or pit

  • Small depression in center of canine disc Represents posterior turn of ganglion cell axons

# Optic chiasm

  • Crossing of optic nerves to contralateral sides

# Degree of decussation in different species

  • Primates: 50%

  • Horse: 80%

  • Cat: 60%

  • Avian: 100%

  • Dog: 70%

# Optic tracts

  • Between chiasm and brain

# Destinations of optic nerve

  • Midbrain - for pupillary light responses

  • Lateral geniculate nucleus - to be transferred to visual cortex

Last Updated: 8/3/2018, 3:30:50 AM

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