Multiple Levels Of Organization Of The Eye Flashcards
Relationship of globe to orbit and adnexa
Critical for vision, appearance, and health of the ye
Adnexa
Tissues around eye supporting its health
Orbit
Cone-shaped cavity containing globe and adnexa as well as other vessels, nerves, glands, fat ect.
Human orbit
Entirely bone
Canine orbit
Temporalis muscle forms lateral orbit wall and the medial pterygoid form part of the floor, zygomatic salivary gland forms more than 1/2 of the orbit floor (ie it is not entirely bone)
Canine rim of the orbit
Aka margin of the orbit, in some species entirely bone but in dog orbital ligament completes lateral margin of orbit rim
Bones comprising orbit walls and rim in dogs
Frontal, maxillary, zygomatic, and variably the lacrimal bones
Periorbita
Cone of thick connective tissue suspending Eye ball, extraocular muscles, vessels, nerves, fat, and adjacent connective tissue within orbit
Canine periorbita
Continuous with periosteum medially and dorsally
Distinct tissue ventrally and laterally
Orbit apex caudally periorbita continuous with dura matter and periosteum at optic canal/ orbital fissure
Orbit septum
Rostral at orbit rim periorbita blend into periosteum of adjacent bones and continue into eye lids as orbit septum
Anterior extent of orbital cavity
Eyelids and orbital septum define this boundary
What keeps eye forward in orbit
Periorbital connective tissue contains fine circularly arranged smooth muscle fibers exerting a constant squeeze on cone
Tenon’s capsule
Loose connective tissue immediately surrounding eyeball deep to conjunctiva
How is the globe suspended within periorbital cone?
Suspended via elaborate fascia and connective tissue networks including loose connective tissue sheets and check ligaments
Fat in orbit
Fat amounts variable between species, fat within and external to periorbita caudally, ventrally, laterally, fat cushions eye and provides room to move
Foramina/ fissures found in orbit
Optic canal, orbital fissure, rostral alar foramen, 2 ethmoidal foramina, fossa for lacriminal sac and others
Eyelids
Dorsal and ventral folds of facial skin with connective tissue for support plus muscles within them that allow lids to open and close
Eyelid function
Protect eye, remove foreign substances from surface of eye, distribute tear film, prevent tear overflow, contribute to tear film, exclude light from eyes, pump tears medially for cleansing of eye and tear remove all and more
Palpebral fissue
Space or opening between eyelids
Medial commissure or canthus
Union of free edges of upper and lower lids
Lateral commissure or canthus
Union of free edges of upper and lower lids laterally
Cilia
Eye lashes
How many rows eyelashes on upper lid of dog
2-4
Medial
Nasal
Lateral
Temporal
Tacticle hairs
Analagos to eyebrows
Lacrimal carbuncle
Small mound of tissue just inside medial canthus often haired; contains glands, serves to divert or redirect tears toward lacrimal puncta
Eyelid muscles
Orbicularis oculi, lavator palpebreal superious, tarsal muscles,+ more we don’t need to know names of
Orbicularis oculi
Oval sphincter like skeletal muscle beneath skin, closes lids in resposne to somatic efferent impulses
Orbicularis oculi innervation
Somatic efferent impulses; innervated by palpebral branch of cranial nerve VII
Orbicularis oculi anchoring
Anchored medially and laterally by ligaments
Absence of medial and lateral ligaments anchoring orbicularis oculi
If these are absent stretched or lacerated then palpebreal fissures can close like other fissures surrounded by sphincters
Levator palpebral superioris
Skeletal muscle, retracts upper lid, thin flat muscle works in unison with dorsal rectus muscle which it is dorsal to
Levator palpebral superioris innervation
Skeletal muscle, innervated by cranial nerve III to retract upper lid
Tarsal muscles
Ie Mueller’s muscle; smooth muscles that retract (open lid) sympathetic innervation
Medial levator annuli oculi
Eyelid miscue, raises eyebrow gives expression
Eyelid gland
Tarsal gland (meibomian glands); there are others we aren’t worrying about for block 1
Tarsal glands
Ie meibomain glands; sebaceous glands of upper and lower lids that form oily, outer layer of tear film, visible through conjunctiva, perpendicular to lid margins, parallel white yellow lines
Tarsal gland opening
Open in a furrow on lid margin (dotted line called gray line)
Eyelid connective tissue
Tarsus (aka tarsal plate) and ligaments
Tarsus
Ie tarsal plate; fiberous thickening surrounding tarsal glands at lid margins that continues into each eyelid blending into orbit septum and periosteum near orbit rim; VERY important to lid support
Ligaments of eye lid connective tissue
Medial and lateral cantonal ligaments extend from tarsus to orbital periosteum
Medial canthal ligament
Well developed in all breeds
Lateral canthal ligament
Poorly developed in some dog breeds, may be muscular or musculofibrous rather than fibrous
What does the conjunctiva do?
It is a mucus membrane lining of space inside palpebral fissure (except cornea) provides moist smooth surface for movements of globes and lids; moisture in eye and much of immune competency of external eye
Conjunctiva histology
Non-keritanized stratified squamous to columnar epithelium with goblet cells; gradually blends into stratified squamous epithelium of corneal surface at limbus and eyelid skin at lid margins
Membrana nictitans
Aka nicitating membrane, police semilunaris, or third eyelid; this is fold of conjunctiva ventromedial to eye
Conjunctival sac
Potential space inside closed eyelids
Divisions of conjunctiva
All continuous one to the other (and continuous at the limbus with the non-keritanized stratified squamous anterior corneal epithelium);
- Palpebreal or tarsal conjunctiva
- Bulbar conjunctiva
- Conjunctiva covering third eyelid bulbar and palpebral surfaces
Palpebral conjunctiva
Ie tarsal conjunctiva; thick, dark pink in color, small vessels visible, lines entire inner surface of eyelids from lid margin 360 degrees continuing to forbid where it turns and reflects onto the globe
Fornix
Approximately at the level of the orbital rim; transitional area between palpebral and bulbar conjunctiva in recess of conjunctival sac
Bulbar conjunctiva
Thin, colorless,and transparent with few vessels normally visible; may be partially pigmented in some breeds especially temporarily
Sclera visibility
Visible through transparent conjunctiva
Third eyelid conjunctiva
Bulbar surface covered in bulbar nictans conjunctival surface and palpebral surface covered in palpebreal surface of the nicitans
Conjunctiva heavily invested with ___
Goblet cells and numerous lymphatic aggregates may be present within submucosa of conjunctiva
Deep to conjunctiva
Bulbar sheath (ie tennon’s capsule) (loose connective tissue); attaches conjunctiva to globe, facilitates expensive free movement of eyeball and has many immune compentent cells that secrete immune surveillance (mast cells, eosinophils, macrophages, and lymphocytes)
Membrana nictians
Fold of conjunctiva from ventromedial fornix supported with t shaped hyaline cartilage, horizontal part keeps cornea clean and shaping tear film, free edge usually pigmented in dogs, almost completely hidden from view in proper position
What surrounds stem of t shaped hyaline cartilage
Seromucoid, superfical gland of third eyelid; ductules from this gland open onto bulbar surface of nicitans in region of lymph follicles found there in submucosa of conjunctiva
Movement of nictians
Highly mobile, can cover entire cornea; movement is primarily passive; can be active movement in cats and birds
Third eyelid retraction
Facilitated by postganglionic sympathetic fibers from cranial cervical ganglion
Horner’s syndrome
Loss of sympathetic innervation anywhere along path to eye leads to prolapse of nictitans, enophthalmos, meiosis, ptosis and reddened conjunctiva
Equine chronic ocular irritation
Can be with or without an ulcer, caused because horse has a pocket or crypt of conjunctiava centrally on bulbar suface that can trap foreign material, insects ect.
Is the nicitans a vestigial structure?
NO it is an essential component of a healthy eye in most domestic species
Prefrontal tear film
Keeps cornea constantly and uniformly moist; prevents desication and consequently keritanization opacification and even loss of anterior epithelium of the cornea
Loss of anterior epithelium of cornea
Corneal ulcer
Lacrimal apparatus
Encompassing term for all of the structures responsible for production, dispersal, and drainage of tears
Tri-laminar tear film
- Superfical layer
- Middle (aqueous layer)
- Inner mucus (mucin) layer
Superfical layer of tear film
Sebaceous (oily), from tarsal (meiboman gland); prevents evaporation and overflow of tear film
Middle aqueous layer of tear film
2 main sources in dog
- Lacrimal gland- seromucoid gland, located dorsolaterally under zygomatic process of frontal bone within lamina of periorbita (contributes 60-70% of total tear production); secretions enter the dorsotemporal conjunctival sac via many tiny ductules
- Superfical gland of their eyelid (or nictitans gland)- also sromucoid in dog, empties onto bulbar surface of nictans
Innervation of lacrimal gland
Responses to parasympathetic stimulus via CN VII
Inner mucus (mucin) layer of tear film
Produced by conjunctival goblet cells which are concentrated in fornicles of conjunctiva
Tear flow
Tears spread across cornea via blinking, surface tension, and eye movements
Blinking
Tears pumped medially toward lacrimal lake
tears sucked into tiny lacrimal puncta via negative pressure
Tears enter lacrimal canaliculi
Then enter the lacrimal sac located in a fossa in lacrimal bone
Then travel via nasolacrimal duct through maxillary bone and sinus to drain in nasal passage via nasolacrimal origice
Keratoconjunctivitis Sicca
Insufficient tears or dry eye
Lacrimation
Overproduction of tears (usually reflex secretions associated with pain or inflammation)
Epiphora
Overflow of tears due to obstruction (anatomical, functional, disease)
Intraocular muscles
Muscles inside eye that control pupil, lens, and facilitate drainage of fluid from inside eye
Extraocluar muscles
Striated muscles with in periorbita with tendenenous attachments to sclera that facial tears movements of eyes
Rectus muscles: dorsal lateral ventral medial
Oblique muscles: dorsal and ventral
Retractor bulbi: cone of muscle fibers surrounding optic nerve (deep to rectus); attach globe caudal to recti
Tenon’s capsule or fascia bulbi
Surrounds extrinsic ocular muscles and the glob from limbus to optic nerve, loose elastic like connective tissue network; facilitates smooth ocular movements
Extraocular muscle function
3 opposing pairs of muscles rotate eye around 3 axes
Anterior axis
Anterior to posterior- (cornea to optic nerve); oblique muscles rotate the eye around the A-P axis and retractors move eye along this axis
Medial axis
Medial to lateral axis; lateral equator to medial equator; dorsal and ventral rectus
Dorsal axis
Dorsal to ventral axis dorsal equator to ventral equator; medial and lateral rectus
Dorsal
Superior
Ventral
Inferior
Strabismus
Deviation of eye; if inactivity of one muscle eye will deviate toward active muscle
Lateral rectus and retrobulbar innervation
Cranial nerve VI (abducens nerve) (LR6)
Dorsal oblique
Ie superior oblique= cranial nerve IV (trochlear nerve) (SO4)
Extraocular muscle innervation
LR6SO4
All other extraocular muscles innervated by cranial nerve III
All other extraocular muscle innervation
Via cranial nerve III
Down and out ventrolateral strabismus
Seen with cranial nerve III paralysis; dorsal oblique muscle in dogs attaches posterior to equator of globe so when III not functioning unopposed function of DO results in ventral globe deviation
What cranial nerves have ocular significance
II,III, IV, V, VI, and VII
CN II
Optic nerve; formed by axons leaving retinal ganglion cells; considered extension or brain and part of CNS
CN III
Oculomotor nerve; provides most of ocular motor function via innervation DR, MR, VR, VO and levator palpebrae superioris; carries preganglionic parasympathetic fibers to the cillia Ray ganglion (for constriction of pupil sphincters and ciliary muscles); controls inwards and upwards movements of eyes
CN VI
Trochlear nerve; innervates DO
CN V
Trigeminal nerve
3 main branches
1. V1- ophthalmic branch (somatic afferent from eye and medial canthal skin); carries potsganglionic sympathetic fibers to the eye
2. V2- maxillary branch, carries SE from lateral canthus
3. V3- mandibular branch; innervates temporalis and masseter muscles
CN VI
Abducens or abducent nerve; innervates LR and RB muscles
CN VII
Facial nerve; innervates orbicularis oculi muscle (eyelid closer); parasympathetic fibers to lacrimal gland travel with CN VII
Arterial supply to eye
External carotid»_space; maxillary > external ophthalmic with anastomoses to internal carotid, internal opthalmic, and middle meningeal
Venous drainage from eye
Primarily via facial, maxillary, and deep facial veins; much facial drainage travels through or adjacent to cavernous sinus at base of brain
Is there a central retinal artery or vein in dogs?
NO; do not apply clamp to the nerve during enuclations
Layers of globe
Outer ( tough fiberous tunic; cornea and sclera)
Vascular (urea= iris, cilliary body, and choroid)
Neural (retina)
Transparent intraocular structures or fluids
Aqueous humor, lens, vitreous
Internal compartments of globe
Anterior chamber, posterior chamber, and viterous chamber/ vitreous compartment
Anterior and posterior chambers
Contain aqueous humor, transparent almost cell free and protein free fluid that bathes lens iris and corneal endothelium
Vitreous or vitreal humor
Transparent composed of intricately arranged collagenous fibrils forming gel like substance that fills majority of posterior portion of eye posterior to lens; secreted in part by retina, embryonic ocular vasculartur, and by lens; serves many functions such as helping to maintain retina in position
Outer tunic
Cornea and sclera
Cornea
Transparent anterior portion of fibrous tunic of eye; .6-.8mm thick, 15-30% globe area; main refractive surface of eye
Limbus
Location of cornea blending seamlessly into sclera
Why is corneal transparency maintained?
Normal cornea is moist
avascular non-keritanized, and non-pigmented epithelium
uniform size and precise arrangement of Type I collagen
In relatively dehydrated state
Has small non-myelinated nerve fibers
Corneal nutrition
Occurs via diffusion from sclera vessels near limbus, from tear film, from serous humor; if any of these are abnormal or altered disease will result
Layers of the cornea
4 histological, 5 functional layers
Histological layers of cornea
Anterior epithelium, stroma, descemet’s membrane, posterior epithelium (corneal endothelium)
Functional layers of the cornea
Tear film (WILL NOT SHOW UP ON HISTO SECTION)
Anterior epithelium
Stroma
Descemet’s membrane
Posterior epithelium (corneal endothelium)
Anterior epithelium
Second layer cornea, 6-8 cell layers of non keritanized stratified squamous epithelium; continuous with conjunctival epithelium; fragile and highly invested with sensory dendrites of cranial nerve V
Stroma
Sometimes called substantia propria; makes up 90% corneal thickness; stromal keratocytes with GAG matrix; produce intricately and precisely arranged collagen fibrils of uniform size; alteration to this arrangement leads to cloudiness
Descemet’s membrane
Modified basement membrane of posterior corneal epithelium; gets thicker throughout life
Posterior epithelium
Ie corneal epithelium; critically important post-mitosis monolayer of cells that are very poorly regenerative; Na-K ATPase pump regulates nutrient diffusion into corneal stroma while pumping water out of it
Sclera
Is very strong because composed of irregular arrangements, sheets and sizes of collagen; opaque white
Lamina cribrosa
Sieve-like area in sclera at posterior pole of the globe where axons from retinal ganglion cells exit the globe, coalesce and form optic nerve
Limbus
Junction between cornea and sclera
Tunics of the eye
Outer (cornea and sclera)
Middle (Uvea)
Inner (Retina, neural)
Uvea
Middle tunic of eye; aka uveal tract; vascular tunic of eye; has three continuous subdivisions (anterior to posterior- iris, ciliary body, and choroid); usually heavily pigmented in dogs
Iris
Autonomically controlled diaphragm with opening in the middle (pupil) which regulates amount of light entering the eye
Shape of pupil
Different in different species depending on arrangement and distribution of iris muscle fibers
Posterior iris surface
Lined by two epithelial cell layers, part of sensory tunic of eye (retinal), these two layers are derived from rim of embryonic optic cup; inner layer becomes sensory retina, outer later becomes retinal pigment epithelium
Anterior iris stroma
Part of posterior iris surface; this is a loose connective tissue matrix, NOT covered by an epithelium thus has a lot of texture or surface architecture
Myoepithlial cells derived from posterior epithelium
Neuroectodermal in origin; form antagonistic muscles that control pupil size and shape, circular sphincter muscle and radial dilator muscle
Circular sphincter muscle
Adjacent to pupil, is under parasympathetic control
Radial dilator muscle
Fibers are sympathetically innervated via postganglionic fibers from cranial cervical ganglion
Pectinate ligament
Attaches iris and ciliary bodies to periopheral cornea or sclera depending on species; this is located in iridocorneal angle area
How does aqueous humor leave eye to enter trabecular meshwork
Mainly via fenestrations in pectinate ligament, then sclera veins back to general circulation
Iris and lens
Iris rests against and conforms to curvature of the anterior lens but is not attached; anterior surface of lens is in same plan as iris
Ciliary body
Circumferential Uvea continues posteriorally from iris base as ciliary body
Ciliary processes
Longitudinal folds of ciliary body seen on cut section
Cut section of ciliary body
Appears on cut section as pigmented thickened ridge between iris and chricoid; has longitudinal folds (ciliary processes) extending inward from it towards center of globe
Production of aqueous humor
Epithelium upon ciliary process modifies blood plasma and produces aqueous humor; produced in part by diffusion and ultrafiltration but most actively facilitated by carbonic anhydride and under sympathetic and parasympathetic influences
Aqueous humor vs plasma
Transparent modified plasma with increase amino acids, ascorbate, bicarbonate, CI, and decreased proteins compared to plasma
Aqueous humor function
Fills anterior and posterior chambers, nuroushes and removes waste from avascular cornea and lens; maintains intraocular pressure and globe shape
Do you visualize ciliary body during exam or normal eye
No
Path of aqueous humor
Produced by epithets cells covering cilliary processes
Enters posterior chamber where it bathes lens
Travels through the pupillary aperture into anterior chamber
Circulates throughout anterior chamber where it bathes the corneal endothelium (via thermal currents, pupil and globe movements)
Exits eye primarily through pectinate ligament and trabecular meshowork in the iridoorneal angel (filtration angle)
Into veins in the sclera
Back to venous circulation
Choroid
Posterior most extentension of vascular tunic; loose highly vascular connective tissue between sclera and retina lining entire posterior hemisphere except at optic nerve and is continuous aneriorally with cilliary body and iris
Choroid function
Provides nourishment and removes waste from retina, pigment within choroid absorbs scattered light (reduces glare)
Tapetum lucidum
Colorful and shiny, cellular or fibrous modification of choroid NOT separate layer (causes eye shine at night) variable size and color; usually triangular shape; found in dorsal 1/2 posterior choroid; increases reflection of light back along same path through retina to enhance low light vision
Area of back wall of eye containing tapetum
Area of fungus (back of eye) containing tapetum is the tapetum fund us and remainder is non tapetum fund us
Adult form of eye cup
Retina is adult form of embryonic neuroectodermal eye cup
Neural/ retinal tunic parts
- Pars iridium retinae
- Pars ciliaris retinae
- Pars optica retinae
Pars iridium retinae
Part lining the iris (part of retinal tunic); two cell layers thick; covers posterior iris and is the origin of iris musculature
Pars ciliaris retinae
Part covering ciliary body (part of retinal tunic); 2 cell layers hick; inner cell layer covers ciliary process and contributes to production of aqueous humor
Pars optica retinae
Visual retina; internal to the choroid; outermost layer of original optic cup forms single cell thick layer known as retinal pigment epithelium (outermost layer of mature retina)
Derivation of 9 layers of retina internal to retinal pigment epithelium
Derived from inner portion of embryonic cup
Impulses generated in rods and cones of retina
Travel to retinal ganglion cells; axons of ganglion cells converge at optic nerve head and penetrate sieve-like lamina cribrosa of sclera to form optic nerve; posterior cordial extensions combine with extensions form sclera to form meninges surrounding the nerve and these are continuous with meninges covering brain/ CNS
Eye is part of CNS or PNS
CNS
Lens capsule
Lens epithelium is on the inside basement membrane is on outside which is backwards but forms lens capsule; has elastic characteristics
Lens
Biconvex posterior transparent structure immediately posterior to iris
Zonular ligament
Hundreds of 2-10nm thick fibrillin rich fibers which suspend lens; circumferential attach anterior and posterior lens capsule to anterior vitreous and cilliary body
Lens function
Responsible for precise focus of light on retina; responsible for the coursing power of eyes
Accommodation
The variable refraction or focusing of th eye (ability of eye to increase its optical power and keep a distant object in focus as it moves closer to the eye
Anterior lens epithelium
Deep to anterior lens capsule; these epithelial cells divide migrate to lens equator and elongate to form lens fibers
Lens nucleus
During development posterior lens epithelium was converted to primary lens fibers these are then surrounded by secondary lens fibers forming lens nucleus
Is there a posterior lens epithelium in the normal postnatal eye?
No
Cataract
Any loss of transparency of the lens whether focal or diffuse
Proteins inside lens capsule
Not recognized by immune system because developing lens encapsulated lens fibers before your immune system was developed so breach in lens capsule later in life exposes foreign protein immune system attacks (this is uveitis)
Mammalian species accomidatin to near and far vision
Vary shape and thickness of lens (some species can move entire lens forward and back in eye)
Resting shape of mammalian lens
Globoid and very thick (elasticity of lens fibers and capsule create the globoid shape)
Maximum refracting power for lens
When there is no traction on it
Globoid shape of lens in functional mammalian eye
Globoid shape is opposed in living functioning mammalian eye by tension of zonular fibers or ligaments attached to lens capsule near lens equator (other end of zonular fibers attaches adjacent to muscles of ciliary body; these muscles are located circumferentially around the lens)
Circumferential cilliary muscle contraction
They move closer to lens relaxing zonules and lens becomes rounder and ticker facilitating near vision (increased magnification results)
Circumferential cilary muscle relaxation
They move farther from the lens and the tension or pull on zolulues flattens the lends making the lens thinner and allowing focus on distant objects
Near focusing
CM contract causing attached zonular fibers to relax leading to a decrease in zonule tension leading to lens returning to globoid shape for near focusing
Far/ distance focusing
CM relax leading to increase zonule tension leading to lens flattens/ thins and optical strength decreases
Aging lens
With age elasticity of lens is lost; hardens due to continued lens fiber growth in contained space throughout life; lens becomes flattened not globoid which means old mammals loose ability to see close objects clearly (issues with NEAR vision)
Why does lens elasticity decrease with age
Because lens density increases with age; lens continually adds layers on outside just beneath capsule the center of lens eventually becomes hard as a result
Lens hardening in humans
Starts around 40-45
Lens hardening in dogs
Causes dogs to develop a gray appearance to their eyes, called nuclear sclerosis around age 7-8
