Special Senses

The two main special senses

• eyes - sense light

• ears - sends vibrations in the air (sound)

Ocular

relating to the eye

Aural

relating to the ear

The eye

• cornea and sclera

  • fibrous tunic surrounding globe

• iris

  • controls pupillary opening

• lens

  • focuses light

• choroid

  • vascular tunic

• retina

  • photoreceptor layer

• tapetum lucidum

  • reflects light

• conjunctiva

  • mucous membrane

Eye chambers and fluids

• chambers

  • anterior chamber - between cornea and iris

  • posterior chamber - between iris and lens

  • vitreous chamber - between lens and retina

• fluids (provide nutrients)

  • aqueous humor

    • fills anterior and posterior chambers

    • watery

  • vitreous humor

    • fills vitreous chamber

    • gelatinous

Cornea

• anterior third of the fibrous tunic surrounding the globe

  • contiguous with the sclera

• transparent

• layers

  • corneal epithelium

    • epithelial cells forming outer layer

  • corneal stroma

    • composed of parallel collagen fibers

  • Descemet’s membrane

    • cell-free membrane

  • Corneal endothelium

    • maintains water balance in the stroma

Corneal transparency

• maintaining transparency is key to corneal function

  1. avascular

     • no vessels to block light

     • nutrition provided by

       • tear film

       • aqueous humor (fluid in anterior chamber)

  2. dehydrated

     • epithelium and cellular pumps maintain dehydration

  3. organization of collagen

     • parallel arrangement

Corneal edema

• fluid in the cornea

• appearance

  • blue

  • indistinct borders

Corneal scarring

• scar tissue in the cornea

• appearance

  • white

  • indistinct borders

Neovascularization

• blood vessels in the cornea

• appearance

  • red and linear

  • sharp borders

Corneal pigmentation

• melanin pigment in the cornea

• appearance

  • brown-black

  • distinct borders

Keratitis

• inflammation of the cornea

• appearance

  • yellow or creamy

  • indistinct borders

Corneal mineral deposits

• calcium in the cornea

• appearance

  • white and crystalline

  • sharp borders

Corneal erosion

partial thickness loss through the corneal epithelium

Corneal ulcer

• full-thickness loss through epithelium into the stroma

• stromal loss of varying depths

Descemetocele

• full thickness loss through stroma

• only Descemet’s membrane is intact

Corneal perforation

• full thickness loss through cornea into anterior chamber

• can involve iris prolapse

Corneal ulceration

• clinical appearance

  • corneal defect

  • corneal edema

  • neovascularization

  • inflammatory infiltrates

• shallow ulcers can be difficult to see

  • fluorescein stain

    • exposed stroma stains green

    • intact epithelium does not stain

• causes

  • trauma

    • conformational defects of eyelids

    • hair irritation

    • foreign body

  • exposure

    • exopthalmos

      • protruding eye

    • cranial nerve defects

  • keratoconjunctivitis sicca

    • dry eye

  • infection

Keratoconjunctivitis sicca

• aka dry eye

• decreased production of a portion of the tear film

• common disease of dogs

• Schirmer Tear Test

  • test amount of tear production

• Consequences

  • acute: corneal ulceration

  • chronic: corneal epidermalization

    • corneal epithelium begins to resemble epidermis

Infectious bovine keratoconjunctivitis

• aka: pink eye

• cause

  • Moraxella bovi

    • Gram negative bacteria

• highly contagious

• appearance

  • unilateral corneal

  • intense inflammatory cell infiltrates

  • neovascularization

Uveal tract

• vascular tunic of the eye

• three components

  1. Iris

     • controls pupillary opening

  2. ciliary body

     • hold lens in place

     • produces aqueous humor

  3. choroid

     • nourishes retina

• features

  • well vascularized

  • abundant pigment

Uveitis

• inflammation of the uveal tract

• inflammatory cells enter through blood vessels

  • neutrophils and macrophages

    • settle in eye chambers

  • lymphocytes and plasma cells

    • remain in tissues around blood vessels

Hypopion

• neutrophils settle in anterior chamber

• seen with uveitis

Retina

• light sensitive layer at the back of the eye

• function

  • transduces visible light into neural impulses

  • transmitted to the visual cortex of the brain

• layers

  • neurosensory retina

    • 8 layers of neurons and axons

    • 1 layer of photoreceptors

    • light getes to photoreceptors last

  • retinal pigment epithelium (RPE)

    • outermost layer

Photoreceptors

• rods

  • high sensitivity

    • low light

    • slight movement

  • low acuity

  • no color

  • ex. cats have a higher percentage or rods

• cones

  • low sensitivity

  • high acuity

    • sharp detail

  • color

  • ex. eagles have a higher percentage of cones

Retinal detachment

• separation of neurosensroy retina from RPE

• can be partial or complete

  • partial: confined to localized area

  • complete: retina billows into the vitreous

    • attached at single point (optic disc)

    • “Morning glory”

• histopathology - differentiate from artifact

  • exudate in space under detached retina

  • tombstoning

    • hypertrophied RPE

Glaucoma

• diverse group of ocular disease

• elevated intraocular pressure

  • buildup of aqueous humor leads to increase in pressure

• damages

  • retinal cells

  • optic nerve

  • other ocular tissues

• results in vision loss

Normal flow of aqueous humor

1. produced at ciliary body

2. flows over lends and through pupil

3. flows across iris

4. exits at the iridocorneal angle

• balance between production and outflow of aqueous fluid

Mechanism of glaucoma

• blockage of outflow through the iridocorneal angle

  • production of aqueous fluid is normal

  • outflow of fluid cannot keep up with production

  • results in buildup of aqueous fluid

  • increased intraocular pressure

• glaucoma is NOT related to a change in production of aqueous humor

Functions of the ear

• transduction of sound waves into neural potentials to send to the brain

• vestibular

  • “sense of balance”

External ear

• pinna (aka auricle)

• external acoustic meatus

  • ear canal

  • surrounded by cartilage and bone

• ceruminous glands

  • produce ear wax

Middle ear

• tympanic membrane

  • “ear drum”

  • thin membrane that responds to sound waves

• tympanic cavity

  • air filled compartment surrounded by bone

• tympanic bulla

  • bulbous ventral portion of tympanic cavity

• auditory ossicles

  • chain of 3 bones for mechanical transduction

    • malleus

    • incus

    • stapes

Inner ear

• cochlea

  • complex structure of membranous and bony labyrinths

  • contains fluid

  • transforms mechanical deflection of fluid waves into neurologic impulses

• vestibular system

  • three semicircular canals filled with fluid

  • movement of fluid in the canals is sensed by special cells

Otitis

• inflammation of the ear

• classified based on the portion affected

  • otitis externa

    • external ear

  • otitis media

    • middle ear

  • otitis interna

    • inner ear

Otitis externa

• portal of entry

  • extension from external environment

  • extension from middle ear

• predisposing factors

  • conformation

  • breed

  • external ear moisture

• infectious agents

  • bacterial and fungal infections

• chronic infections leads to the thickening of the epidermis

• very itchy and uncomfortable

• can spread to middle and inner ear

Otitis media

• portal of entry

  • extension through perforation in tympanic membrane

  • ascension of auditory tube

• can cause or result from rupture of tympanic membrane

• tympanic bullae can fill with exudate

• may cause lysis/erosion of surrounding bone

• can cause facial nerve paralysis

Otitis interna

• portal of entry

  • extension from middle ear

  • hematogenous spread

  • migration along neural pathways

• often associated with neurologic deficits