Ear

External ear/pinna

• collect sound waves

• conduct sound waves to tympanic membrane

• determine origin/direction of sound

External ear canal

• composed of 2 elastic cartilages (auricular + annular)

• rostral helix, lateral tragus, caudal antitragus (separated by intertragic incisure)

• vertical + horizontal ear canal (dogs) separated by auricular projection on the dorsal aspect, must lift ear to visualize horizontal canal

• hair present, decreasing from distal to proximal

• contains sebaceous glands (ducts open to hair follicles) + ceruminous glands (apocrine glands open to hair follicles or surface of canal)

Auricular cartilage

• expands to form pinna, funnel shapped

Annular cartilage

• fits within base of auricular, overlaps osseous external auditory meatus (allows flexibility)

Cerumen

• Emulsion of desquamated keratinized epithelial cells and glandular secretions that coats the ear canal

• Removal by epithelial migration

Canine middle ear

• tympanic membrane

• auditory ossicles (malleus, incus, stapes)

• tympanic cavity

• Eustachian tube

• incomplete bulla septum

• should be air-filled, contains normal microbiota

Tympanic membrane

• outer & inner epithelium w/collagen core (no hair or glands)

• dorsal pars flaccida (loosely attached, vascular, normally flat but bulging can be normal) + ventral pars tensa (concave, semitransparent, complaint)

• at a 45 degree angle, pars tensa more distal

Stria mallearis

• visual projection of malleus through the tympanic membrane

Auditory ossicles

• malleus: manubrium embedded in tympanic membrane

• incus

• stapes: footplate attached to oval window

• tensor tympani (inserts on malleus) + stapedius muscle (inserts on stapes)

Tympanic cavity

• round window (covered by membrane)

• oval window (covered by stapes)

• Eustachian tube (connects middle ear to nasopharynx)

• air-filled cavity

Feline middle ear

• bulla septum almost completely separates dorsal and ventral compartments, connected via foramen between septum and petrous bone

• dorsolateral (smaller): ossicles, ostium (opening) of Eustachian tube, tympanic membrane

• ventromedial: air-filled bulla

Promontory

• medial wall of tympanic cavity that protects semicircular canals + cochlea

• oval window on dorsolateral aspect adjacent to pars flaccida, covered by thin diaphragm & stapes footplate

• round window on caudolateral aspect, covered by thin membrane

Inner ear

• housed in bony labyrinth

• contains vestibule, semicircular canals, & cochlea

Cochlea

• housed in petrous temporal bone

• membranous labyrinth contained within bony labyrinth, contains oval & round windows

• scala vestibuli (superior), media, and tympani (inferior) - perilymph in vestibuli + tympani, endolymph in media

• spiral ligament containing stria vascularis (blood vessels & endolymph) on outer wall

Reissner’s membrane

• floor of scala vestibuli & roof of scala media

Basilar membrane

• floor of of the scala media & roof of scala tympani

• contains Organ of Corti

Organ of Corti

• rests on basilar membrane, covers entire length of cochlea

• tectorial membrane covers hair cells

• reticular lamina anchors outer hair cells to structure

• sensory inner & outer hair cells

• supporting cells

Hair cells

• inner: actual receptor cells for hearing

• outer: adjust tuning and sensitivity of inner hair cells (more susceptible to damage)

• stereocilia touch tectorial membrane (tallest is kinocilium)

Sound conduction

• sound waves vibrate tympanic membrane, which vibrates ossicles

• compression wave: stapes footplate pushes basilar membrane down and round window out

• rarefaction wave: when stapes goes out, the basilar membrane goes up and the round window closes (depolarization)

Information from auditory system

• intensity: depends on rate of action potential firing

• frequency: depends on part of Organ of Corti stimulated

• location: determined by higher central auditory nervous centers (requires biaural hearing)

Auditory nerve

• connects cochlea to brainstem via internal auditory meatus in petrous temporal bone

• part of CN VIII (vestibulocochlear nerve)

Central auditory nervous system

• sound transmitted from cochlea to brainstem to auditory cortex via nuclei/generator centers

• sound transmitted ipsilaterally and contralaterally

Hearing loss

• conductive: sound blocked from reaching inner ear (often physically blocked, typically reversible)

• sensorineural: abnormalities/damage to cochlea or nerve (may be congenital, noise-induced, age-related)