Lecture 8 Ear Anatomy

three main parts of the anatomy of the ear

external ear, middle ear, internal ear

three structures of the external ear

pinna (auricle), external auditory canal (ear canal), tympanic membrane (eardrum)

where does the external auditory canal (EAC) extend from

concha to tympanic membrane

what makes up the lateral one-third of the external auditory canal (EAC)?

membranous external acoustic meatus

what makes up the posterior two-third of the external auditory canal (EAC)?

bony cartilaginous canal

what is the important structure in the tympanic membrane

umbo

what is the umbo of the tympanic membrane

the concavity on the lateral side created by the attachment of the lower end and the handle of the malleus

what is the middle ear made up of

tympanic cavity, epitympanic recess, eustachian tube

where is the tympanic cavity located

directly medial to the tympanic membrane

where is the epitympanic recess located

it is the space superior to the tympanic membrane

what is the Eustachian tube

Passage between tympanic cavity and nasopharynx

where does the eustachian tube run towards

downwards, forwards and medially from middle ear at 45 degree angle

how long is the Eustachian tube

36 mm long with two unequal cones connected at the apices

the Eustachian tube is separated into what parts

lateral bony (closest to middle ear) and medial cartilage (structural backbone of tube)

how long is the lateral bony part of the Eustachian tube

12 mm (1/3rd)

how long is the medial cartilage part of the Eustachian tube

24 mm (2/3rd)

contents of the middle ear

ear ossicles, muscles, mucosal folds, nerves, vessels

what are the three ossicles in the middle ear

malleus, incus, stapes

what are the two muscles in the middle ear

tensor tympani and stapedius

where can we find the tensor tympani

it is attached to the handle of the malleus

innvervation of the tensor tympani

mandibular division of trigeminal nerve (CN V3)

function of tensor tympani

Tenses the tympanic membrane by pulling the malleus medially

Reduces the amplitude of vibrations reaching the inner ear

Particularly dampens low-frequency sounds

what structure is responsible for this function: Tenses the tympanic membrane by pulling the malleus medially

tensor tympani

what structure is responsible for this function: Reduces the amplitude of vibrations reaching the inner ear

tensor tympani

what structure is responsible for this function: dampens low-frequency sounds

tensor tympani

where can we find the stapedius in the middle ear

it is attached to the neck of the stapes

function of stapedius

Stabilizes the stapes at the oval window

Reduces excessive movement of the stapes

Protects the inner ear from loud sounds (acoustic reflex)

what structure is responsible for this function: Stabilizes the stapes at the oval window

stapedius

what structure is responsible for this function: Reduces excessive movement of the stapes

stapedius

what structure is responsible for this function: Protects the inner ear from loud sounds (acoustic reflex)

stapedius

innervation of the stapedius

facial nerve (CN VII)

general function of middle ear

transfer sound vibrations from your eardrum to your inner ear

what special structure does the middle ear have

eustachian tubes

function of eustachian tubes in the middle ear

help equalize the air pressure in your ears

the two parts of the inner ear

outer bony labyrinth and inner membranous labyrinth

what can we find between the outer bony labyrinth and inner membranous labyrinth

perilymph

what does the membranous labyrinth contain

endolymph

what is the inner ear innervated by

vestibulocochlear nerve (CN VIII)

two parts of the nerve supply in the inner ear

vestibular nerve and cochlear nerve

what is the vestibular nerve of the inner ear responsible for

balance

what is the cochlear nerve of the inner ear responsible for

hearing

which nerve pass through the inner ear but does not innervate any of the structures present

facial nerve (CN VII)

3 parts of the bony labyrinth from inside

semi-circular canals, vestibule, cochlea

what is the membranous labyrinth in the inner ear consisted of

closed system of inter-communicating membranous sacs and ducts within bony labyrinth filled with endolymph

what are the three canals of the semicircular canals that is oriented perpendicularly

anterior (superior), posterior, horizontal (lateral)

what is the anterior (superior) canal in the semicircular canal responsible for

detects nodding (yes motion)

what is the posterior canal in the semicircular canal responsible for

detects tilting (head to shoulder motion)

what is the horizontal (lateral) canal in the semicircular canal responsible for

detects rotation (no motion)

what does each canal in the semicircular canal contain

ampulla with crista ampullaris (sensory epithelium) and cupula

what is cupula in the semicircular canal

a gelatinous structure deflected by endolymph movement during rotation

functions of semcircular canal

Equilibrium and postural stability

Stable vision (via the vestibulo-ocular reflex, VOR)

Spatial orientation and awareness of head position

what structure is responsible for this function: Equilibrium and postural stability

semcircular canal

what structure is responsible for this function: Stable vision (via the vestibulo-ocular reflex, VOR)

semcircular canal

what structure is responsible for this function: spatial orientation and awareness of head position

semcircular canal

what is the vestibule

the central chamber connecting the cochlea and semicircular canals

what does the vestibule contain

utricle and saccule

function of utricle in the vestibule

Detects horizontal linear acceleration

function of saccule in the vestibule

Detects vertical linear acceleration

both utricle and saccule have maculae - what is maculae?

sensory regions with hair cells embedded in a gelatinous otolithic membrane topped with calcium carbonate crystals (otoconia)

how does otoncia shift in our ear

when the head moves linearly, inertia causes it to shift

what happens when otoconia shift?

it bends hair cells and triggers signals to the brain about head position

where does the vestibular signal get tranmitted to

the oculomotor nuclei (CN III, IV, and VI) in the brainstem

what does the oculomotor nuclei (CN III, IV, and VI) in the brainstem control

the extraocular muscles to move the eyes in the opposite direction of the head rotation

what is the purpose of compensatory eye movement (e.g. head turns right —> eyes move left)

keeps the visual fixation stable in the target

what is cochlea

A spiral-shaped organ (~2.5 turns) that converts sound vibrations into neural impulses

three chambers of the cochlea

Scala vestibuli (perilymph)

Scala media/cochlear duct (endolymph, contains Organ of Corti)

Scala tympani (perilymph)

what does Organ of Corti contains

it contains hair cells that transduce sound-induced mechanical vibrations into electrical signals sent via the auditory nerve

what is the simplified flow summary in our ear

Tympanic membrane → Ossicles → Oval window → Cochlea (Scala vestibuli) → Basilar membrane (Organ of Corti) → Scala tympani → Round window

the waves move from the vestibule of the bony labyrinth to where in the inner ear

scala vestibuli of the cochlea

where do vibrations deflect in our ear

basilar membrane and organ of Corti

hair cells transduce mechancal movement into what

neural impulses

Signals travel through the cochlear branch of CN VIII and land on where

auditory cortex

where do pressure waves continue into

scala tympani

purpose of round window in our inner ear

it bulges outward to relieve pressure, allowing smooth fluid motion

what can vestibular dysfunction cause

nystagmus, vertigo, oscillopsia, and impaired fixation

what optometric assessment tests can be used for vestibular dysfunction

Vestibulo-Ocular Reflex (VOR) testing

Dynamic Visual Acuity (DVA) test

Head Impulse Test (HIT)

Ocular Motility and Gaze Stability assessment

Visual Vertigo or Motion Sensitivity Questionnaire