Clinical anatomy & physiology of the ear

YANG Jun, MD, Ph.D.

09/18/09

Otology & neurotologySurgical management on hearing loss

Conductive hearing loss: tympanoplasty, ossicular chain reconstruction, stapes surgery

Sensorineural hearing loss : implantable hearing-aids, cochlear implatation

Tumor in the lateral skull base,such as acoustic neuroma

Facial nerve: facial paralysis, facial spasmSurgical management on vertigoTrigeminal neuralgiaRepaire of CSF leakage

Temporal boneLocation : lateral skullNeighbour : parietal bone, sphenoid bone, occipital boneComposition: squamous part, tympanic part, pars

mastoidea, petrosal part

Anatomy of the external earauricle

anterior notch of ear-an incision can be made less subcutaneous tissue

difficult absorption of hematoma prone to cold injury

Anatomy of the auricle

Anatomy of the external earexternal auditory canal

2.5-3.5cm2.5-3.5cm outerouter1/31/3 ：： cartilage innerinner2/32/3 ：： bonebone Stenosis: juncture of bone and cartilage, bony part (0.5cm

from the tympanic anulus)

Anatomy of the middle ear

Tympanic cavityEustachian tubeTympanic sinusMastoid cavity

Tympanic cavity

Attic, mesotympanum, hypotympanum Six walls: interior, exterior, anterior, posterior,

superior, inferior

Tympanic cavity

颈静脉球颈静脉球

颈内动脉颈内动脉蜗窗小窝蜗窗小窝

鼓膜鼓膜鼓索神经鼓索神经

砧骨砧骨锤骨锤骨

鼓膜张肌鼓膜张肌附着处附着处 咽鼓管鼓口咽鼓管鼓口

鼓膜张肌鼓膜张肌半管半管

鼓岬鼓岬大脑颞叶大脑颞叶

匙突匙突

鼓索神经孔鼓索神经孔面神经面神经锥隆起锥隆起鼓窦入口鼓窦入口外半规管凸外半规管凸面神经管凸面神经管凸 镫骨底板镫骨底板

Exterior wall-tympanic membraneTympanic membrane

Semi-transparent film, 1cm2, 1mmUpper is pars flaccid, lower is pars tensaThree layer construction: epithelial lamina, fibrous

lamina, mucous layer

tympanic membrane

Interior wallNamely exterior wall of the inner earCenter-promontorium tympaniPost-superior : vestibular window-vestibulePost-inferior : cochlear window-scala tympani horizontal part of facial nerve canalprominence of lateral semicircular canalcochleariform process

Anterior wall

Namely carotid wallInferior part is separated with

the carotid arteryTwo openings at the superior

part: semicanal for tensor tympani (upper), semicanal for auditory tube (lower)

Posterior wallMinipore at the posterior wall-

aditus ad antrum tympanicumincudal fossa- juncture of

horizontal part and perpendicular part

pyramidal eminence-about at height of vestibular window

facial recess-posterior tympanotomy

Superior wallNamely tegmen tympaniBe separated with the temporal

lobe of the cerebrum in the middle fossa

The petrosquamous fissure in infant is not closed-one of the route by which infection from the middle ear could get into

Inferior wallNamely jugular wallBe separated with the

jugular bulbblue drum

Content in the tympanic cavityossicles （ smallest bone in the human body ） :

malleus, incus, stapes- ossicular chainligamenta ossiculorum auditus: ligament of the

malleus, incus and stapesmuscle in the tympanic cavity: tensor tympani muscle,

stapedial musclechorda tympani nerve

Ossicular chain

Eustachian tubePassageway between tympanic

cavity and nasopharynx, outer 1/3-bony part, inner 2/3- cartilaginous part. Isthmic portion-junction of bony part and cartilaginous part.

The opening at the nasopharynx is open when muscle contraction in order to adjust air pressure in the tympanic cavity.

Infection is prone to enter the tympanic cavity because of Horizontal, short and wide Eustachian tube in child.

Tympanic sinus and mastoid cavityTympanic sinus: pneumatic space and passage between

the attic and mastoid cavityMastoid cavity: cells in the temporal bone-pneumatic

type, diploetic type, constrictive type and mixed type

CT scan of temporal bone

Anatomy of the inner ear Also labyrinth, containing apparatus responsible for

hearing and balance The inner ear is divided into bony labyrinth and

membranous labyrinthPerilymph is full of the space between bony labyrinth

and membranous labyrinth, endolymph is full of the membranous labyrinth

 

bony labyrinth

Compact boneVestibule, semicircular canal,

cochlea

VestibuleBetween the cochlea and the semicircular canalFive openings from three bony semicircular canalssaccular recess, utricular recessExterior wall- vestibular window: sealed by footplate

of the stapes

Bony semicircular canalsThree curved bony ducts that form right angle

mutually- lateral, superior and posterior semicircular canal

A common crus is formed by the superior and posterior semicircular canal, therefore, five openings from three semicircular canals enter the vestibule

Membranous labyrinthComposed of membranous duct and membranous sacfixation at bony labyrinth by fiber bundledividing into utricle, saccule, membranous semicircular

canal and membranous cochlea (scala media)cross-connection each other

Membranous labyrinth Utricle

Utricular recessMacula utriculi-sense of balanceFive openings in the posterior wall connect with three

semicircular canalsConnection with the utriculosaccular duct and endolymphatic

duct in the anterior wall. Vestibular aqueduct. Endolymphytic sac (within dura behind the petrosal part of the temporal bone)

Membranous labyrinthSaccule

Saccular recessMacula sacculi-sense of balanceConnection with utriculosaccular duct and endolymphatic duct

Membranous labyrinthMembranous semicircular canal Connection with the utricle

Membranous labyrinthMembranous cochlea (scala media)

Between the osseous spiral lamina and the lateral wall of the osseous cochlear canal, also between scala vestibuli and scala tympani, containing endolymph

Basilar membrane: from free edge of the osseous spiral laminaOrgan of Corti : hearing receptor composed of outer hair cells

and inner hair cells

Physiology of the earHearing Balance

Route of sound conductingAir conduction

Sound wave auricle external auditory canal

vestibular window perilymph/endolymph organ of

Corti auditory nerve nucleus auditory cortex

Route of sound conductingBone conduction

Sound wave makes the perilymph vibrate through skull route, then stimulates the organ of Corti by which hearing generate. Translatory mode of bone conduction Compressional mode of bone conduction

Physiological functions of the external ear

Gathering sound Discriminating direction ResonanceProtectionSound wave pressurizing

Physiological functions of the middle ear

Transformation and gainStructure for sound transmission and transformation:

tympanic membrane and ossicular chain

Physiological functions of the tympanic membrane

Valid area of vibration : 55 mm2 Area of the footplate: 3.2 mm2 17times

Function Middle ear—amplification from area ratio

•Pressure = Force/area

•Area of tympanic membrane ~17 > area stapes

•Gain of area ratio ~24 dB

Physiological functions of the ossicular chain

Lever manubrium of malleus long crus of incus 1.3:1

1.3×17=22.1 27dB

Function of Middle ear—pressure amplification-Function of Middle ear—pressure amplification-ossiclesossicles

Energy loss at air-fluid interface-99.9% loss (-30 dB)Energy loss at air-fluid interface-99.9% loss (-30 dB)

•Malleus longer than incus-amplify pressure ~1.7X (+2 Malleus longer than incus-amplify pressure ~1.7X (+2 dB)dB)

Physiological functions of muscles in the tympanic cavity

stapedial muscle: decreasing pressure of perilymph

Physiological functions of muscles in the Eustachian tube

Keeping balance of pressure in the middle ear DrainagePrevention of retrograde infectionNoise abatement

Auditory physiology

TransmissionSensation

Basilar membrane displacement for a 1 kHz tone

Basilar membrane displacement for a 250 Hz tone

Basilar membrane displacement for a 4 kHz tone

Cochlear mechanical response due mass and stiffness Cochlear mechanical response due mass and stiffness gradientgradient

•Mass & stiffness gradient gives rise to a so-called “traveling Mass & stiffness gradient gives rise to a so-called “traveling wave”wave”

•Characteristic frequency—frequency which produces the Characteristic frequency—frequency which produces the largest amplitude of responselargest amplitude of response

•Apex-maximum response to low frequenciesApex-maximum response to low frequencies

•Base maximum response to high frequenciesBase maximum response to high frequencies

Mass-increases from base (stapes) to apex

Stiffness-increases from apex to base

1

23

4

Envelope of traveling wave

Characteristic frequency

Stereocilia on OHCs attached to tectorial membrane

Stereocilia on IHCs free standing

Motion of basilar membrane towards scala vestibuli deflects stereocilia in excitatory direction

Tectorial membrane deflects OHC stereocilia

Viscous fluid drag of fluid deflects IHC stereocilia

•Model: OHC contraction cause organ of Corti to distort as shown herer •Cell motility feeds back enhancing basilar membrane motion thereby increasing traveling wave amplitude and making the “cochlea active”

Model of organ of Corti Responds to OHC Electromotility

OHC contracts in-phase with deflection of the hair bundle toward the tallest stereocilia. The current through the cell increases with deflection in this direction. If the current is modulated slowly (compared to 1 kHz), then the voltage across the lateral membrane will be in-phase with the current. Conformational changes in many voltage sensitive molecules situated within the lateral membrane cause the length of the cell to change. The diameter of the cell increases slightly as the cell contracts to maintain constant cell volume.

Balance physiology

Semicircular canal ： Perception of positive or

negative angular acceleration

Saccule and utricle : Perception of linear acceleration

Macula sacculi: Perception of static balancing and

linear acceleration on the coronal plane

Macula utriculi: Perception of static balancing and

linear acceleration on the vertical plane