Acoustic reflexAcoustic reflex Protective functionProtective function

Due to muscle contraction in response to intense soundDue to muscle contraction in response to intense sound

Threshold of reflex: Around 80 dB SL (sensation level).Threshold of reflex: Around 80 dB SL (sensation level).

Reflex results in attenuation of loud sounds by about 10-Reflex results in attenuation of loud sounds by about 10-30 dB30 dB

More effective at low frequencies (less than 2 kHz)More effective at low frequencies (less than 2 kHz)

Not very effective for short duration sounds: Latency Not very effective for short duration sounds: Latency about 10-150ms about 10-150ms

Measurement of acoustic reflexMeasurement of acoustic reflex

Middle ear muscles contract: Stiffness Middle ear muscles contract: Stiffness increasesincreases

More sound reflected backMore sound reflected back

Lack of acoustic reflex: Implies problem in Lack of acoustic reflex: Implies problem in middle ear muscles/part(s) of acoustic middle ear muscles/part(s) of acoustic reflex pathwayreflex pathway

Inner earInner ear

Series of interconnecting Series of interconnecting canals or ‘labyrinths’ in the canals or ‘labyrinths’ in the temporal bonetemporal bone

Two types:Two types:

Osseous Osseous

BonyBony Bigger cross-sectional areaBigger cross-sectional area Contains fluid called perilymphContains fluid called perilymph

MembraneousMembraneous

Soft tissueSoft tissue Situated within the bony Situated within the bony

labyrinthlabyrinth Contains fluid called Contains fluid called

endolymphendolymph

http://research.meei.harvard.edu/Otopathology/3dmodels/download.html

Labyrinths containLabyrinths contain

Three partsThree parts

Semicircular canals Semicircular canals

Vestibular Vestibular

system system

VestibuleVestibule

CochleaCochlea Auditory Auditory systemsystem

Vestibular systemVestibular system

Function: Balance and equilibriumFunction: Balance and equilibrium

Sensory cells: Hair cellsSensory cells: Hair cells

Auditory system: CochleaAuditory system: Cochlea

Name derived from snail-Name derived from snail-like shapelike shape

Tube of decreasing Tube of decreasing diameter coiled around diameter coiled around itselfitself

Coiled around a central Coiled around a central bony canal called modiolus bony canal called modiolus

Broad base, narrow apexBroad base, narrow apex

Length: About 35 mm Length: About 35 mm

In human beings: 2 and 5/8 In human beings: 2 and 5/8 coilscoils

Isolated cochlear turns from the inner ear of fetal sheep: Photo courtesy of Gerhardt, Isolated cochlear turns from the inner ear of fetal sheep: Photo courtesy of Gerhardt, K. K.

Cochlear macrostructureCochlear macrostructure

Partially divided Partially divided throughout its throughout its length by a thin length by a thin spiral shelf of bone spiral shelf of bone called called osseous osseous spiral laminaspiral lamina

At the outer At the outer (lateral) wall of the (lateral) wall of the cochlea: cochlea: Spiral Spiral ligamentligament

Between the osseous spiral Between the osseous spiral lamina and the spiral lamina and the spiral ligament: ligament: Basilar Basilar membranemembrane

Runs all the way along the Runs all the way along the length of the cochlea, length of the cochlea, except for a small opening except for a small opening at the apex called the at the apex called the helicotremahelicotrema

Reissner’s membraneReissner’s membrane projects diagonally from projects diagonally from spiral lamina to outer bony spiral lamina to outer bony wall of cochleawall of cochlea

Joins the basilar membrane Joins the basilar membrane at the helicotremaat the helicotrema

Between the osseous spiral Between the osseous spiral lamina and the spiral lamina and the spiral ligament: ligament: Basilar Basilar membranemembrane

Runs all the way along the Runs all the way along the length of the cochlea, length of the cochlea, except for a small opening except for a small opening at the apex called the at the apex called the helicotremahelicotrema

Reissner’s membraneReissner’s membrane projects diagonally from projects diagonally from spiral lamina to outer bony spiral lamina to outer bony wall of cochleawall of cochlea

Joins the basilar membrane Joins the basilar membrane at the helicotremaat the helicotrema

http://www.iurc.montp.inserm.fr/cric/audition/english/ear/fear.htm

http://epl.meei.harvard.edu/~hwang/3Dviewer/3Dviewer.html

Cross-section of the cochleaCross-section of the cochlea

Basilar membrane and Basilar membrane and Reissner’s membrane Reissner’s membrane divide the cochlear divide the cochlear canal into three ducts:canal into three ducts:

Scala vestibuliScala vestibuli Scala tympaniScala tympani Scala mediaScala media

Scala Vestibuli Scala Vestibuli

Above Reissner’s Above Reissner’s membranemembrane

Extends from oval Extends from oval window in the window in the vestibule to the vestibule to the helicotremahelicotrema

Contains perilymphContains perilymph

Scala TympaniScala Tympani

Below the basilar Below the basilar membranemembrane

Extends from round Extends from round window to the window to the helicotremahelicotrema

Contains perilymphContains perilymph

Scala media/cochlear sacScala media/cochlear sac

Bound below by Bound below by basilar membranebasilar membrane

Bound above by Bound above by Reissner’s membraneReissner’s membrane

Bound on the outer Bound on the outer side by side by Stria Stria vascularisvascularis

Contains endolymphContains endolymph