Acoustic reflex Protective function Protective function Due to muscle contraction in response to intense sound Due 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 dB Reflex results in attenuation of loud sounds by about 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 about ms Not very effective for short duration sounds: Latency about ms

Measurement of acoustic reflex Middle ear muscles contract: Stiffness increases Middle ear muscles contract: Stiffness increases More sound reflected back More sound reflected back Lack of acoustic reflex: Implies problem in middle ear muscles/part(s) of acoustic reflex pathway Lack of acoustic reflex: Implies problem in middle ear muscles/part(s) of acoustic reflex pathway

Inner ear Series of interconnecting canals or ‘labyrinths’ in the temporal bone Series of interconnecting canals or ‘labyrinths’ in the temporal bone Two types: Two types: Osseous Osseous Bony Bony Bigger cross-sectional area Bigger cross-sectional area Contains fluid called perilymph Contains fluid called perilymph Membraneous Membraneous Soft tissue Soft tissue Situated within the bony labyrinth Situated within the bony labyrinth Contains fluid called endolymph Contains fluid called endolymph

Labyrinths contain Three parts Three parts Semicircular canals Semicircular canals Vestibular system Vestibular system Vestibule Vestibule Cochlea Auditory system Cochlea Auditory system

Vestibular system Function: Balance and equilibrium Function: Balance and equilibrium Sensory cells: Hair cells Sensory cells: Hair cells

Auditory system: Cochlea Name derived from snail-like shape Name derived from snail-like shape Tube of decreasing diameter coiled around itself Tube of decreasing diameter coiled around itself Coiled around a central bony canal called modiolus Coiled around a central bony canal called modiolus Broad base, narrow apex Broad base, narrow apex Length: About 35 mm Length: About 35 mm In human beings: 2 and 5/8 coils In human beings: 2 and 5/8 coils

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

Cochlear macrostructure Partially divided throughout its length by a thin spiral shelf of bone called osseous spiral lamina Partially divided throughout its length by a thin spiral shelf of bone called osseous spiral lamina At the outer (lateral) wall of the cochlea: Spiral ligament At the outer (lateral) wall of the cochlea: Spiral ligament

Between the osseous spiral lamina and the spiral ligament: Basilar membrane Between the osseous spiral lamina and the spiral ligament: Basilar membrane Runs all the way along the length of the cochlea, except for a small opening at the apex called the helicotrema Runs all the way along the length of the cochlea, except for a small opening at the apex called the helicotrema Reissner’s membrane projects diagonally from spiral lamina to outer bony wall of cochlea Reissner’s membrane projects diagonally from spiral lamina to outer bony wall of cochlea Joins the basilar membrane at the helicotrema Joins the basilar membrane at the helicotrema

Between the osseous spiral lamina and the spiral ligament: Basilar membrane Between the osseous spiral lamina and the spiral ligament: Basilar membrane Runs all the way along the length of the cochlea, except for a small opening at the apex called the helicotrema Runs all the way along the length of the cochlea, except for a small opening at the apex called the helicotrema Reissner’s membrane projects diagonally from spiral lamina to outer bony wall of cochlea Reissner’s membrane projects diagonally from spiral lamina to outer bony wall of cochlea Joins the basilar membrane at the helicotrema Joins the basilar membrane at the helicotrema

Cross-section of the cochlea Basilar membrane and Reissner’s membrane divide the cochlear canal into three ducts: Basilar membrane and Reissner’s membrane divide the cochlear canal into three ducts: Scala vestibuli Scala vestibuli Scala tympani Scala tympani Scala media Scala media

Scala Vestibuli Above Reissner’s membrane Above Reissner’s membrane Extends from oval window in the vestibule to the helicotrema Extends from oval window in the vestibule to the helicotrema Contains perilymph Contains perilymph

Scala Tympani Below the basilar membrane Below the basilar membrane Extends from round window to the helicotrema Extends from round window to the helicotrema Contains perilymph Contains perilymph

Scala media/cochlear sac Bound below by basilar membrane Bound below by basilar membrane Bound above by Reissner’s membrane Bound above by Reissner’s membrane Bound on the outer side by Stria vascularis Bound on the outer side by Stria vascularis Contains endolymph Contains endolymph