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EAC
-2.5 cm long S-shaped curved anteriorly
-lateral 1/3 cerumen cartilaginous
-resonance at 3500 Hz, gain of 15 dB for adults
*resonance is 8000Hz for baby until
2.5 years of age
Innervation
•Anteriorly auriculotemporal nerve V3
•posterior superior CN 7
•posterior inferior and floor CN 9 (Jacobsen) +10 (Arnold)
•clockwise
Head Shadow effect
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Head blocks sound
Shorter wavelengths > 2000Hz can’t bend
and will have interaural INTENSITY difference
Longer wavelengths can bend around head
and will have interaural TIME difference
Middle ear must compensate for loss of
energy from air to fluid transition
(impedence)
Impedence match
Area of TM to area of footplate 17 : 1
Handle of malleus and long process of
incus 1.3 : 1
Shape of TM allows difference in reception
of oval and round window
22 : 1 advantage 25-30 dB gained
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Transformer ratio
Cochleariform process houses tensor
tympani which attaches to handle of
malleus (CN V) and points to facial
Stapedieus emanates from the pyrimidal
process CN 7
Both are smallest striated mm of body
Stapedial reflex is bilateral
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Protects cochlea esp. <2000Hz from sounds
> 90dB
Delay of 10 ms
COCHLEA
2.5 turns
Helicotrema connects vestibuli and
tympani at apex
3 compartments
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Scala vestibuli, media, tympani
Endolymph
Intracellular fluid low Na; high K
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Perilymph
Vestibuli and tympani (extracellular) high Na; low K
Footplate attached to vestibule of labyrinth
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Contiguous with scala vestibuli
Walls of scala media
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Reissner’s membrane
Basilar membrane – organ of Corti
Lateral wall – stria vascularis
Na-K ATPase
Cochlear implant into scala tympani
Organ of Corti
Outer hair cells (3)
Inner hair cells
Supporting cells
Tectorial membrane
Inner Hair Cells
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Type 1 neurons
many spiral ganglion cells to 1 inner hair cell
Efferents project lateraly
Outer Hair Cells
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More numerous
Innervated by type 2 afferent neurons
One spiral ganglion cell branches to many outer
Efferents project medially
Sound vibrates basilar membrane
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Stiffer at base than apex
Tonotopically constructed with high freq
maximal displacement at base
Cochlear amplification
Outer hair cells enhance frequency pick up
Hair cells have stereocilia
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Directly contact tectorial membrane in outer
Deflection causes K+ influx depolarization
Auditory Nerve CN 8
Auditory nerve function measured by tuning
curves of type I cells (inner)
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Sound is presented an frequency and intensity
adjusted until change in firing rate
Nadir is where nerve is best at that frequency
Characteristic frequency
SNHL loses tips
Presbycusis caused by dysfunction of stria
vascularis
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Normal two-tone suppression
OAE
Sound detected in EAC emanating from
cochlea
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Spontaneous OAE
Transient evoked OAE
Frequency matched deficit
20-30 dB loss will lose OAE
Used in newborn hearing
Auditory Central Nervous System
Cell bodies in spiral ganglion with afferent
to hair cells, axons sent to cochlear
nucleus
Mostly contralateral innervation to superior
olivary complex, small ipsilateral
contribution
Stimulation of the contralateral ear is
usually stimulatory and ipsilateral is
inhibitory
Medial portion of superior olivary complex
is where crossed efferents to outer cell
originates
Lateral superior olivary complex is where
efferents to uncrossed inner originates
Next synapse is at inferior colliculus
(crossing)
Medial geniculate body (crossing)
Slyvian fissure of the auditory cortex in
temporal lobe (no crossing)
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There is tonotopic orginaztion
Auditory Brainstem Response
7 waves measured after given stimulus
I and II: 8th nerve
III : Cochlear nucleus
IV: superior olive
V : inferior colliculus
Auditory steady-state Response
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Continuous tones used with