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Cummings Ch 128:
Auditory System Anatomy
Sameer Ahmed
9/25/2013
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Bony labyrinth

Hardest bone of the body

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Comprised of the cochlea, vestibule, and SCC's
Cochlea
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

2 and ¾ turns (also will see 2 and ½ turns in other
texts)
Modiolous

Core of the cochlea

Highly porous bone to allow passage of auditory nerve fibers
to hair cells
Osseus spiral lamina

Extends from the modiolous

Divides upper and lower cochlear chambers into scala
vestibuli and scala tympani

Also serves as a point of attachment for the basilar membrane
(separates scala media from scala tympani)

Membranous labyrinth – houses organ of corti

Superiorly by Reissner's membrance, Inferiorly
by basilar membrane, laterally by spiral ligament

Stria Vascularis: highly vascular tissue responsible for
metabolic environment of scala media

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Tectorial membrane

Lies over the inner and outer hair cells
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Gelatinous structure
Deiter's cells, Hansen cells, Claudius' cells →
supporting cells of the Organ of Corti
Perilymph: High Na, Low K


Endolymph: High K, Low Na


Between osseus and membranous labyrinth
Found within the membranous labyrinth
Enlarged vestibular aqueduct, defined as a
diameter larger than 1.5 mm at the vestibular
aqueduct midpoint

Minor head trauma can lead to progressive SNHL

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The inner and outer hair cells function as receptor cells
that transduce mechanical movement into an
electrochemical signal to stimulate the auditory nerve.

3500 inner hair cells

12,000 outer hair cells
Afferent innervation of these hair cells comes from 30,000
auditory nerve fibers (cochlear portion of CN 8)

Type I fibers are bipolar, large diameter, and myelinated,


Constitute nearly 95% of the total number of fibers.
 Each type I fiber has a direct and independent synapse
on the body of a single inner hair cell, and each inner hair
cell is innervated by approximately 20 such fibers.
Type II fibers constitute the remaining 5%, and are smaller
and may be myelinated or unmyelinated.

Type II fibers synapse directly on the outer hair cells, with a single
fiber diverging to form branches that synapse with multiple other
outer hair cells.
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Auditory Nerve Pathway Mnemonic

ECOLI

E ighth nerve

C ochlear nucleus

superior O livary nucleus
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L ateral lemniscus
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I nferior colliculus

Pathway
1) Stimulation of hair cells from vibration of basilar
membrane stimulates bipolar neurons of spiral
ganglion that form cochlear division of CN 8
2) Cochlear neurons then synpase in the cochlear
nuclei (anterior ventral, posterior vental, and dorsal)
3) Bilateral innervation from the cochlear nucleus (via
acoustic striae) synpase on the superior Olivary
nucleus
4) Superior olivary nuclei → lateral lemniscus →
inferior colliculus → thalamus (medial geniculate
body) –> auditory cortex at Sylvian fissure of
temporal lobe (Brodmann area 41)

Auditory regions within the transverse gyri of Heschl