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Neuro Objectives 24
Bony labyrinth: cochlea, vestibule, semicircular canals; filled with perilymph
Membranous labyrinth: cochlear duct, utricle, saccule, semicircular ducts;
filled with endolymph
Path of sound wave from outer to inner ear: Outer ear → malleus → incus →
stapes → oval window → perilymph → endolymph → tectorial membrane →
hair cells → CN VIII
Conductive hearing loss: disorder in outer or middle ear, characterized by an
air-bone gap
Sensorineural hearing loss: damage to hair cells; characterized by a failure to
detect otoacoustic emissions, and a lack of air-bone gap
Mechanical arrangement of receptors/accessory structures in the cochlea:
Tonotopic organization: lower frequencies are detected by hair cells nearer to
the helicotrema; higher frequencies are detected by hair cells nearer the stapes
Inner hair cell function: convey sound to the CNS
Outer hair cell function: cochlear amplification of vibration, providing a boost
in mechanical stimulus for the organ of Corti
Otoacoustic emissions: normal emissions from properly functioning hair cells in
response to sound
Pathway of acoustic reflex: sound enters (greater than 80 dB), hair cell → CN
VIII → cochlear nucleus → superior olivary nucleus (bilaterally) → CN VII →
stapedius muscle → stiffening of stapes and ossicular chain
Pathway of otoacoustic emission suppression: sound enters (greater than 60
dB), hair cell → CN VIII → cochlear nucleus → superior olivary nucleus
(bilaterally) → neurons to outer hair cells → suppression of otoacoustic emissions
Functions of efferent control: To hear sounds in a noisy environment (changes
transmission sensitivities to “block out” background noise)
 Note: Loss of acoustic reflex will lead to hyperacousis in the damaged
side. Since there is no stiffening of the ossicular chain (quieting effect),
sounds will sound unusually loud in the damaged side.
Central auditory pathway: hair cell → CN VII → cochlear nucleus → (bilateral
projection through the trapezoid body) → superior olivary nucleus or lateral
lemniscus → inferior colliculus → inferior brachium → medial geniculate
nucleus → auditory cortex (superior temporal gyrus)
Areas causing unilateral damage: hair cells, CN VIII, and the cochlear nucleus
Cochlear nucleus (primary afferent termination): dorsolateral to the inferior
cerebellar peduncle in the pontomedullary junction
Trapezoid body (fibers allowing for bilateral projection): dorsal grouping of
pontine fibers in the caudal pons
Superior olivary nucleus (relay for afferent pathways to cortex): ventromedial
to facial nucleus in caudal pons
Lateral lemniscus (tract for secondary afferents to inferior colliculus): dorsal to
spinothalamic tract in caudal pons to the caudal midbrain where they synapse in
the inferior colliculus
Inferior colliculus (secondary afferent termination): dorsal to periaqueductal
gray in caudal midbrain
Inferior brachium (tract for afferent fibers to the thalamus): ventrolateral to the
superior colliculus in the middle midbrain
Medial geniculate nucleus (thalamic nucleus for auditory afferents):
dorsolateral in the rostral midbrain