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Neuro Objectives 24 1. Bony labyrinth: cochlea, vestibule, semicircular canals; filled with perilymph Membranous labyrinth: cochlear duct, utricle, saccule, semicircular ducts; filled with endolymph 2. 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 3. 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 4. 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. 5. 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 6. 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