Sound energy vibrates the tympanic membrane. This movement is amplified by the bones of the middle ear so that vibrations occur in a special fluid-filled organ known as the cochlear. Specialized cochlear receptor cells are innervated by sensory axons of the primary auditory neurons (cranial VIII) located in a peripheral ganglion (spiral ganglion) near the inner ear. Centrally directed afferent axons of these neurons enter the brain stem and synapse with secondary sensory neurons located in the cochlear nuclei on the dorsolateral aspect of the rostral medulla oblongata (Figs. 2c and 2d). The secondary sensory neurons have axonal projections either via the lateral lemniscus directly to the inferior colliculus or via a synapse in the superior olivary nucleus and then via the lateral lemniscus to the inferior colliculus. From the inferior colliculus, ascending axons project to the medial geniculate nucleus of the thalamus and then to the auditory cortex.

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