I. OVERVIEW. The auditory system is an exteroceptive special somatic afferent system that can detect sound frequencies from 20 Hz to 20,000 Hz. It is derived from the otic vesicle, which is a derivative of the otic placode, a thickening of the surface ectoderm.
II. THE AUDITORY PATHWAY (Figure 111) consists of the following structures.
A. The hair cells of the organ of Corti are innervated by the peripheral processes of bipolar cells of the spiral ganglion. They are stimulated by vibrations of the basilar membrane.
1. Inner hair cells are the chief sensory elements; they synapse with dendrites of myelinated neurons whose axons comprise 90% of the cochlear nerve.
2. Outer hair cells synapse with dendrites of unmyelinated neurons whose axons comprise 10% of the cochlear nerve. The OHCs reduce the threshold of the IHCs.
B. The bipolar cells of the spiral (cochlear) ganglion project peripherally to the hair cells of the organ of Corti. They project centrally as the cochlear nerve to the cochlear nuclei.
C. The cochlear nerve [cranial nerve (CN) VIII] extends from the spiral ganglion to the cerebellopontine angle, where it enters the brain stem.
D. The cochlear nuclei receive input from the cochlear nerve. They project contralateral ly to the superior olivary nucleus and lateral lemniscus.
E. The superior olivary nucleus, which plays a role in sound localization, receives input from the cochlear nuclei. It projects to the lateral lemniscus.
F. The trapezoid body is located in the pons. It contains decussating fibers from the ventral cochlear nuclei.
G. The lateral lemniscus receves input from the contralateral cochlear nuclei and superior olivary nuclei.
H. The nucleus of inferior colliculus receives input from the lateral lemniscus. It projects through the brachium of the inferior colliculus to the medial geniculate body.
I. The medial geniculate body receives input from the nucleus of inferior colliculus. It projects through the internal capsule as the auditory radiation to the primary auditory cortex, the transverse temporal gyri of Heschl.
J. The transverse temporal gyri of Heschl contain the primary auditory cortex (Brod-manns areas 41 and 42). The gyri are located in the depths of the lateral sulcus.
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