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Hearing is one of our most precious possessions. Sounds alert us to danger; spoken words are the universal means of communication; music is one of our most exalted aesthetic pleasures. The loss of this sense excludes the individual from much of what is happening, and adjustment to this deprivation imposes a profound reorientation. Prevention of deafness is a goal toward which medicine strives. Likewise, vestibular function ensures one’s ability to stand steadily, stabilize eye position during head movement, and move about gracefully. Hence an understanding of the functions of the eighth cranial nerves and their derangements by disease is as much the legitimate concern of the neurologist as the otologist. As a general rule, the association of vertigo and deafness signifies a disease process of the end organ or eighth nerve. The precise locus of the disease is determined by tests of labyrinthine and auditory function, described further on, and by findings on neurologic examination and imaging studies that implicate the primary and secondary connections of the eighth cranial nerve.

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The vestibulocochlear, or eighth, cranial nerve has two separate components: the cochlear nerve, which subserves hearing or acoustic function, and the vestibular nerve, which is concerned with equilibrium (balance) and orientation of the body and eyes to the surrounding world. The acoustic division has its cell bodies in the spiral ganglion of the cochlea. This ganglion is composed of bipolar cells, the peripheral processes of which convey auditory impulses from the specialized neuroepithelium of the inner ear, the spiral organ of Corti. This is the end organ of hearing, wherein sound is transduced into nerve impulses. It consists of approximately 15,000 neuroepithelial (hair) cells that rest on the basilar membrane, which extends along the entire 2.5 turns of the cochlea. Projecting from the inner surface of each hair cell are approximately 60 very fine filaments, or stereocilia, which are embedded in the tectorial membrane, a gelatinous structure overlying the organ of Corti (Fig. 15-1). Sound causes the basilar membrane to vibrate; upward displacement of the basilar membrane bends the relatively fixed stereocilia and provides a stimulus adequate for activating the hair cells. The stimulus is then transmitted to the sensory fibers of the cochlear nerve, which end synaptically at the base of each hair cell. Each afferent auditory fiber and the hair cell with which it is connected have a minimum threshold at one frequency (“characteristic” or “best” frequency). The basilar membrane vibrates at different frequencies throughout its length, according to the frequency of the sound stimulus. In this way the fibers of the cochlear nerve respond to the full range of audible sound and can differentiate and resolve complexes of sounds.

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Figure 15-1.
Graphic Jump Location

The auditory and vestibular systems. A. The right ear, viewed from the front, showing the external ear and auditory canal, the middle ear and its ossicles, and the inner ear. B. ...

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