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 hearing excludes the individual from much of what is happening, and adjustment to this deprivation imposes profound challenge. 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 concern of the neurologist as the otologist. As a general rule, the association of vertigo and deafness signifies a disease of the end organs for hearing and vestibular function, or of the 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.
The vestibulocochlear, or eighth, cranial nerve has two separate components: the cochlear nerve, which subserves hearing, 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.
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. The main parts of the right inner ear, viewed from the front. The perilymph is located between the wall of the bony labyrinth and the membranous labyrinth. In the cochlea, the perilymphatic space takes the form of two coiled tubes—the scala vestibuli and scala tympani. The endolymph is located within the membranous labyrinth, which includes the three semicircular canals, utricle, and saccule. C. The organ of Corti. This is the end organ of hearing; ...