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.
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.
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. ...