128 Audition
Why should the system have two types of hair cells (inner
and outer)? More than 90% of the inner hair cells are inner-
vated by afferent auditory nerve fibers, indicating that the
inner haircells are the biological transducers for sound.
The outer hair cells appear to preform a very different task
(Dallos et al., 1996). The outer hair cells change their size
(primarily in length) in reaction to stimulation, and the
change in length happens on a cycle-by-cycle basis, even for
high frequencies of 20,000 Hz and above. The shearing of the
stereocilia of outer hair cells causes a neural action leading
to a deformation of the walls of the outer hair cells, result-
ing in a change in length (Brownell, Bader, Bertrand, & de
Ribaupierre, 1985; Geisler, 1998; Pickles, 1988). The length
change most likely alters the connections between the basilar
and tectorial membranes in a dynamic fashion, which in turn
affects the shearing of the inner hair cell stereocilia (Zajic &
Schacht, 1991). This type of positive feedback system
appears to feed energy back into the cochlea, making the
haircell function as an active process. The high sensitivity,
fine frequency resolution, and nonlinear properties of the bio-
mechanical action of the cochlear partition depend on viable
outer hair cells. Thus, the outer hair cells act like a motor,
varying the biomechanical connections within the cochlea
that allow for the inner hair cells to transduce vibration into
neural signals with high sensitivity and great frequency
selectivity (Dallos et al., 1996).
A consequence of the motile outer hair cells may be the
otoacoustic emissions that are measurable in the sealed outer
ear canal of many animals and humans (Kemp, 1978). If a brief
transient is presented to the ear and a recording is made in the
closed outer ear canal, an echo to the transient can be recorded.
This echo or emission is cochlear in origin. Emissions occur in
response to transients (transient-evoked otoacoustic emis-
sions; TOAE), steady-state sounds (usually measured as dis-
tortion product otoacoustic emissions; DPOAE), and they can
occur spontaneously (spontaneous otoacoustic emissions;
SOAE) in the absence of any externally presented sound.
Otoacoustic emissions are also dependent on neural efferent
influences on the outer hair cells. Presumably, the emissions
result either from the spontaneous motion of outer hair cells or
from other actions of the active processes associated with
outer hair cell motility. These emissions can be used to access
the viability of the cochlea and are used as a noninvasive mea-
sure of hearing function, especially in infant hearing screening
programs (Lonsbury-Martin & Martin, 1990).
Auditory Nerve
Each inner hair cell is connected to about ten auditory
nerve fibers, which travel in the XIIIth cranial nerve in a
topographical organization to the first synapse in the cochlear
nucleus of the auditory brain stem (Webster et al., 1992).
Thus, the auditory nerve fibers carry information about the
activity of the inner hair cells, which are monitoring the bio-
mechanical displacements of the cochlear partition (Fay &
Popper, 1992; Geisler, 1998; Pickles, 1988). Figure 5.7
shows tuning curves for individual auditory nerve fibers. AFigure 5.7 Tuning curves for six single auditory neurons with different
characteristic frequencies. The stimulus level in dB SPL calibrated at the
tympanic membrane needed to reach each neuron’s threshold is plotted as a
function of stimulus frequency. Note the steep slope on the high-frequency
side of the tuning curve and the shallow slope on the low-frequency side,
suggesting a high degree of frequency selectivity. Source: From Yost
(2000), adapted from Liberman and Kiang (1978), with permission.Threshold intensity (relative dB) required to stimulate unit above spontaneous firing rate0.1 1.0 10.0 80 20 80 20 80 20 80 20 80 20 80 20Frequency - kHz