CHAPTER 13
Hearing & Equilibrium 211bony walls of the scala vestibuli are rigid, but Reissner’s mem-
brane is flexible. The basilar membrane is not under tension,
and it also is readily depressed into the scala tympani by the
peaks of waves in the scala vestibuli. Displacements of the fluid
in the scala tympani are dissipated into air at the round window.
Therefore, sound produces distortion of the basilar membrane,
and the site at which this distortion is maximal is determined by
the frequency of the sound wave. The tops of the hair cells in the
organ of Corti are held rigid by the reticular lamina, and the
hairs of the outer hair cells are embedded in the tectorial mem-
brane (Figure 13–4). When the stapes moves, both membranes
move in the same direction, but they are hinged on different ax-
es, so a shearing motion bends the hairs. The hairs of the inner
hair cells are not attached to the tectorial membrane, but they
are apparently bent by fluid moving between the tectorial mem-
brane and the underlying hair cells.
FUNCTIONS OF THE INNER
& OUTER HAIR CELLS
The inner hair cells are the primary sensory cells that generate
action potentials in the auditory nerves, and presumably they
are stimulated by the fluid movements noted above.
The outer hair cells, on the other hand, have a different func-
tion. These respond to sound, like the inner hair cells, but depo-
larization makes them shorten and hyperpolarization makes
them lengthen. They do this over a very flexible part of the basal
membrane, and this action somehow increases the amplitude
and clarity of sounds. These changes in outer hair cells occur in
parallel with changes in
prestin,
a membrane protein, and this
protein may well be the motor protein of outer hair cells.
The outer hair cells receive cholinergic innervation via an
efferent component of the auditory nerve, and acetylcholine
hyperpolarizes the cells. However, the physiologic function of
this innervation is unknown.
ACTION POTENTIALS IN
AUDITORY NERVE FIBERS
The frequency of the action potentials in single auditory nerve fi-
bers is proportional to the loudness of the sound stimuli. At low
sound intensities, each axon discharges to sounds of only one
frequency, and this frequency varies from axon to axon depend-
ing on the part of the cochlea from which the fiber originates. At
higher sound intensities, the individual axons discharge to a wid-
er spectrum of sound frequencies, particularly to frequencies
lower than that at which threshold simulation occurs.
The major determinant of the pitch perceived when a sound
wave strikes the ear is the place in the organ of Corti that is
maximally stimulated. The traveling wave set up by a tone pro-
duces peak depression of the basilar membrane, and conse-
quently maximal receptor stimulation, at one point. As noted
above, the distance between this point and the stapes is
inversely related to the pitch of the sound, with low tones pro-
ducing maximal stimulation at the apex of the cochlea and high
tones producing maximal stimulation at the base. The path-
ways from the various parts of the cochlea to the brain are dis-
tinct. An additional factor involved in pitch perception at
sound frequencies of less than 2000 Hz may be the pattern of
the action potentials in the auditory nerve. When the frequency
is low enough, the nerve fibers begin to respond with an
impulse to each cycle of a sound wave. The importance of this
volley effect,
however, is limited; the frequency of the action
potentials in a given auditory nerve fiber determines princi-
pally the loudness, rather than the pitch, of a sound.
Although the pitch of a sound depends primarily on the fre-
quency of the sound wave, loudness also plays a part; low tones
(below 500 Hz) seem lower and high tones (above 4000 Hz)
seem higher as their loudness increases. Duration also affects
pitch to a minor degree. The pitch of a tone cannot be perceived
unless it lasts for more than 0.01 s, and with durations between
0.01 and 0.1 s, pitch rises as duration increases. Finally, the
pitch of complex sounds that include harmonics of a given fre-
quency is still perceived even when the primary frequency
(missing fundamental) is absent.CENTRAL PATHWAY
The afferent fibers in the auditory division of the eighth cranial
nerve end in
dorsal
and
ventral cochlear nuclei
(Figure 13–12).
From there, auditory impulses pass by various routes to the
infe-
rior colliculi,
the centers for auditory reflexes, and via the medi-
al geniculate body in the thalamus to the auditory cortex. Other
impulses enter the reticular formation. Information from both
ears converges on each superior olive, and beyond this, most of
the neurons respond to inputs from both sides. The primary au-
ditory cortex is Brodmann’s area 41 (see Figure 13–13). In hu-
mans, low tones are represented anterolaterally and high tones
posteromedially in the auditory cortex.
In the primary auditory cortex, most neurons respond to
inputs from both ears, but strips of cells are stimulated by input
from the contralateral ear and inhibited by input from the ipsi-
lateral ear. There are several additional auditory receiving areas,
just as there are several receiving areas for cutaneous sensation.
The auditory association areas adjacent to the primary auditory
receiving areas are widespread.
The olivocochlear bundle is a prominent bundle of efferent
fibers in each auditory nerve that arises from both ipsilateral
and contralateral superior olivary complexes and ends pri-
marily around the bases of the outer hair cells of the organ of
Corti.AUDITORY RESPONSES OF NEURONS
IN THE MEDULLA OBLONGATAThe responses of individual second-order neurons in the
cochlear nuclei to sound stimuli are like those of the individu-
al auditory nerve fibers. The frequency at which sounds of the