Auditory Anatomy and Physiology 127Figure 5.5 Light micrograph of a cross section of a chinchilla organ of
Corti. Clearly shown are: IHC:inner hair cells; OHC:the three rows of outer
hair cells. The stereocilla (Sc) of the outer and inner hair cells protrude
through the recticular lamina that helps support the hair cells. Other support-
ing structures are shown. Source: From Yost (2000), photographs courtesy
of Dr. Ivan Hunter-Duvar, Hospital for Sick Children, Toronto.
Figure 5.6 Instantaneous patterns and envelopes of traveling waves of
three different frequencies shown on a schematic diagram of the cochlea.
Note that the point of maximum displacement, as shown by the high point of
the envelope, is near the apex for low frequencies and near the base for
higher frequencies. Also note that low frequencies stimulate the apical end as
well as the basal end, but that displacement from higher frequencies is con-
fined to the base. Source: From Yost (2000), adapted from similar draw-
ings Zemlin (1981), with permission.partition; and inner hair cells, which are aligned in a single
row. Several different supporting structures buttress the hair
cells on the basilar membrane.
The vibration of the stapes causes the oval window to
vibrate the fluids of the cochlea (Dallos, Popper, & Fay,
1996). This vibration sets up a pressure differential across the
cochlear partition, causing the cochlear partition to vibrate.
This vibration causes a shearing action between the basilar
membrane upon which the hair cells set, and the tectorial
membrane, which makes contact with the stereocilia (the
hairs, so to speak, that protrude from the top of the hair cells;
see Figure 5.5) such that the stereocilia are bent. The shearing
of the stereocilia opens transduction channels, presumably
toward the tips of the stereocilia, which initiates a generator
potential in the hair cell and a resulting action potential in the
auditory nerve fiber that innervates the hair cells (Pickles,
1988). Thus, the mechanical vibration of the stereocilia is
transduced into a neural signal.
The properties of the cochlear partition involving its width
and tension, as well as the fact that the cochlear partition does
not terminate at the end of the cochlea, all result in a particu-
lar motion being imparted to the cochlear partition when it is
vibrated by the action of the stapes (Dallos et al., 1996). The
cochlear partition motion is described as a traveling wave,
such that the vibration of the cochlear partition is distributed
across the partition in a frequency-specific manner. High-
frequency sounds generate maximal displacement toward the
base of the partition where the stapes is, and the vibration does
not travel very far along the partition. Low-frequency sounds
travel along the partition towards its apex (end opposite of the
stapes), such that maximal displacement is toward the apical
end of the cochlear partition. Figure 5.6 provides a schematic
depiction of the traveling wave for three different frequencies.
The biomechanical traveling wave, therefore, sorts frequency
according to the location of maximal displacement along the
cochlear partition: High frequencies cause maximal vibration
at the base, low frequencies at the apex, and middle frequen-
cies at intermediate partition locations. Thus, the place of
maximal displacement codes for the frequency content of the
stimulating sound wave. If a sound wave is the sum of two
frequency components, then there will be two locations of
maximal displacement; three frequency components would
generate a maximum of three, and so forth. The hair cells are
distributed along the cochlear partition as if they were sensors
of the cochlear displacement. Thus, different hair cells code
for the frequency content of the incoming sound.