Ganong's Review of Medical Physiology, 23rd Edition

208 SECTION III Central & Peripheral Neurophysiology
Tip link
K+
Ca2+ FIGURE 13–6 Schematic representation of the role of tip links in the responses of hair cells. When a stereocilium is pushed toward a
taller stereocilium, the tip line is stretched and opens an ion channel in its taller neighbor. The channel next is presumably moved down the taller
stereocilium by a molecular motor, so the tension on the tip link is released. When the hairs return to the resting position, the motor moves back
up the stereocilium. (Modified from Kandel ER, Schwartz JH, Jessel TM [editors]: Principles of Neuroscience, 4th ed. McGraw-Hill, 2000.)
pushed toward the higher, the open time of these channels increases.
K + —the most abundant cation in endolymph—and
Ca 2+ enter via the channel and produce depolarization. There is
still considerable uncertainty about subsequent events. However,
one hypothesis is that a molecular motor in the higher neighbor
next moves the channel toward the base, releasing tension
in the tip link (Figure 13–6). This causes the channel to close
and permits restoration of the resting state. The motor apparently
is myosin-based. Depolarization of hair cells causes them
to release a neurotransmitter, probably glutamate, which initiates
depolarization of neighboring afferent neurons.
The K + that enters hair cells via the mechanically sensitive
cation channels is recycled (Figure 13–7). It enters supporting
Scala vestibuli
Cl− Na
125
+ 150
K + 5
Organ of Corti
Scala tympani
Cl− Na
130
+ 1
K + 150
Cl− Na
125
+ 150
K + 3
SL and
SV
FIGURE 13–7 Ionic composition of perilymph in the scala
vestibuli, endolymph in the scala media, and perilymph in the
scala tympani. SL, spiral ligament. SV, stria vascularis. The dashed arrow
indicates the path by which K + recycles from the hair cells to the
supporting cells to the spiral ligament and is then secreted back into
the endolymph by cells in the stria vascularis.
cells and then passes on to other supporting cells by way of
tight junctions. In the cochlea, it eventually reaches the stria
vascularis and is secreted back into the endolymph, completing
the cycle.
The processes of the hair cells project into the endolymph
whereas the bases are bathed in perilymph. This arrangement
is necessary for the normal production of generator potentials.
The perilymph is formed mainly from plasma. On the
other hand, endolymph is formed in the scala media by the
stria vascularis and has a high concentration of K + and a low
concentration of Na + (Figure 13–7). Cells in the stria vascularis
have a high concentration of Na + –K + pump. In addition,
it appears that a unique electrogenic K + pump in the stria
vascularis accounts for the fact that the scala media is electrically
positive by 85 mV relative to the scala vestibuli and
scala tympani.
HEARING
SOUND WAVES
Myosin
Sound is the sensation produced when longitudinal vibrations
of the molecules in the external environment—that is, alternate
phases of condensation and rarefaction of the molecules—strike
the tympanic membrane. A plot of these
movements as changes in pressure on the tympanic membrane
per unit of time is a series of waves (Figure 13–8); such
movements in the environment are generally called sound
waves. The waves travel through air at a speed of approximately
344 m/s (770 mph) at 20 °C at sea level. The speed of sound
increases with temperature and with altitude. Other media in
which humans occasionally find themselves also conduct
sound waves but at different speeds. For example, the speed of
sound is 1450 m/s at 20 °C in fresh water and is even greater in
salt water. It is said that the whistle of the blue whale is as loud
as 188 decibels and is audible for 500 miles.