Ganong's Review of Medical Physiology, 23rd Edition

208
SECTION III
Central & Peripheral Neurophysiology

pushed toward the higher, the open time of these channels in-
creases. 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. Howev-
  er, one hypothesis is that a molecular motor in the higher neigh-
  bor 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 appar-
  ently is myosin-based. Depolarization of hair cells causes them
  to release a neurotransmitter, probably glutamate, which ini-
  tiates 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

  
  

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, complet-

ing 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 poten-

tials. 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 vascu-

laris 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 electri-

cally positive by 85 mV relative to the scala vestibuli and

scala tympani.

HEARING

SOUND WAVES

Sound is the sensation produced when longitudinal vibrations

of the molecules in the external environment—that is, alter-

nate phases of condensation and rarefaction of the mole-

cules—strike the tympanic membrane. A plot of these

movements as changes in pressure on the tympanic mem-

brane 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 approximate-

ly 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.

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.)

Myosin

Ca^2 + K

+

Tip link

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 ar-
row 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.

Organ of Corti

Scala vestibuli

Cl− 125

Na+ 150

K+ 5

Scala tympani

Cl− 130

Na+ 1

K+ 150

SL and

SV

Cl− 125

Na+ 150

K+ 3