Ganong's Review of Medical Physiology, 23rd Edition

210
SECTION III
Central & Peripheral Neurophysiology

waves in the fluid of the inner ear (Figure 13–10). The action
of the waves on the organ of Corti generates action potentials
in the nerve fibers.
In response to the pressure changes produced by sound
waves on its external surface, the tympanic membrane moves
in and out. The membrane therefore functions as a
resonator
that reproduces the vibrations of the sound source. It stops
vibrating almost immediately when the sound wave stops
.
The motions of the tympanic membrane are imparted to the
manubrium of the malleus. The malleus rocks on an axis
through the junction of its long and short processes, so that
the short process transmits the vibrations of the manubrium
to the incus. The incus moves in such a way that the vibra-
tions are transmitted to the head of the stapes. Movements of
the head of the stapes swing its foot plate to and fro like a door
hinged at the posterior edge of the oval window. The auditory
ossicles thus function as a lever system that converts the reso-
nant vibrations of the tympanic membrane into movements of
the stapes against the perilymph-filled scala vestibuli of the
cochlea (Figure 13–10). This system increases the sound pres-
sure that arrives at the oval window, because the lever action
of the malleus and incus multiplies the force 1.3 times and the
area of the tympanic membrane is much greater than the area
of the foot plate of the stapes. Some sound energy is lost as a
result of resistance, but it has been calculated that at frequen-
cies below 3000 Hz, 60% of the sound energy incident on the
tympanic membrane is transmitted to the fluid in the cochlea.

TYMPANIC REFLEX

When the middle ear muscles (tensor tympani and stapedius)
contract, they pull the manubrium of the malleus inward and
the footplate of the stapes outward (Figure 13–2). This de-

creases sound transmission. Loud sounds initiate a reflex con-

traction of these muscles called the

tympanic reflex.

Its

function is protective, preventing strong sound waves from

causing excessive stimulation of the auditory receptors. How-

ever, the reaction time for the reflex is 40 to 160 ms, so it does

not protect against brief intense stimulation such as that pro-

duced by gunshots.

BONE & AIR CONDUCTION

Conduction of sound waves to the fluid of the inner ear via the

tympanic membrane and the auditory ossicles, the main path-

way for normal hearing, is called

ossicular conduction.

Sound

waves also initiate vibrations of the secondary tympanic mem-

brane that closes the round window. This process, unimpor-

tant in normal hearing, is

air conduction.

A third type of

conduction,

bone conduction,

is the transmission of vibra-

tions of the bones of the skull to the fluid of the inner ear. Con-

siderable bone conduction occurs when tuning forks or other

vibrating bodies are applied directly to the skull. This route

also plays a role in transmission of extremely loud sounds.

TRAVELING WAVES

The movements of the foot plate of the stapes set up a series of

traveling waves in the perilymph of the scala vestibuli. A dia-

gram of such a wave is shown in Figure 13–11. As the wave

moves up the cochlea, its height increases to a maximum and

then drops off rapidly. The distance from the stapes to this point

of maximum height varies with the frequency of the vibrations

initiating the wave. High-pitched sounds generate waves that

reach maximum height near the base of the cochlea; low-

pitched sounds generate waves that peak near the apex. The

FIGURE 13–10
Schematic representation of the auditory
ossicles and the way their movement translates movements of
the tympanic membrane into a wave in the fluid of the inner ear.
The wave is dissipated at the round window. The movements of the
ossicles, the membranous labyrinth, and the round window are indi-
cated by dashed lines.

Stapes

Oval

window

Reissner's

membrane

Basilar

membrane

Round

window

Auditory tube

Organ

of Corti

Pivot

Malleus

Incus

FIGURE 13–11

Traveling waves. Top:

The solid and the short-

dashed lines represent the wave at two instants of time. The long-

dashed line shows the “envelope” of the wave formed by connecting

the wave peaks at successive instants.

Bottom:

Displacement of the

basilar membrane by the waves generated by stapes vibration of the

frequencies shown at the top of each curve.

22

0102030

24 26

Distance from stapes (mm)

1600 Hz 800 Hz 400 Hz 50 Hz

Distance from stapes (mm)

28 30 32

Relative

amplitude

Displace-

ment of

basilar

membrane