Sensory Physiology
219
smaller than the tympanic membrane, the force per unit area
(i.e., the pressure) is increased 15 to 20 times. Additional
advantage is gained through the lever action of the middle-
ear bones. The amount of energy transmitted to the inner
ear can be lessened by the contraction of two small skeletal
muscles in the middle ear. The
tensor tympani
attaches to
the malleus and tympanic membrane, and its contraction
dampens their movement. The
stapedius
attaches to the sta-
pes and similarly controls its mobility. These muscles con-
tract refl exively to protect the delicate receptor apparatus of
the inner ear from continuous, loud sounds. They cannot,
however, protect against sudden, intermittent loud sounds,
which is why it is crucial for people to wear ear protection
in environments like a gun fi ring range. These muscles also
contract refl exively when you vocalize to reduce the loudness
of your own voice, and optimize hearing over certain fre-
quency ranges.
The entire system described thus far involves the trans-
mission of sound energy into the cochlea, where the recep-
tor cells are located. The cochlea is almost completely divided
lengthwise by a fl uid-fi lled membranous tube, the
cochlear
duct,
which follows the cochlear spiral (see Figure 7–35) and
contains the sensory receptors of the auditory system. On
either side of the cochlear duct are endolymph-fi lled compart-
ments: the
scala vestibuli,
which is on the side of the cochlear
Temporal
bone
Malleus
Incus
Semicircular canal
Vestibulocochlear nerve
Cochlea
Auditory
(eustachian)
tube
Middle
ear
cavity
Stapes
(in oval
window)
Tympanic
membrane
Pinna
(auricle)
External
auditory
canal
Vestibular branch
Cochlear branch
Figure 7–34
The human ear. In this and the following two fi gures, violet indicates the outer ear, green the middle ear, and blue the inner ear. The malleus,
incus, and stapes are bones and components of the middle ear compartment. Actually, the auditory tube is closed except during pharynx
movements such as swallowing or yawning.
Malleus
Helicotrema
Incus
Stapes at oval window
External
auditory
canal
Tympanic
membrane
Round window
Middle ear cavity
Scala
tympani
Scala
vestibuli
Cochlear
duct
Basilar
membrane
Cochlea
Figure 7–35
Relationship between the middle ear bones and the cochlea.
Movement of the stapes against the membrane covering the oval
window sets up pressure waves in the fl uid-fi lled scala vestibuli.
These waves cause vibration of the cochlear duct and the basilar
membrane. Some of the pressure is transmitted around the
helicotrema directly into the scala tympani. The cochlea is shown
uncoiled for clarity.
Redrawn from Kandel and Schwartz.
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