Human Physiology, 14th edition (2016)

288 Chapter 10

However, vibrations of the basilar membrane are damp-

ened by the fluid of the cochlea, and because of this we would

be nearly deaf were it not for the actions of the outer hair cells,

which act as cochlear amplifiers. The outer hair cells are near

the center of the basilar membrane, are more than three times

as numerous as the inner hair cells, and change length: they

get longer when hyperpolarized and shorter when depolarized

by motor neurons. These length changes magnify the effects

of sound on basilar membrane vibrations and inner hair cell

stimulation up to a thousand times. This allows us to hear far

softer sounds than would otherwise be possible, and serves

to significantly sharpen the frequency response of the basilar

membrane, thereby sharpening pitch perception.

Neural Pathways for Hearing

Sensory neurons in the spiral ganglion of each ear send their

axons in the vestibulocochlear nerve (VIII) to one of two

cochlear nuclei in the junction of the medulla and pons of the

brain stem. Neurons in the cochlear nuclei send axons either

directly to the inferior colliculi of the midbrain or to the superior

olive, a collection of brain stem nuclei. Axons from the superior

olive pass through the lateral lemniscus to the inferior collicu-

lus. Whatever the route, all auditory paths synapse in the inferior

colliculus. Neurons in the inferior colliculus then send axons to

the medial geniculate body of the thalamus, which in turn proj-

ects to the auditory cortex of the temporal lobe ( fig. 10.24 ).

The cochlea is a frequency analyzer, in that different fre-

quencies (pitches) of sound stimulate different sensory neurons

stimulates the associated sensory neurons. The K^1 that entered
the hair cells at their apical surface can then move passively out
through channels in their basal surface, which face perilymph
in the scala tympani. Perilymph, as previously mentioned, has a
low K^1 concentration typical of extracellular fluids.
The greater the displacement of the basilar membrane and
the bending of the stereocilia, the greater the amount of trans-
mitter released by the inner hair cell, and therefore the greater
the generator potential produced in the sensory neuron. By this
means, a greater bending of the stereocilia will increase the
frequency of action potentials produced by the fibers of the
cochlear nerve that are stimulated by the hair cells. Experi-
ments suggest that the stereocilia need bend only 0.3 nano-
meters to be detected at the threshold of hearing! A greater
bending will result in a higher frequency of action potentials,
which will be perceived as a louder sound.
As mentioned earlier, traveling waves in the basilar mem-
brane reach a peak in different regions, depending on the pitch
(frequency) of the sound. High-pitched sounds produce a peak
displacement closer to the base, while sounds of lower pitch
cause peak displacement further toward the apex (see figs. 10.21
and 10.23 ). Those neurons that originate in hair cells located
where the displacement is greatest will be stimulated more than
neurons that originate in other regions. This mechanism pro-
vides a neural code for pitch discrimination.
Because the basilar membrane of the cochlear duct is
shaped into a spiral, the base of the cochlea—its first turn—is
where the basilar membrane vibrates in response to sounds of
high frequency (high pitch). By contrast, the smaller apex (top)
of the cochlea is where the basilar membrane vibrates most in
response to low-frequency (low-pitch) sounds. In figure 10.21 ,
a high-frequency sound that is audible to the human ear is rep-
resented as 20,000 Hz (hertz). In figure 10.23 , this is shown as
20 kHz, and very low pitches are shown in the apical portions
of the cochlea as fractions of a kHz; for example, 0.5 kHz is
equivalent to the 500 Hz frequency depicted in figure 10.21.

Figure 10.23 Portions of the cochlea that detect
different frequencies. The numbers represent sound frequencies
in kilohertz (kHz); thus, 20  5  20,000 Hz, and 0.1  5  100 Hz. The
basilar membrane within the organ of Corti (spiral organ) vibrates
maximally at the locations determined by the sound frequency. This
stimulates inner hair cells at those locations, which activate sensory
neurons of cranial nerve VIII that convey action potentials to the
brain. The brain then interprets action potentials from different
regions of the cochlea as sounds of different pitches.

20

10

5

2

1

0.5

0.2

0.1

Figure 10.24 Neural pathways for hearing. These

pathways extend from the spiral organ in the cochlea to the

auditory cortex. The superior olive and lateral lemniscus are not

shown.

Inferior

colliculus

Midbrain

Medulla

oblongata

Vestibulocochlear

nerve From spiral organ

(of Corti)

Cochlear nucleus

Medial geniculate

body of thalamus

Auditory cortex

(temporal lobe)

Thalamus