Auditory Nerve Pathways
The cochlear branches of the vestibulocochlear nerves enter
the auditory nerve pathways that extend into the medulla
oblongata and proceed through the midbrain to the thala-
mus. From there they pass into the auditory cortices of the
temporal lobes of the cerebrum, where they are interpreted.
On the way, some of these ±
bers cross over, so that impulses
arising from each ear are interpreted on both sides of the
brain. Consequently, damage to a temporal lobe on one side
of the brain is not necessarily accompanied by complete
hearing loss in the ear on that side
g. 12.16)
Table 12.4
summarizes the pathway of vibrations
through the parts of the middle and inner ears. Clinical
Application 12.5 examines types of hearing loss.
Units called
(dB) measure sound intensity as a logarithmic
scale. The decibel scale begins at 0 dB, the intensity of the sound
least perceptible by a normal human ear. A sound of 10 dB is 10
times as intense as the least perceptible sound; a sound of 20 dB
is 100 times as intense; and a sound of 30 dB is 1,000 times as
intense. A whisper has an intensity of about 40 dB, normal conver-
sation measures 60–70 dB, and heavy traF
c produces about 80 dB.
A sound of 120 dB, such as a rock concert, produces discomfort;
and a sound of 140 dB, such as a jet plane at takeo±
, causes pain.
²requent or prolonged exposure to sounds with intensities above
90 dB can cause permanent hearing loss—as many aging rock stars
have discovered.
Describe the outer, middle, and inner ears.
Explain how sound waves are transmitted through the parts of
the ear.
Describe the tympanic re³
Distinguish between the osseous and membranous labyrinths.
Explain the function of the spiral organ.
Auditory cortex
(temporal lobe)
Medial geniculate
body of thalamus
FIGURE 12.16
The auditory nerve pathway extends into the
medulla oblongata, proceeds through the midbrain to the thalamus,
and passes into the auditory cortex of the cerebrum.
Steps in the Generation of Sensory Impulses from the Ear
1. Sound waves enter the external acoustic meatus.
7. A receptor cell depolarizes; its membrane becomes more permeable to
calcium ions.
2. Waves of changing pressures cause the tympanic membrane to
reproduce the vibrations coming from the sound-wave source.
8. In the presence of calcium ions, vesicles at the base of the receptor cell
release neurotransmitter.
3. Auditory ossicles amplify and transmit vibrations to the end of the stapes.
9. Neurotransmitter stimulates the ends of nearby sensory neurons.
4. Movement of the stapes at the oval window transmits vibrations to the
perilymph in the scala vestibuli.
10. Sensory impulses are triggered on ´
bers of the cochlear branch of the
vestibulocochlear nerve.
5. Vibrations pass through the vestibular membrane and enter the
endolymph of the cochlear duct.
11. The auditory cortex of the temporal lobe interprets the sensory impulses.
6. Di±
erent frequencies of vibration in endolymph move speci´
c regions of
the basilar membrane,
stimulating speci´
c sets of receptor cells.
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