Chapter 15
The Special Senses
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15
frequent action potentials as greater loudness. In addition, as
more neurotransmitter is released, more of the 10 or so bipolar
cells connected to a given hair cell are recruited to fire action
potentials.
Localization of Sound
Several brain stem nuclei (most im-
portantly the superior olivary nuclei) help us localize a sound’s
source in space by means of two cues: the
relative intensity
and
the
relative timing
of sound waves reaching the two ears. If the
sound source is directly in front, in back, or overhead, the in-
tensity and timing cues are the same for both ears. However,
when sound comes from one side, it activates the receptors of
the nearer ear slightly earlier and also more vigorously (because
of the greater intensity of the sound waves entering that ear).
Check Your Understanding
12.
Apart from the bony boundaries, which structure separates
the external from the middle ear? Which two (nonbone)
structures separate the middle from the inner ear?
13.
Which structure inside the spiral organ allows us to
differentiate sounds of different pitch?
14.
If the brain stem did not receive input from both ears, what
would you not be able to do?
For answers, see Appendix H.
stem to the outer hair cells via the efferent fibers, which re-
lease neurotransmitters that cause the outer hair cells to
stiffen. Tis dampens the motion of the basilar membrane
and spreads the sound energy over a wider area.
The Auditory Pathway to the Brain
Te ascending auditory pathway transmits auditory informa-
tion primarily from the cochlear receptors (the inner hair cells)
to the cerebral cortex. Impulses generated in the cochlea pass
through the
spiral ganglion
, where the auditory bipolar cells
reside, and along the afferent fibers of the cochlear nerve to the
cochlear nuclei
of the medulla
(Figure 15.32)
.
From there, neurons project to the
superior olivary nucleus
,
which lies at the junction of the medulla and pons. Beyond this,
axons ascend in the
lateral lemniscus
(a fiber tract) to the
infe-
rior colliculus
(auditory reflex center in the midbrain), which
projects to the
medial geniculate nucleus
of the thalamus. Ax-
ons of the thalamic neurons then project to the
primary audi-
tory cortex
, which provides conscious awareness of sound.
Te auditory pathway is unusual because not all of the fibers
from each ear cross over to the other side of the brain. For this
reason, each auditory cortex receives impulses from both ears.
Auditory Processing
If you are at a Broadway musical, the sound of the instruments,
the actors’ voices, rustling of clothing, and closing of doors are
all intermingled in your awareness. Yet, your auditory cortex can
distinguish the separate parts of this auditory jumble. Whenever
the difference between sound wavelengths is sufficient for dis-
crimination, you hear two separate and distinct tones. In fact,
the analytic powers of the auditory cortex are so great that we are
able to pick single instruments out of a whole orchestra.
Cortical processing of sound stimuli is complex. For exam-
ple, certain cortical cells depolarize at the beginning of a par-
ticular tone, and others depolarize when the tone ends. Some
cortical cells depolarize continuously, and others appear to have
high thresholds (low sensitivity), and so on. Here we will con-
centrate on the more straightforward aspects of cortical percep-
tion of pitch, loudness, and sound location.
Perception of Pitch
As we explained, sound waves of different
frequencies activate hair cells in different positions along the
length of the basilar membrane, and impulses from specific hair
cells are interpreted as specific pitches. When a sound is com-
posed of tones of many frequencies, it activates several popula-
tions of cochlear hair cells and cortical cells simultaneously, and
we perceive multiple tones.
Detection of Loudness
Louder sounds cause larger move-
ments of the tympanic membrane, auditory ossicles, and oval
window, and pressure waves of greater amplitude in the fluids
of the cochlea. Tese larger waves in turn cause larger move-
ments of the basilar membrane, larger deflections of the hairs
on the hair cells, and larger graded potentials in the hair cells.
As a result, they release more neurotransmitter and generate
more frequent action potentials. Te brain interprets more
Medial geniculate
nucleus of thalamus
Primary auditory
cortex in temporal lobe
Inferior colliculus
Lateral lemniscus
Superior olivary
nucleus (pons-
medulla junction)
Spiral organ
Bipolar cell
Spiral ganglion
of cochlear nerve
Vestibulocochlear
nerve
Medulla
Midbrain
Cochlear nuclei
Vibrations
Vibrations
Figure 15.32
The auditory pathway.
This simplified diagram
shows only the pathway from the right ear.
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