The Special Senses
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
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
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
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?
Which structure inside the spiral organ allows us to
differentiate sounds of different pitch?
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
, where the auditory bipolar cells
reside, and along the aﬀerent ﬁbers of the cochlear nerve to the
of the medulla
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
(a ﬁber tract) to the
(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
, 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.
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
nucleus of thalamus
cortex in temporal lobe
of cochlear nerve
The auditory pathway.
This simpliﬁed diagram
shows only the pathway from the right ear.