CHAPTER 53

UNIT X
e Sense of Hearing

is chapter describes the mechanisms whereby the ear malleus moves, the incus moves with it. e opposite end
receives sound waves, discriminates their frequencies, of the incus articulates with the stem of the stapes, and the
and transmits auditory information into the central ner- faceplate of the stapes lies against the membranous laby-
vous system, where its meaning is deciphered. rinth of the cochlea in the opening of the oval window.
e tip end of the handle of the malleus is attached
to the center of the tympanic membrane, and this point
TYMPANIC MEMBRANE AND THE
of attachment is constantly pulled by the tensor tympani
OSSICULAR SYSTEM
muscle, which keeps the tympanic membrane tensed.
is tension allows sound vibrations on any portion of
CONDUCTION OF SOUND FROM THE
the tympanic membrane to be transmitted to the ossicles,
TYMPANIC MEMBRANE TO THE COCHLEA
which would not occur if the membrane were lax.
Figure 53-1 shows the tympanic membrane (commonly e ossicles of the middle ear are suspended by liga-
called the eardrum) and the ossicles, which conduct sound ments in such a way that the combined malleus and incus
from the tympanic membrane through the middle ear act as a single lever, having its fulcrum approximately at
to the cochlea (the inner ear). Attached to the tympanic the border of the tympanic membrane.
membrane is the handle of the malleus. e malleus is e articulation of the incus with the stapes causes
bound to the incus by minute ligaments, so whenever the the stapes to (1) push forward on the oval window and

OUTER EAR MIDDLE EAR INNER EAR

Malleus Stapes Scala tympani

Incus
Scala vestibuli
Vestibular nerve
Cochlear nerve

Auditory canal
External acoustic
meatus Cochlea

Tensor tympani
Stapedius
muscle
muscle
Tympanic Oval Round
membrane window window

Eustachian tube

Figure 53-1. The outer ear, tympanic membrane, and ossicular system of the middle ear and inner ear.

663
UNIT X The Nervous System: B. The Special Senses

on the cochlear fluid on the other side of window every Another function of the tensor tympani and stapedius
time the tympanic membrane moves inward; and (2) muscles is to decrease a person’s hearing sensitivity to his
pull backward on the fluid every time the malleus moves or her own speech. is effect is activated by collateral
outward. nerve signals transmitted to these muscles at the same
time that the brain activates the voice mechanism.
“Impedance Matching” by the Ossicular System. e
amplitude of movement of the stapes faceplate with each
sound vibration is only three-fourths as much as the ampli- TRANSMISSION OF SOUND THROUGH
tude of the handle of the malleus. erefore, the ossicular BONE
lever system does not increase the movement distance of
Because the inner ear, the cochlea, is embedded in a bony
the stapes, as is commonly believed. Instead, the system ac-
cavity in the temporal bone, called the bony labyrinth, vibra-
tually reduces the distance but increases the force of move-
tions of the entire skull can cause fluid vibrations in the
ment about 1.3 times. In addition, the surface area of the
cochlea. erefore, under appropriate conditions, a tuning
tympanic membrane is about 55 square millimeters, where-
fork or an electronic vibrator placed on any bony protuber-
as the surface area of the stapes averages 3.2 square millim-
ance of the skull, but especially on the mastoid process near
eters. is 17-fold difference times the 1.3-fold ratio of the
the ear, causes the person to hear the sound. However, the
lever system causes about 22 times as much total force to be
energy available even in loud sound in the air is not sufficient
exerted on the fluid of the cochlea as is exerted by the sound
to cause hearing via bone conduction unless a special electro-
waves against the tympanic membrane. Because fluid has
mechanical sound-amplifying device is applied to the bone.
far greater inertia than air does, increased amounts of force
are necessary to cause vibration in the fluid. erefore, the
tympanic membrane and ossicular system provide imped- COCHLEA
ance matching between the sound waves in air and the
sound vibrations in the fluid of the cochlea. e impedance FUNCTIONAL ANATOMY OF THE
matching is about 50% to 75% of perfect for sound frequen- COCHLEA
cies between 300 and 3000 cycles/sec, which allows utiliza-
e cochlea is a system of coiled tubes, shown in
tion of most of the energy in the incoming sound waves.
Figure 53-1 and in cross section in Figure 53-2. It con-
In the absence of the ossicular system and tympanic
sists of three tubes coiled side by side: (1) the scala ves-
membrane, sound waves can still travel directly through
tibuli; (2) the scala media; and (3) the scala tympani. e
the air of the middle ear and enter the cochlea at the oval
scala vestibuli and scala media are separated from each
window. However, the sensitivity for hearing is then 15 to 20
other by Reissner’s membrane (also called the vestibular
decibels less than for ossicular transmission—equivalent to
membrane), shown in Figure 53-2B; the scala tympani
a decrease from a medium to a barely perceptible voice level.
and scala media are separated from each other by the
Attenuation of Sound by Contraction of the Tensor basilar membrane. On the surface of the basilar mem-
Tympani and Stapedius Muscles. When loud sounds brane lies the organ of Corti, which contains a series of
are transmitted through the ossicular system and from electromechanically sensitive cells, the hair cells. ey are
there into the central nervous system, a reflex occurs af- the receptive end organs that generate nerve impulses in
ter a latent period of only 40 to 80 milliseconds to cause response to sound vibrations.
contraction of the stapedius muscle and, to a lesser extent, Figure 53-3 diagrams the functional parts of the
the tensor tympani muscle. e tensor tympani muscle uncoiled cochlea for conduction of sound vibrations.
pulls the handle of the malleus inward while the stape- First, note that Reissner’s membrane is missing from this
dius muscle pulls the stapes outward. ese two forces figure. is membrane is so thin and so easily moved that
oppose each other and thereby cause the entire ossicular it does not obstruct the passage of sound vibrations from
system to develop increased rigidity, thus greatly reducing the scala vestibuli into the scala media. erefore, as far as
the ossicular conduction of low-frequency sound, mainly fluid conduction of sound is concerned, the scala vestibuli
frequencies below 1000 cycles/sec. and scala media are considered to be a single chamber. As
is attenuation reflex can reduce the intensity of discussed later, Reissner’s membrane maintains a special
lower frequency sound transmission by 30 to 40 decibels, kind of fluid in the scala media that is required for normal
which is about the same difference as that between a loud function of the sound-receptive hair cells.
voice and a whisper. e function of this mechanism is Sound vibrations enter the scala vestibuli from the
believed to be twofold—to protect the cochlea from dam- faceplate of the stapes at the oval window. e faceplate
aging vibrations caused by excessively loud sound and to covers this window and is connected with the window’s
mask low-frequency sounds in loud environments. Mask- edges by a loose annular ligament so that it can move
ing usually removes a major share of the background inward and outward with the sound vibrations. Inward
noise and allows a person to concentrate on sounds above movement causes the fluid to move forward in the scala
1000 cycles/sec, where most of the pertinent information vestibuli and scala media, and outward movement causes
in voice communication is transmitted. the fluid to move backward.

664
 Chapter 53 The Sense of Hearing

Helicotrema distal ends, except that the distal ends are embedded in
(connection between scala the loose basilar membrane. Because the fibers are stiff
vestibuli and scala tympani)
Scala vestibuli and free at one end, they can vibrate like the reeds of a
harmonica.
Stria vascularis e lengths of the basilar fibers increase progressively,
beginning at the oval window and going from the base of

UNIT X
Scala
media
the cochlea to the apex, increasing from a length of about
0.04 millimeter near the oval and round windows to 0.5
Organ
of Corti
millimeter at the tip of the cochlea (the “helicotrema”), a
12-fold increase in length.
Scala e diameters of the fibers, however, decrease from
tympani the oval window to the helicotrema, so their overall stiff-
ness decreases more than 100-fold. As a result, the stiff,
Modiolus
short fibers near the oval window of the cochlea vibrate
Spiral best at a very high frequency, whereas the long, limber
ganglion Cochlear nerve fibers near the tip of the cochlea vibrate best at a low
frequency.
A us, high-frequency resonance of the basilar mem-
Reissner's
Tectorial membrane brane occurs near the base, where the sound waves
membrane enter the cochlea through the oval window. However,
low-frequency resonance occurs near the helicotrema,
Stria mainly because of the less stiff fibers but also because of
vascularis Scala
vestibuli Spiral
increased “loading” with extra masses of fluid that must
Scala vibrate along the cochlear tubules.
limbus
media
Spiral
prominence TRANSMISSION OF SOUND WAVES IN
THE COCHLEA—“TRAVELING WAVE”
Organ Scala
tympani When the foot of the stapes moves inward against the oval
of Corti
window, the round window must bulge outward because
Basilar the cochlea is bounded on all sides by bony walls. e ini-
membrane tial effect of a sound wave entering at the oval window is to
Spiral ganglion
B cause the basilar membrane at the base of the cochlea to
Figure 53-2. The cochlea (A) and section through one of the turns bend in the direction of the round window. However, the
of the cochlea (B). elastic tension that is built up in the basilar fibers as they
bend toward the round window initiates a fluid wave that
“travels” along the basilar membrane toward the helico-
Oval Scala vestibuli trema. Figure 53-4A shows movement of a high-frequency
Stapes window and scala media
wave down the basilar membrane, Figure 53-4B shows a
medium-frequency wave, and Figure 53-4C shows a very
low-frequency wave. Movement of the wave along the basi-
lar membrane is comparable to the movement of a pressure
wave along the arterial walls, discussed in Chapter 15; it is
Round Scala Basilar Helicotrema also comparable to a wave that travels along the surface of
window tympani membrane a pond.
Figure 53-3. Movement of fluid in the cochlea after forward thrust Vibration Patterns of the Basilar Membrane for
of the stapes.
Different Sound Frequencies. Note in Figure 53-4 the
different patterns of transmission for sound waves of dif-
Basilar Membrane and Resonance in the Cochlea. ferent frequencies. Each wave is relatively weak at the
e basilar membrane is a fibrous membrane that sepa- outset but becomes strong when it reaches the portion
rates the scala media from the scala tympani. It contains of the basilar membrane that has a natural resonant fre-
20,000 to 30,000 basilar fibers that project from the bony quency equal to the respective sound frequency. At this
center of the cochlea, the modiolus, toward the outer point, the basilar membrane can vibrate back and forth
wall. ese fibers are stiff, elastic, reedlike structures that with such ease that the energy in the wave is dissipated.
are fixed at their basal ends in the central bony structure Consequently, the wave dies at this point and fails to
of the cochlea (the modiolus) but are not fixed at their travel the remaining distance along the basilar membrane.

665
UNIT X The Nervous System: B. The Special Senses

a
c
A High frequency

d
B Medium frequency A

Frequency (cycles/sec)
8000 4000 2000 1000 600 400 200

C Low frequency
0 5 10 15 20 25 30 35
Figure 53-4. “Traveling waves” along the basilar membrane for
high- (A), medium- (B), and low-frequency (C) sounds. B Distance from stapes (millimeters)
Figure 53-5. A, Amplitude pattern of vibration of the basilar mem-
brane for a medium-frequency sound (a–d). B, Amplitude patterns
us, a high-frequency sound wave travels only a short for sounds of frequencies between 200 and 8000 cycles/sec, showing
the points of maximum amplitude on the basilar membrane for the
distance along the basilar membrane before it reaches
different frequencies.
its resonant point and dies, a medium-frequency sound
wave travels about halfway and then dies, and a very low-
frequency sound wave travels the entire distance along whereby the scala tympani and scala vestibuli communi-
the membrane. cate (Figure 53-2).
Another feature of the traveling wave is that it travels e principal method whereby sound frequencies are
fast along the initial portion of the basilar membrane but discriminated from one another is based on the “place” of
becomes progressively slower as it goes farther into the maximum stimulation of the nerve fibers from the organ
cochlea. e cause of this difference is the high coefficient of Corti lying on the basilar membrane, as explained in
of elasticity of the basilar fibers near the oval window and a the next section.
progressively decreasing coefficient farther along the mem-
brane. is rapid initial transmission of the wave allows the FUNCTION OF THE ORGAN OF CORTI
high-frequency sounds to travel far enough into the cochlea
e organ of Corti, shown in Figure 53-2 and Figure
to spread out and separate from one another on the basilar
53-6, is the receptor organ that generates nerve impulses
membrane. Without this rapid initial transmission, all the
in response to vibration of the basilar membrane. Note
high-frequency waves would be bunched together within
that the organ of Corti lies on the surface of the basilar
the first millimeter or so of the basilar membrane, and their
fibers and basilar membrane. e actual sensory recep-
frequencies could not be discriminated.
tors in the organ of Corti are two specialized types of
Vibration Amplitude Pattern of the Basilar nerve cells called hair cells—a single row of internal (or
Membrane. e dashed curves of Figure 53-5A show “inner”) hair cells, numbering about 3500 and measuring
the position of a sound wave on the basilar membrane about 12 micrometers in diameter, and three or four rows
when the stapes is (a) all the way inward, (b) has moved of external (or “outer”) hair cells, numbering about 12,000
back to the neutral point, (c) is all the way outward, and and having diameters of only about 8 micrometers. e
(d) has moved back again to the neutral point but is mov- bases and sides of the hair cells synapse with a network
ing inward. e shaded area around these different waves of cochlear nerve endings. Between 90% and 95% of these
shows the extent of vibration of the basilar membrane endings terminate on the inner hair cells, emphasizing
during a complete vibratory cycle. is is the amplitude their special importance for detection of sound.
pattern of vibration of the basilar membrane for this par- e nerve fibers stimulated by the hair cells lead to the
ticular sound frequency. spiral ganglion of Corti, which lies in the modiolus (cen-
Figure 53-5B shows the amplitude patterns of vibra- ter) of the cochlea. e spiral ganglion neuronal cells send
tion for different frequencies, demonstrating that the axons—a total of about 30,000—into the cochlear nerve
maximum amplitude for sound at 8000 cycles/sec occurs and then into the central nervous system at the level of
near the base of the cochlea, whereas that for frequencies the upper medulla. e relation of the organ of Corti to
less than 200 cycles/sec is all the way at the tip of the basi- the spiral ganglion and to the cochlear nerve is shown in
lar membrane near the helicotrema, the minute opening Figure 53-2.

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 Chapter 53 The Sense of Hearing

Tectorial membrane Tectorial membrane Stereocilia Reticular lamina

Outer hair cells Inner hair cells

Modiolus

UNIT X
Hair cells

A Rods of Corti Basilar fiber

Basilar fiber Tip link protein

Spiral ganglion
K+
Cochlear nerve
Figure 53-6. The organ of Corti, showing especially the hair cells and K+ channel
the tectorial membrane pressing against the projecting hairs. K+

Excitation of the Hair Cells. Note in Figure 53-6 that K+

minute hairs, or stereocilia, project upward from the hair
cells and either touch or are embedded in the surface gel Stereocilia
coating of the tectorial membrane, which lies above the K+
stereocilia in the scala media. ese hair cells are similar
to the hair cells found in the macula and cristae ampul-
laris of the vestibular apparatus, discussed in Chapter 56.
Bending of the hairs in one direction depolarizes the hair
cells, and bending in the opposite direction hyperpolar-
izes them. is in turn excites the auditory nerve fibers
synapsing with their bases. K+ K+ K+ K+
Figure 53-7A shows the mechanism whereby vibra- Membrane
tion of the basilar membrane excites the hair endings. e depolarization
outer ends of the hair cells are fixed tightly in a rigid struc-
ture composed of a flat plate, called the reticular lamina,
supported by triangular rods of Corti, which are attached
tightly to the basilar fibers. e basilar fibers, the rods of
Corti, and the reticular lamina move as a rigid unit.
Ca2+ Ca2+
Upward movement of the basilar fiber rocks the retic-
ular lamina upward and inward toward the modiolus.
en, when the basilar membrane moves downward,
the reticular lamina rocks downward and outward. e
inward and outward motion causes the hairs on the hair
cells to shear back and forth against the tectorial mem- Glutamate
brane. us, the hair cells are excited whenever the basilar Afferent
membrane vibrates. neuron
Spiral
Auditory Signals Are Transmitted Mainly by the ganglion
Inner Hair Cells. Even though there are three to four times
as many outer hair cells as inner hair cells, about 90% of
the auditory nerve fibers are stimulated by the inner cells
rather than by the outer cells. Nonetheless, if the outer B
cells are damaged while the inner cells remain fully func- Figure 53-7. A, Stimulation of the hair cells by to and fro movement
tional, a large amount of hearing loss occurs. erefore, it of the hairs projecting into the gel coating of the tectorial membrane.
has been proposed that the outer hair cells in some way B, Transduction of mechanical energy into neural signals by the hair
control the sensitivity of the inner hair cells at different cells. When the stereocilia are bent in the direction of the longer
ones, K+ channels are opened, causing depolarization, which in turn
sound pitches, a phenomenon called “tuning” of the re-
opens voltage-gated Ca2+ channels. The influx of Ca2+ augments the
ceptor system. In support of this concept, a large number depolarization and elicits release of the excitatory transmitter gluta-
of retrograde nerve fibers pass from the brain stem to the mate, which depolarizes the sensory nerve.

667
UNIT X The Nervous System: B. The Special Senses

vicinity of the outer hair cells. Stimulating these nerve fib- e importance of the endocochlear potential is that the
ers can actually cause shortening of the outer hair cells tops of the hair cells project through the reticular lamina and
and possibly also change their degree of stiffness. ese are bathed by the endolymph of the scala media, whereas per-
effects suggest a retrograde nervous mechanism for con- ilymph bathes the lower bodies of the hair cells. Furthermore,
the hair cells have a negative intracellular potential of −70
trol of the ear’s sensitivity to different sound pitches, acti-
millivolts with respect to the perilymph but −150 millivolts
vated through the outer hair cells.
with respect to the endolymph at their upper surfaces, where
Hair Cell Receptor Potentials and Excitation of Audi- the hairs project through the reticular lamina and into the
tory Nerve Fibers. e stereocilia (i.e., the “hairs” pro- endolymph. It is believed that this high electrical potential at
truding from the ends of the hair cells) are stiff structures the tips of the stereocilia sensitizes the cell an extra amount,
thereby increasing its ability to respond to the slightest sound.
because each has a rigid protein framework. Each hair cell
has about 100 stereocilia on its apical border. ese ste-
DETERMINATION OF SOUND
reocilia become progressively longer on the side of the hair
FREQUENCY—THE “PLACE” PRINCIPLE
cell away from the modiolus. e tops of the shorter ste-
reocilia are attached by thin filaments to the back sides of From earlier discussions in this chapter, it is apparent that
their adjacent longer stereocilia. erefore, whenever the low-frequency sounds cause maximal activation of the basilar
cilia are bent in the direction of the longer ones, the tips of membrane near the apex of the cochlea, and high-frequency
the smaller stereocilia are tugged outward from the surface sounds activate the basilar membrane near the base of the
of the hair cell. is causes a mechanical transduction that cochlea. Intermediate-frequency sounds activate the mem-
opens 200 to 300 cation-conducting channels, allowing for brane at intermediate distances between the two extremes.
the rapid movement of positively charged potassium ions Furthermore, there is spatial organization of the nerve fibers
from the surrounding scala media fluid into the stereocilia, in the cochlear pathway, all the way from the cochlea to the
which causes depolarization of the hair cell membrane (see cerebral cortex. Recording of signals in the auditory tracts of
Figure 53-7B). e depolarization opens voltage-sensitive the brain stem and in the auditory receptive fields of the cere-
calcium channels and causes influx of calcium ions, which bral cortex shows that specific brain neurons are activated
augments the depolarization. Repolarization of the hair by specific sound frequencies. erefore, the major method
cell occurs mainly by exit of potassium ions through cal- used by the nervous system to detect different sound frequen-
cium ion–sensitive potassium channels. cies is to determine the positions along the basilar membrane
us, when the basilar fibers bend toward the scala that are stimulated the most, called the place principle for the
vestibuli, the hair cells depolarize, and in the opposite determination of sound frequency.
direction they hyperpolarize, thereby generating an alter- Referring again to Figure 53-5, one can see that the
nating hair cell receptor potential that, in turn, stimulates distal end of the basilar membrane at the helicotrema is
the cochlear nerve endings that synapse with the bases stimulated by all sound frequencies below 200 cycles/sec.
of the hair cells. It is believed that the rapidly acting neu- erefore, it has been difficult to understand from the place
rotransmitter glutamate is released by the hair cells at principle how one can differentiate between low sound fre-
these synapses during depolarization. quencies in the range of 200 down to 20 cycles/sec. ese
low frequencies have been postulated to be discriminated
Endocochlear Potential. To explain even more fully the mainly by the so-called volley or frequency principle. at
electrical potentials generated by the hair cells, we need to is, low-frequency sounds, from 20 to 1500 to 2000 cycles/
explain another electrical phenomenon called the endo- sec, can cause volleys of nerve impulses synchronized at
cochlear potential. e scala media is filled with a fluid called the same frequencies, and these volleys are transmitted by
endolymph, in contradistinction to the perilymph present in
the cochlear nerve into the cochlear nuclei of the brain. It is
the scala vestibuli and scala tympani. e scala vestibuli and
scala tympani communicate directly with the subarachnoid
further suggested that the cochlear nuclei can distinguish
space around the brain, so the perilymph is almost identical the different frequencies of the volleys. In fact, destruction
to cerebrospinal fluid. Conversely, the endolymph that fills of the entire apical half of the cochlea, which destroys the
the scala media is an entirely different fluid secreted by the basilar membrane where all lower frequency sounds are
stria vascularis, a highly vascular area on the outer wall of normally detected, does not totally eliminate discrimina-
the scala media. Endolymph contains a high concentration tion of the lower frequency sounds.
of potassium and a low concentration of sodium, which is
exactly opposite to the contents of perilymph. DETERMINATION OF LOUDNESS
An electrical potential of about +80 millivolts exists all
the time between endolymph and perilymph, with posi- Loudness is determined by the auditory system in at least
tivity inside the scala media and negativity outside. is three ways.
is called the endocochlear potential, and it is generated by First, as the sound becomes louder, the amplitude
continual secretion of positive potassium ions into the sca- of vibration of the basilar membrane and hair cells also
la media by the stria vascularis. increases so that the hair cells excite the nerve endings at
more rapid rates.

668
 Chapter 53 The Sense of Hearing

Second, as the amplitude of vibration increases, it Vibration Sound

causes more and more of the hair cells on the fringes of 100 Pricking

(0 decibel = 1 dyne/cm2)
80 (in middle ear)
the resonating portion of the basilar membrane to become

Pressure (decibels)
stimulated, thus causing spatial summation of impulses— 60
that is, transmission through many nerve fibers rather 40 Tactual
20 threshold
than through only a few.

UNIT X
ird, the outer hair cells do not become stimulated 0
Threshold
significantly until vibration of the basilar membrane –20
of hearing
–40
reaches high intensity, and stimulation of these cells pre-
–60
sumably apprises the nervous system that the sound is Reference
loud. –80 -
pressure = 73.8
1 2 5 10 20 100 500 2000 10,000
Detection of Changes in Loudness—The Power Law.
Frequency (cycles/sec)
As pointed out in Chapter 47, a person interprets changes
Figure 53-8. Relation of the threshold of hearing and of somesthetic
in intensity of sensory stimuli approximately in propor-
perception (pricking and tactual threshold) to the sound energy level
tion to an inverse power function of the actual intensity. at each sound frequency.
In the case of sound, the interpreted sensation changes
approximately in proportion to the cube root of the ac-
sound pressure level, the sound range is 500 to 5000 cy-
tual sound intensity. To express this concept in another
cles/sec; only with intense sounds can the complete range
way, the ear can discriminate differences in sound inten-
of 20 to 20,000 cycles be achieved. In old age, this fre-
sity from the softest whisper to the loudest possible noise,
quency range is usually shortened to 50 to 8,000 cycles/
representing an approximately 1 trillion times increase
sec or less, as discussed later in this chapter.
in sound energy or 1 million times increase in amplitude
of movement of the basilar membrane. Yet, the ear inter-
prets this much difference in sound level as approximately CENTRAL AUDITORY MECHANISMS
a 10,000-fold change. us, the scale of intensity is greatly
“compressed” by the sound perception mechanisms of the AUDITORY NERVOUS PATHWAYS
auditory system, which allows a person to interpret dif-
Figure 53-9 shows the major auditory pathways. Nerve
ferences in sound intensities over a far wider range than
fibers from the spiral ganglion of Corti enter the dorsal
would be possible were it not for compression of the in-
and ventral cochlear nuclei located in the upper part of the
tensity scale.
medulla. At this point, all the fibers synapse, and second-
Decibel Unit. Because of the extreme changes in sound order neurons pass mainly to the opposite side of the
intensities that the ear can detect and discriminate, sound brain stem to terminate in the superior olivary nucleus. A
intensities are usually expressed in terms of the logarithm few second-order fibers also pass to the superior olivary
of their actual intensities. A 10-fold increase in sound en- nucleus on the same side.
ergy is called 1 bel, and 0.1 bel is called 1 decibel. One From the superior olivary nucleus, the auditory path-
decibel represents an actual increase in sound energy of way passes upward through the lateral lemniscus. Some
1.26 times. of the fibers terminate in the nucleus of the lateral lem-
Another reason for using the decibel system to express niscus, but many fibers bypass this nucleus and travel on
changes in loudness is that in the usual sound intensity to the inferior colliculus, where all or almost all the audi-
range for communication, the ears can barely distinguish tory fibers synapse. From there, the pathway passes to the
an approximately 1-decibel change in sound intensity. medial geniculate nucleus, where all the fibers do synapse.
Finally, the pathway proceeds via auditory radiation to
Threshold for Hearing Sound at Different
the auditory cortex, located mainly in the superior gyrus
Frequencies. Figure 53-8 shows the pressure thresholds
of the temporal lobe.
at which sounds of different frequencies can barely be
Several important points should be noted. First, signals
heard by the ear. is figure demonstrates that a 3,000 cy-
from both ears are transmitted through the pathways of
cles/sec sound can be heard even when its intensity is as
both sides of the brain, with a preponderance of transmis-
low as 70 decibels below 1 dyne/cm2 sound pressure level,
sion in the contralateral pathway. In at least three places
which is one ten-millionth microwatt per square centim-
in the brain stem, crossing over occurs between the two
eter. Conversely, a 100 cycles/sec sound can be detected
pathways: (1) in the trapezoid body; (2) in the commissure
only if its intensity is 10,000 times as great as this.
between the two nuclei of the lateral lemnisci; and (3) in
Frequency Range of Hearing. e frequencies of sound the commissure connecting the two inferior colliculi.
that a young person can hear are between 20 and 20,000 Second, many collateral fibers from the auditory
cycles/sec. However, referring again to Figure 53-8, we tracts pass directly into the reticular activating system of
see that the sound range depends to a great extent on the brain stem. is system projects diffusely upward in
loudness. If the loudness is 60 decibels below 1 dyne/cm2 the brain stem and downward into the spinal cord and

669
UNIT X The Nervous System: B. The Special Senses

Firing Rates at Different Levels of the Auditory

Pathways. Single nerve fibers entering the cochlear nuclei
from the auditory nerve can fire at rates up to at least 1000/
sec, with the rate being determined mainly by the loud-
ness of the sound. At sound frequencies up to 2000 to 4000
cycles/sec, the auditory nerve impulses are often synchro-
nized with the sound waves, but they do not necessarily
occur with every wave.
In the auditory tracts of the brain stem, the firing is
usually no longer synchronized with the sound frequency,
Primary auditory except at sound frequencies below 200 cycles/sec. Above
cortex the level of the inferior colliculi, even this synchronization
Midbrain is mainly lost. ese findings demonstrate that the sound
Medial
geniculate
signals are not transmitted unchanged directly from the ear
nucleus to the higher levels of the brain; instead, information from
(thalamus) the sound signals begins to be dissected from the impulse
traffic at levels as low as the cochlear nuclei. We will have
more to say about this subject later, especially in relation to
Inferior perception of direction from which sound comes.
Midbrain colliculus
FUNCTION OF THE CEREBRAL CORTEX IN
HEARING
e projection area of auditory signals to the cerebral cor-
tex is shown in Figure 53-10, which demonstrates that
Nucleus of the auditory cortex lies principally on the supratempo-
Pons the lateral ral plane of the superior temporal gyrus but also extends
lemniscus
onto the lateral side of the temporal lobe, over much of
the insular cortex, and even onto the lateral portion of the
parietal operculum.
Two separate subdivisions are shown in Figure 53-
Pons 10—the primary auditory cortex and the auditory asso-
ciation cortex (also called the secondary auditory cortex).
Superior
olivary
e primary auditory cortex is directly excited by projec-
Dorsal
acoustic stria nucleus tions from the medial geniculate body, whereas the audi-
Intermediate
tory association areas are excited secondarily by impulses
acoustic site from the primary auditory cortex, as well as by some pro-
Medulla jections from thalamic association areas adjacent to the
Cochlear nuclei medial geniculate body.
Sound Frequency Perception in the Primary
N. VIlI Trapezoid Auditory Cortex. At least six tonotopic maps have been
body described in the primary auditory cortex and auditory
association areas. In each of these maps, high-frequency
sounds excite neurons at one end of the map, whereas
Figure 53-9. Auditory nervous pathways. N., Nerve.
low-frequency sounds excite neurons at the opposite end.
In most maps, the low-frequency sounds are located ante-
activates the entire nervous system in response to loud riorly, as shown in Figure 53-10, and the high-frequency
sounds. Other collaterals go to the vermis of the cerebel- sounds are located posteriorly. is setup is not true for
lum, which is also activated instantaneously in the event all the maps.
of a sudden noise. Why does the auditory cortex have so many different
ird, a high degree of spatial orientation is maintained tonotopic maps? e answer, presumably, is that each of
in the fiber tracts from the cochlea all the way to the cor- the separate areas dissects out some specific feature of
tex. In fact, there are three spatial patterns for termination the sounds. For example, one of the large maps in the pri-
of the different sound frequencies in the cochlear nuclei, mary auditory cortex almost certainly discriminates the
two patterns in the inferior colliculi, one precise pattern sound frequencies and gives the person the psychic sen-
for discrete sound frequencies in the auditory cortex, and sation of sound pitches. Another map is probably used to
at least five other less precise patterns in the auditory cor- detect the direction from which the sound comes. Other
tex and auditory association areas. auditory cortex areas detect special qualities, such as the

670
 Chapter 53 The Sense of Hearing

Low frequency High frequency Indeed, the parietal portion of the auditory association
cortex partly overlaps somatosensory area II, which could
provide an opportunity for the association of auditory
information with somatosensory information.
Discrimination of Sound “Patterns” by the Auditory

UNIT X
Cortex. Complete bilateral removal of the auditory cortex
does not prevent a cat or monkey from detecting sounds
or reacting in a crude manner to sounds. However, it does
greatly reduce or sometimes even abolish the animal’s
ability to discriminate different sound pitches and espe-
cially patterns of sound. For example, an animal that has
Association Primary been trained to recognize a combination or sequence of
tones, one following the other in a particular pattern, los-
es this ability when the auditory cortex is destroyed; fur-
thermore, the animal cannot relearn this type of response.
erefore, the auditory cortex is especially important in
the discrimination of tonal and sequential sound patterns.
Destruction of both primary auditory cortices in the
Association human being greatly reduces one’s sensitivity for hear-
Primary ing. Destruction of one side only slightly reduces hearing
in the opposite ear; it does not cause deafness in the ear
because of many crossover connections from side to side
in the auditory neural pathway. However, it does affect
one’s ability to localize the source of a sound because
comparative signals in both cortices are required for
sound localization.
Figure 53-10. Auditory cortex. Lesions that affect the auditory association areas but
not the primary auditory cortex do not decrease a per-
sudden onset of sounds, or perhaps special modulations,
son’s ability to hear and differentiate sound tones or even
such as noise versus pure frequency sounds.
to interpret at least simple patterns of sound. However,
e frequency range to which each individual neuron
the person is often unable to interpret the meaning of the
in the auditory cortex responds is much narrower than
sound heard. For example, lesions in the posterior portion
that in the cochlear and brain stem relay nuclei. Referring
of the superior temporal gyrus, which is called Wernicke’s
to Figure 53-5B, note that the basilar membrane near the
area and is part of the auditory association cortex, often
base of the cochlea is stimulated by sounds of all frequen-
make it impossible for a person to interpret the meanings
cies and, in the cochlear nuclei, this same breadth of sound
of words even though he or she hears them perfectly well
representation is found. Yet, by the time the excitation
and can even repeat them. ese functions of the auditory
has reached the cerebral cortex, most sound-responsive
association areas and their relation to the overall intellec-
neurons respond only to a narrow range of frequencies
tual functions of the brain are discussed in Chapter 58.
rather than to a broad range. erefore, somewhere along
the pathway, processing mechanisms “sharpen” the fre- DETERMINATION OF THE DIRECTION
quency response. is sharpening effect is believed to be FROM WHICH SOUND COMES
caused mainly by lateral inhibition, discussed in Chapter
47 in relation to mechanisms for transmitting informa- A person determines the horizontal direction from which
tion in nerves. at is, stimulation of the cochlea at one sound comes by two principal means: (1) the time lag
frequency inhibits sound frequencies on both sides of this between the entry of sound into one ear and its entry into
primary frequency; this inhibition is caused by collateral the opposite ear; and (2) the difference between the inten-
fibers angling off the primary signal pathway and exert- sities of the sounds in the two ears.
ing inhibitory influences on adjacent pathways. is same e first mechanism functions best at frequencies
effect is important in sharpening patterns of somesthetic below 3000 cycles/sec, and the second mechanism oper-
images, visual images, and other types of sensations. ates best at higher frequencies because the head is a
Many of the neurons in the auditory cortex, especially greater sound barrier at these frequencies. e time lag
in the auditory association cortex, do not respond only mechanism discriminates direction much more exactly
to specific sound frequencies in the ear. It is believed than the intensity mechanism because it does not depend
that these neurons “associate” different sound frequen- on extraneous factors but only on the exact interval of
cies with one another or associate sound information time between two acoustical signals. If a person is look-
with information from other sensory areas of the cortex. ing straight toward the source of the sound, the sound

671
UNIT X The Nervous System: B. The Special Senses

reaches both ears at exactly the same instant, whereas if is mechanism for detection of sound direction indi-
the right ear is closer to the sound than the left ear is, the cates again how specific information in sensory signals is
sound signals from the right ear enter the brain ahead of dissected out as the signals pass through different levels of
those from the left ear. neuronal activity. In this case, the “quality” of sound direc-
ese two mechanisms cannot tell whether the sound tion is separated from the “quality” of sound tones at the
is emanating from in front of or behind the person or from level of the superior olivary nuclei.
above or below. is discrimination is achieved mainly by
Centrifugal Signals From the Central Nervous
the pinnae (the visible outer part), which act as funnels System to Lower Auditory Centers
to direct the sound into the two ears. e shape of the
pinna changes the quality of the sound entering the ear, Retrograde pathways have been demonstrated at each level
of the auditory nervous system from the brain cortex to the
depending on the direction from which the sound comes.
cochlea in the ear. e final pathway is mainly from the su-
It changes the quality by emphasizing specific sound fre- perior olivary nucleus to the sound-receptor hair cells in
quencies from the different directions. the organ of Corti.
Neural Mechanisms for Detecting Sound Direction. ese retrograde fibers are inhibitory. Indeed, direct
Destruction of the auditory cortex on both sides of the brain stimulation of discrete points in the olivary nucleus has
been shown to inhibit specific areas of the organ of Corti,
causes loss of almost all ability to detect the direction from
reducing their sound sensitivities by 15 to 20 decibels. One
which sound comes. Yet, the neural analyses for this detec- can readily understand how this mechanism could allow
tion process begin in the superior olivary nuclei in the brain someone to direct their attention to sounds of particular
stem, even though the neural pathways all the way from qualities while rejecting sounds of other qualities. is
these nuclei to the cortex are required for interpretation of characteristic is readily demonstrated when one listens to
the signals. e mechanism is believed to be the following. a single instrument in a symphony orchestra.
e superior olivary nucleus is divided into two sec-
Types of Deafness
tions: (1) the medial superior olivary nucleus; and (2) the
lateral superior olivary nucleus. e lateral nucleus is con- Deafness is usually divided into two types: (1) that caused by
cerned with detecting the direction from which the sound impairment of the cochlea, the auditory nerve, or the central
is coming, presumably by simply comparing the difference nervous system circuits from the ear, which is usually classi-
fied as “nerve deafness,” and (2) that caused by impairment
in intensities of the sound reaching the two ears and send-
of the physical structures of the ear that conduct sound itself
ing an appropriate signal to the auditory cortex to esti- to the cochlea, which is usually called “conduction deafness.”
mate the direction. If either the cochlea or the auditory nerve is destroyed,
e medial superior olivary nucleus, however, has a the person becomes permanently deaf. However, if the
specific mechanism for detecting the time lag between cochlea and nerve are still intact but the tympanum-
acoustical signals entering the two ears. is nucleus con- ossicular system has been destroyed or ankylosed (“frozen”
tains large numbers of neurons that have two major den- in place by fibrosis or calcification), sound waves can still be
drites, one projecting to the right and the other to the left. conducted into the cochlea by means of bone conduction
e acoustical signal from the right ear impinges on the from a sound generator applied to the skull over the ear.
right dendrite, and the signal from the left ear impinges Audiometer. To determine the nature of hearing dis-
on the left dendrite. e intensity of excitation of each abilities, an audiometer is used. is instrument is an
neuron is highly sensitive to a specific time lag between earphone connected to an electronic oscillator capable of
emitting pure tones ranging from low frequencies to high
the two acoustical signals from the two ears. e neurons
frequencies, and it is calibrated so that zero-intensity-level
near one border of the nucleus respond maximally to a
sound at each frequency is the loudness that can barely be
short time lag, whereas those near the opposite border heard by the normal ear. A calibrated volume control can
respond to a long time lag; those in between respond to increase the loudness above the zero level. If the loudness
intermediate time lags. must be increased to 30 decibels above normal before it
us, a spatial pattern of neuronal stimulation develops can be heard, the person is said to have a hearing loss of 30
in the medial superior olivary nucleus, with sound from decibels at that particular frequency.
directly in front of the head stimulating one set of olivary In performing a hearing test using an audiometer, one
neurons maximally and sounds from different side angles tests about 8 to 10 frequencies covering the auditory spec-
stimulating other sets of neurons on opposite sides. is trum, and the hearing loss is determined for each of these
spatial orientation of signals is then transmitted to the frequencies. en the so-called audiogram is plotted, as
shown in Figure 53-11 and 53-12, depicting hearing loss
auditory cortex, where sound direction is determined by
at each of the frequencies in the auditory spectrum. e au-
the locus of the maximally stimulated neurons. All these diometer, in addition to being equipped with an earphone
signals for determining sound direction are believed to for testing air conduction by the ear, is equipped with a
be transmitted through a different pathway and excite a mechanical vibrator for testing bone conduction from the
different locus in the cerebral cortex from the transmis- mastoid process of the skull into the cochlea.
sion pathway and termination locus for tonal patterns of Audiogram in Nerve Deafness. In nerve deafness,
sound. which includes damage to the cochlea, the auditory nerve,

672
 Chapter 53 The Sense of Hearing

−10 is essentially normal, but conduction through the ossicular

Normal system is greatly depressed at all frequencies, but more so
10 X X X at low frequencies. In some cases of conduction deafness,
X X
20
Loss (decibels) * * * * * X
the faceplate of the stapes becomes “ankylosed” by bone
overgrowth to the edges of the oval window. Here, the per-
30
40
* X son becomes totally deaf for ossicular conduction but can

UNIT X
regain almost normal hearing by the surgical removal of the
50
stapes and its replacement with a minute Teflon or metal
60
prosthesis that transmits the sound from the incus to the
70
X Air conduction oval window.
80
90 * Bone conduction

100
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673