The Sense of Hearing pdf - The Nervous System

allows utilization of most of the energy in the incoming sound waves.

perfect for sound frequencies between 300 and 3000 cycles per second, which

of the cochlea. Indeed, the impedance matching is about 50 to 75 per cent of

matching

Therefore, the tympanic membrane and ossicular system provide

membrane. Because fluid has far greater inertia than air does, it is easily under-

averages 3.2 square millimeters. This 17-fold difference times the 1.3-fold ratio

brane is about 55 square millimeters, whereas the surface area of the stapes

movement about 1.3 times. In addition, the surface area of the tympanic mem-

Instead, the system actually reduces the distance but increases the

does not increase the movement distance of the stapes, as is commonly believed.

amplitude of the handle of the malleus. Therefore, the ossicular lever system

The amplitude of movement of the

time the tympanic membrane moves inward, and to pull backward on the fluid

The articulation of the incus with the stapes causes the stapes to push forward

imately at the border of the tympanic membrane.

the combined malleus and incus act as a single lever, having its fulcrum approx-

The ossicles of the middle ear are suspended by ligaments in such a way that

ossicles, which would not be true if the membrane were lax.

which keeps the tympanic membrane tensed. This allows sound

tympani muscle,

panic membrane, and this point of attachment is constantly pulled by the

The tip end of the handle of the malleus is attached to the center of the tym-

oval window

membranous labyrinth

stapes,

ments, so that whenever the malleus moves, the incus moves with it. The oppo-

The malleus is bound to the

malleus.

(the inner ear). Attached to the tympanic membrane

cochlea

ossicles,

eardrum

Figure 52–1 shows the

to the Cochlea

Conduction of Sound from the Tympanic Membrane

Tympanic Membrane and the Ossicular System

the central nervous system, where its meaning is

quencies, and transmits auditory information into

ear receives sound waves, discriminates their fre-

This chapter describes the mechanisms by which the

The Sense of Hearing

651

deciphered.

tympanic membrane (commonly called the

) and

the

which conduct sound from the tympanic membrane through the

middle ear to the
is the handle of the

incus by minute liga-

site end of the incus articulates with the stem of the

and the faceplate of

the stapes lies against the

of the cochlea in the opening

of the

tensor

vibrations on any portion of the tympanic membrane to be transmitted to the

on the oval window and on the cochlear fluid on the other side of window every

every time the malleus moves outward.

“Impedance Matching” by the Ossicular System.

stapes faceplate with each sound vibration is only three fourths as much as the

force of

of the lever system causes about 22 times as much total force to be exerted on
the fluid of the cochlea as is exerted by the sound waves against the tympanic

stood that increased amounts of force are needed to cause vibration in the fluid.

impedance

between the sound waves in air and the sound vibrations in the fluid

First, note that Reissner’s membrane is missing from

uncoiled cochlea for conduction of sound vibrations.

Figure 52–4 diagrams the functional parts of the

vibrations.

They are the receptive end organs

hair cells.

cells, the

basilar membrane.

), shown in Figure 52–3;

Reissner’s membrane

The scala vestibuli and scala media are sepa-

52–3. It consists of three tubes coiled side by side: (1)

Figure 52–1 and in cross section in Figures 52–2 and

The cochlea is a system of coiled tubes, shown in

Functional Anatomy of the Cochlea

to the bone.

the person to hear the sound. However, the energy

especially on the mastoid process near the ear, causes

tor placed on any bony protuberance of the skull, but

priate conditions, a tuning fork or an electronic vibra-

vibrations in the cochlea itself.Therefore, under appro-

labyrinth,

bony cavity in the temporal bone, called the

cochlea,

Because the inner ear, the

Transmission of Sound Through Bone

sensitivity to his or her own speech. This effect is acti-

stapedius muscles is to decrease a person’s hearing

second, where most of the pertinent information

concentrate on sounds above 1000 cycles per

environments. This usually removes a major share

low-frequency sounds in loud

2. To

1. To

between a loud voice and a whisper. The function of

bels, which is about the same difference as that

lower-frequency sound transmission by 30 to 40 deci-

This

cycles per second.

frequency sound, mainly frequencies below 1000

ossicular system to develop increased rigidity, thus

stapedius muscle pulls the stapes outward. These two

The tensor tympani muscle

tensor tympani muscle.

and, to a lesser extent, the

central nervous system, a reflex occurs after a latent

When loud sounds are transmitted

Attenuation of Sound by Contraction of the Tensor Tympani and

at the oval window. However, the sensitivity for

membrane, sound waves can still travel directly

The Nervous System: B. The Special Senses

652

Unit X

In the absence of the ossicular system and tympanic

through the air of the middle ear and enter the cochlea

hearing is then 15 to 20 decibels less than for ossicu-
lar transmission—equivalent to a decrease from a
medium to a barely perceptible voice level.

Stapedius Muscles.

through the ossicular system and from there into the

period of only 40 to 80 milliseconds to cause contrac-
tion of the stapedius muscle

pulls the handle of the malleus inward while the

forces oppose each other and thereby cause the entire

greatly reducing the ossicular conduction of low-

attenuation reflex can reduce the intensity of

this mechanism is believed to be twofold:

protect the cochlea from damaging vibrations

caused by excessively loud sound.

mask

of the background noise and allows a person to

in voice communication is transmitted.

Another function of the tensor tympani and

vated by collateral nerve signals transmitted to these
muscles at the same time that the brain activates the
voice mechanism.

is embedded in a

bony

vibrations of the entire skull can cause fluid

available even in loud sound in the air is not sufficient
to cause hearing via bone conduction unless a special
electromechanical sound-amplifying device is applied

Cochlea

the scala vestibuli, (2) the scala media, and (3) the scala
tympani.
rated from each other by

(also

called the vestibular membrane
the scala tympani and scala media are separated from
each other by the

On the surface of

the basilar membrane lies the organ of Corti, which
contains a series of electromechanically sensitive

that generate nerve impulses in response to sound

Cochlear
nerve

Spiral
ganglion

Cochlea

Oval window

Scala tympani

Stapes

Incus

Malleus

Scala vestibuli

Round
window

Auditory canal

Tympanic membrane

inner ear.

Tympanic membrane, ossicular system of the middle ear, and

Figure 52–1

Basilar

membrane

Spiral organ

of Corti

Vestibular membrane

Scala vestibuli

Stria

vascularis

Scala media

Scala tympani

Cochlear nerve

Spiral ganglion

Spiral

ligament

of the Human Body. Philadelphia: Lea & Febiger, 1948.)

Cochlea. (Redrawn from Gray H, Goss CM [eds]: Gray’s Anatomy

Figure 52–2

must vibrate along the cochlear tubules.

of increased “loading” with extra masses of fluid that

low-

enter the cochlea through the oval window. But

brane occurs near the base, where the sound waves

Thus,

best at a low frequency.

long, limber fibers near the tip of the cochlea vibrate

cochlea vibrate best at a very high frequency, while the

result, the stiff, short fibers near the oval window of the

overall stiffness decreases more than 100-fold. As a

the oval window to the helicotrema, so that their

of the fibers, however,

The

cochlea (the “helicotrema”), a 12-fold increase in

the base of the cochlea to the apex, increasing from a

The

one end, they can vibrate like the reeds of a

membrane. Because the fibers are stiff and free at

olus) but are not fixed at their distal ends, except

toward the outer wall. These fibers are stiff, elastic,

modiolus,

from the bony center of the cochlea, the

contains 20,000 to 30,000

rates the scala media from the scala tympani. It

The

Basilar Membrane and Resonance in the Cochlea.

forward in the scala vestibuli and scala media, and

tions. Inward movement causes the fluid to move

window’s edges by a loose annular ligament so that it

faceplate of the stapes at the oval window. The face-

later in the chapter.)

function of the sound-receptive hair cells, as discussed

Reissner’s membrane is to maintain a special kind of

sidered to be a single chamber. (The importance of

cerned, the scala vestibuli and scala media are con-

Therefore, as far as fluid conduction of sound is con-

this figure. This membrane is so thin and so easily

Chapter 52

The Sense of Hearing

653

moved that it does not obstruct the passage of sound
vibrations from the scala vestibuli into the scala media.

fluid in the scala media that is required for normal

Sound vibrations enter the scala vestibuli from the

plate covers this window and is connected with the

can move inward and outward with the sound vibra-

outward movement causes the fluid to move back-
ward.

basilar membrane is a fibrous membrane that sepa-

basilar fibers that project

reedlike structures that are fixed at their basal ends
in the central bony structure of the cochlea (the modi-

that the distal ends are embedded in the loose basilar

harmonica.

lengths of the basilar fibers increase progres-

sively beginning at the oval window and going from

length of about 0.04 millimeter near the oval and
round windows to 0.5 millimeter at the tip of the

length.

diameters

decrease from

high-frequency resonance of the basilar mem-

frequency resonance occurs near the helicotrema,
mainly because of the less stiff fibers but also because

Scala vestibuli

Tectorial membrane

Scala tympani

Basilar membrane

Organ of Corti

Spiral

prominence

Spiral limbus

Reissner's membrane

Spiral ganglion

Scala media

Stria vascularis

Histology, 11th ed. Philadelphia:

Bloom & Fawcett: A Textbook of

Colard Keene. From Fawcett DW:

Section through one of the turns

Figure 52–3

of the cochlea. (Drawn by Sylvia

WB Saunders, 1986.)

Scala vestibuli

and scala media

Oval

window

Basilar

membrane

Scala

tympani

Round

window

Stapes

Helicotrema

Movement of fluid in the cochlea after forward thrust of the stapes.

Figure 52–4

tion for different frequencies, demonstrating that the

Figure 52–6

frequency.

cycle. This is the

point but is moving inward. The shaded area around

outward, and (d) has moved back again to the neutral

moved back to the neutral point, (c) is all the way

when the stapes (a) is all the way inward, (b) has

The dashed curves of Figure 52–6

Amplitude Pattern of Vibration of the Basilar Mem-

be discriminated from one another.

the basilar membrane, and their frequencies could not

Without this, all the high-frequency waves would be

separate from one another on the basilar membrane.

mission of the wave allows the high-frequency sounds

farther along the membrane. This rapid initial trans-

farther into the cochlea. The cause of this is the high

travels the entire distance along the membrane.

and then dies, and a very low frequency sound wave

medium-frequency sound wave travels about halfway

before it reaches its resonant point and dies, a

membrane. Thus, a high-frequency sound wave travels

sipated. Consequently, the wave dies at this point and

this point, the basilar membrane can vibrate back and

quency equal to the respective sound frequency. At

frequencies. Each wave is relatively weak at the outset

Note in Figure 52–5 the different

rial walls, which is discussed in Chapter 15; it is also

a very low frequency wave. Movement

Figure 52–5

a medium-frequency wave; and

brane; Figure 52–5

ment of a high-frequency wave down the basilar mem-

as shown in Figure 52–5. Figure 52–5

round window initiates a fluid wave that “travels”

round window. However, the elastic tension that is

walls. The initial effect of a sound wave entering at the

window, the

oval

When the foot of the stapes moves inward against the

Cochlea—“Traveling Wave”

Transmission of Sound Waves in the

The Nervous System: B. The Special Senses

654

Unit X

round window must bulge outward

because the cochlea is bounded on all sides by bony

oval window is to cause the basilar membrane at
the base of the cochlea to bend in the direction of the

built up in the basilar fibers as they bend toward the

along the basilar membrane toward the helicotrema,

A shows move-

of the wave along the basilar membrane is compara-
ble to the movement of a pressure wave along the arte-

comparable to a wave that travels along the surface of
a pond.

Pattern of Vibration of the Basilar Membrane for Different
Sound Frequencies.

patterns of transmission for sound waves of different

but becomes strong when it reaches that portion of the
basilar membrane that has a natural resonant fre-

forth with such ease that the energy in the wave is dis-

fails to travel the remaining distance along the basilar

only a short distance along the basilar membrane

Another feature of the traveling wave is that it

travels fast along the initial portion of the basilar
membrane but becomes progressively slower as it goes

coefficient of elasticity of the basilar fibers near the
oval window and a progressively decreasing coefficient

to travel far enough into the cochlea to spread out and

bunched together within the first millimeter or so of

brane.

A show the

position of a sound wave on the basilar membrane

these different waves shows the extent of vibration of
the basilar membrane during a complete vibratory

amplitude pattern of vibration of

the basilar membrane for this particular sound

B shows the amplitude patterns of vibra-

High frequency

Medium frequency

Low frequency

medium-, and low-frequency sounds.

“Traveling waves” along the basilar membrane for high-,

Figure 52–5

4000

2000

1000

600

400

200

Distance from stapes (millimeters)

8000

Frequency

different frequencies.

frequencies between 200 and 8000 cycles per second, showing

Amplitude patterns for sounds of

a medium-frequency sound.

Amplitude pattern of vibration of the basilar membrane for

Figure 52–6

the points of maximum amplitude on the basilar membrane for the

of the inner hair cells at different sound pitches, a

loss occurs. Therefore, it has been proposed that the

remain fully functional, a large amount of hearing

cells rather than by the outer cells. Yet, despite this, if

outer hair cells as inner hair cells, about 90 per cent of

Auditory Signals Are Transmitted Mainly by the Inner Hair Cells.

whenever the basilar membrane vibrates.

the tectorial membrane. Thus, the hair cells are excited

The inward and outward motion causes the

outward.

ward, the reticular lamina rocks downward and

olus. Then, when the basilar membrane moves down-

inward

basilar fibers, the rods of Corti, and the reticular

which are attached tightly to the basilar fibers. The

rigid structure composed of a flat plate, called the

The outer ends of the hair cells are fixed tightly in a

tion of the basilar membrane excites the hair endings.

Figure 52–8 shows the mechanism by which vibra-

ing with their bases.

This in turn excites the auditory nerve fibers synaps-

hairs in one direction depolarizes the hair cells, and

which are discussed in Chapter 55. Bending of the

and cristae ampullaris of the vestibular apparatus,

lies above the stereocilia in the scala media. These hair

tectorial membrane,

minute hairs, or

Note in Figure 52–7 that

cochlear nerve is shown in Figure 52–2.

system at the level of the upper medulla. The relation

cochlear nerve

(center) of the cochlea. The spiral ganglion neuronal

The nerve fibers stimulated by the hair cells lead to

endings terminate on the inner hair cells, which

nerve endings. Between 90 and 95 per cent of these

of only about 8 micrometers. The bases and sides of

cells,

ter, and three or four rows of

hair cells,

of the basilar fibers and basilar membrane. The actual

brane. Note that the organ of Corti lies on the surface

52–7, is the receptor organ that generates nerve

The organ of Corti, shown in Figures 52–2, 52–3, and

from the organ of Corti lying on the basilar membrane,

“place” of maximum stimulation of the nerve fibers

The principal method by which sound frequencies

helicotrema, where the scala vestibuli opens into the

that for frequencies less than 200 cycles per second is

second occurs near the base of the cochlea, whereas

maximum amplitude for sound at 8000 cycles per

Chapter 52

The Sense of Hearing

655

all the way at the tip of the basilar membrane near the

scala tympani.

are discriminated from one another is based on the

as explained in the next section.

Function of the Organ of Corti

impulses in response to vibration of the basilar mem-

sensory receptors in the organ of Corti are two spe-
cialized types of nerve cells called hair cells—a single
row of internal (or “inner”)

numbering about

3500 and measuring about 12 micrometers in diame-

external (or “outer”) hair

numbering about 12,000 and having diameters

the hair cells synapse with a network of cochlea

emphasizes their special importance for the detection
of sound.

the spiral ganglion of Corti, which lies in the modiolus

cells send axons—a total of about 30,000—into the

and then into the central nervous

of the organ of Corti to the spiral ganglion and to the

Excitation of the Hair Cells.

stereocilia, project upward from the

hair cells and either touch or are embedded in the
surface gel coating of the

which

cells are similar to the hair cells found in the macula

bending in the opposite direction hyperpolarizes them.

retic-

ular lamina, supported by triangular rods of Corti,

lamina move as a rigid unit.

Upward movement of the basilar fiber rocks the

reticular lamina upward and

toward the modi-

hairs on the hair cells to shear back and forth against

Even though there are three to four times as many

the auditory nerve fibers are stimulated by the inner

the outer cells are damaged while the inner cells

outer hair cells in some way control the sensitivity

Tectorial membrane

Basilar fiber

Outer hair cells

Inner hair cells

Spiral ganglion

Cochlear nerve

membrane pressing against the projecting hairs.

Organ of Corti, showing especially the hair cells and the tectorial

Figure 52–7

Hairs

Tectorial membrane

Basilar fiber

Reticular lamina

Rods of Corti

Modiolus

projecting into the gel coating of the tectorial membrane.

Stimulation of the hair cells by to-and-fro movement of the hairs

Figure 52–8

at more rapid rates.

increases, so that the hair cells excite the nerve endings

First, as the sound becomes louder, the amplitude of

least three ways.

Determination of Loudness

tion of the lower-frequency sounds.

mally detected, does not totally eliminate discrimina-

membrane where all lower-frequency sounds are nor-

apical half of the cochlea, which destroys the basilar

cies of the volleys. In fact, destruction of the entire

nuclei of the brain. It is further suggested that the

nized at the same frequencies, and these volleys are

second, can cause volleys of nerve impulses synchro-

frequency sounds, from 20 to 1500 to 2000 cycles per

That is, low-

frequency principle.

range from 200 down to 20. It is postulated that these

200 cycles per second. Therefore, it has been difficult

Yet, referring again to Figure 52–6, one can see that

frequency.

membrane that are most stimulated. This is called

frequencies. Therefore, the

cochlea to the cerebral cortex. Recording of signals in

fibers in the cochlear pathway, all the way from the

Furthermore, there is spatial organization of the nerve

mediate distances between the two extremes.

frequency sounds activate the membrane at inter-

brane near the base of the cochlea. Intermediate-

and high-frequency sounds activate the basilar mem-

that low-frequency sounds cause maximal activation

From earlier discussions in this chapter, it is apparent

Principle

extra amount, thereby increasing its ability to respond

endolymph. It is believed that this high electrical poten-

the hair cells. Furthermore, the hair cells have a nega-

media, whereas perilymph bathes the lower bodies of

The importance of the endocochlear potential is that

into the scala media by the stria vascularis.

endocochlear potential,

This is called the

positivity inside the scala media and negativity outside.

all the time between endolymph and perilymph, with

of sodium, which is exactly opposite to the contents of

wall of the scala media. Endolymph contains a high

stria vascularis,

brospinal fluid. Conversely, the endolymph that fills

The scala vestibuli and scala tympani communicate

The scala media is filled with a

endocochlear potential:

electrical potentials generated by the hair cells, we need

To explain even more fully the

sible that the transmitter substance is glutamate, but

cells at these synapses during depolarization. It is pos-

with the bases of the hair cells. It is believed that a

alternating hair cell receptor potential. This, in turn,

direction they hyperpolarize, thereby generating an

vestibuli, the hair cells depolarize, and in the opposite

Thus, when the basilar fibers bend toward the scala

ization of the hair cell membrane.

media fluid into the stereocilia, which causes depolar-

channels, allowing rapid movement of positively

the surface of the hair cell. This causes a mechanical

cilia are bent in the direction of the longer ones, the

adjacent longer stereocilia. Therefore, whenever the

the modiolus, and the tops of the shorter stereocilia are

100 stereocilia on its apical border. These become pro-

has a rigid protein framework. Each hair cell has about

The stereocilia (the “hairs” protruding from the

Hair Cell Receptor Potentials and Excitation of Auditory Nerve

pitches, activated through the outer hair cells.

for control of the ear’s sensitivity to different sound

These effects suggest a retrograde nervous mechanism

cells and possibly also change their degree of stiffness.

vicinity of the outer hair cells. Stimulating these nerve

In support of this concept, a large number of retro-

phenomenon called “tuning” of the receptor system.

The Nervous System: B. The Special Senses

656

Unit X

grade nerve fibers pass from the brain stem to the

fibers can actually cause shortening of the outer hair

Fibers.

ends of the hair cells) are stiff structures because each

gressively longer on the side of the hair cell away from

attached by thin filaments to the back sides of their

tips of the smaller stereocilia are tugged outward from

transduction that opens 200 to 300 cation-conducting

charged potassium ions from the surrounding scala

stimulates the cochlear nerve endings that synapse

rapidly acting neurotransmitter is released by the hair

this is not certain.

Endocochlear Potential.

to explain another electrical phenomenon called the

fluid called endolymph, in contradistinction to the peri-
lymph present in the scala vestibuli and scala tympani.

directly with the subarachnoid space around the brain,
so that the perilymph is almost identical with cere-

the scala media is an entirely different fluid secreted by
the

a highly vascular area on the outer

concentration of potassium and a low concentration

perilymph.

An electrical potential of about +80 millivolts exists

and it is gen-

erated by continual secretion of positive potassium ions

the tops of the hair cells project through the reticular
lamina and are bathed by the endolymph of the scala

tive intracellular potential of –70 millivolts with respect
to the perilymph but –150 millivolts with respect to the
endolymph at their upper surfaces where the hairs
project through the reticular lamina and into the

tial at the tips of the stereocilia sensitizes the cell an

to the slightest sound.

Determination of Sound Frequency—
The “Place”

of the basilar membrane near the apex of the cochlea,

the auditory tracts of the brain stem and in the audi-
tory receptive fields of the cerebral cortex shows that
specific brain neurons are activated by specific sound

major method used by

the nervous system to detect different sound frequen-
cies is to determine the positions along the basilar

the place principle for the determination of sound

the distal end of the basilar membrane at the heli-
cotrema is stimulated by all sound frequencies below

to understand from the place principle how one can
differentiate between low sound frequencies in the

low frequencies are discriminated mainly by the so-
called volley or

transmitted by the cochlear nerve into the cochlear

cochlear nuclei can distinguish the different frequen-

Loudness is determined by the auditory system in at

vibration of the basilar membrane and hair cells also

body, (2) in the commissure between the two nuclei of

occurs between the two pathways: (1) in the trapezoid

In at least three places in the brain stem, crossing over

derance of transmission in the contralateral pathway.

pathways of both sides of the brain, with a prepon-

Several important points should be noted. First,

gyrus of the temporal lobe.

where all the fibers do synapse. Finally, the

nucleus,

there, the pathway passes to the

all or almost all the auditory fibers synapse. From

nucleus and travel on to the inferior colliculus, where

nucleus of the lateral lemniscus,

lateral lemniscus.

on the same side. From the superior olivary nucleus,

superior olivary nucleus.

all the fibers synapse, and second-order neurons pass

located in the upper part of the medulla. At this point,

ventral cochlear nuclei

Figure 52–10 shows the major auditory pathways. It

Central Auditory Mechanisms

the chapter.

50 to 8000 cycles per second or less, as discussed later in

In old age, this frequency range is usually shortened to

the complete range of 20 to 20,000 cycles be achieved.

to 5000 cycles per second; only with intense sounds can

sound pressure level, the sound range is 500

extent on loudness. If the loudness is 60 decibels below

52–9, we see that the sound range depends to a great

cycles per second. However, referring again to Figure

The frequencies of sound that

as this.

timeter. Conversely, a 100-cycle-per-second sound can

This figure demonstrates that a 3000-cycle-per-second

different frequencies can barely be heard by the ear.

Figure

sound intensity.

change

intensity range for communication, the ears can barely

express changes in loudness is that, in the usual sound

sound energy of 1.26 times.

and 0.1 bel is called 1

logarithm of their actual intensities. A 10-fold increase

intensities that the ear can detect and discriminate,

compression of the intensity scale.

system. This allows a person to interpret differences in

Thus, the scale of intensity is greatly “compressed” by

in sound level as approximately a 10,000-fold change.

membrane. Yet the ear interprets this much difference

approximately 1 trillion

possible noise, representing an

another way, the ear can discriminate differences in

root of the actual sound intensity. To express this

intensity. In the case of sound, the interpreted sensa-

pointed out in Chapter 46, a person interprets changes

Detection of Changes in Loudness—The Power Law.

reaches high intensity, and stimulation of these cells

Third, the outer hair cells do not become stimulated

fibers rather than through only a few.

impulses—that is, transmission through many nerve

become stimulated, thus causing

Second, as the amplitude of vibration increases, it

Chapter 52

The Sense of Hearing

657

causes more and more of the hair cells on the fringes
of the resonating portion of the basilar membrane to

spatial summation of

significantly until vibration of the basilar membrane

presumably apprises the nervous system that the
sound is loud.

in intensity of sensory stimuli approximately in pro-
portion to an inverse power function of the actual

tion changes approximately in proportion to the cube

sound intensity from the softest whisper to the loudest

times increase in sound energy or 1 million times
increase in amplitude of movement of the basilar

the sound perception mechanisms of the auditory

sound intensities over an extremely wide range—a far
wider range than would be possible were it not for

Decibel Unit.

Because of the extreme changes in sound

sound intensities are usually expressed in terms of the

in sound energy is called 1 bel,
decibel. One decibel represents an actual increase in

Another reason for using the decibel system to

distinguish an approximately 1-decibel

Threshold for Hearing Sound at Different Frequencies.

52–9 shows the pressure thresholds at which sounds of

sound can be heard even when its intensity is as low as
70 decibels below 1 dyne/cm

sound pressure level,

which is one ten-millionth microwatt per square cen-

be detected only if its intensity is 10,000 times as great

Frequency Range of Hearing.

a young person can hear are between 20 and 20,000

1 dyne/cm

Auditory Nervous Pathways

shows that nerve fibers from the spiral ganglion of
Corti enter the dorsal and

mainly to the opposite side of the brain stem to ter-
minate in the

A few second-

order fibers also pass to the superior olivary nucleus

the auditory pathway passes upward through the

Some of the fibers terminate in the

but many bypass this

medial geniculate

pathway proceeds by way of the auditory radiation to
the auditory cortex, located mainly in the superior

signals from both ears are transmitted through the

5 10 20

100

500 2000 10,000

Vibration

Sound

Pressure in db

(0 decibel = 1 dyne/cm

)

Frequency

1 2

– 80

–60

–40

–20

100

Pricking
(in middle ear)

Tactual

threshold

Threshold
of hearing

Reference

pressure =

73.8

each sound frequency.

tion (pricking and tactual threshold) to the sound energy level at

Relation of the threshold of hearing and of somesthetic percep-

Figure 52–9

sounds are located posteriorly. This is not true for all

orly, as shown in Figure 52–11, and the high-frequency

In most, the low-frequency sounds are located anteri-

frequency sounds excite neurons at the opposite end.

excite neurons at one end of the map, whereas low-

areas. In each of these maps, high-frequency sounds

Sound Frequency Perception in the Primary Auditory Cortex.

body.

by projections from the medial geniculate body,

). The primary auditory cortex is directly excited

52–11: the

Two separate subdivisions are shown in Figure

, and even onto the

lateral side of the temporal lobe,

cortex is shown in Figure 52–11, which demonstrates

The projection area of auditory signals to the cerebral

in Hearing

Function of the Cerebral Cortex

direction from which sound comes.

about this later, especially in relation to perception of

as low as the cochlear nuclei. We will have more to say

the brain; instead, information from the sound signals

this synchronization is mainly lost. These findings

per second.Above the level of the inferior colliculi, even

quency, except at sound frequencies below 200 cycles

In the auditory tracts of the brain stem, the firing is

not necessarily occur with every wave.

often synchronized with the sound waves, but they do

4000 cycles per second, the auditory nerve impulses are

ness of the sound. At sound frequencies up to 2000 to

second, the rate being determined mainly by the loud-

Firing Rates at Different Levels of the Auditory Pathways.

tion areas.

the auditory cortex, and

one precise pattern

two patterns

to the cortex. In fact, there are

Third, a high degree of spatial orientation is main-

taneously in the event of a sudden noise.

response to loud sounds. Other collaterals go to the

This system projects diffusely

Second, many collateral fibers from the auditory

the lateral lemnisci, and (3) in the commissure con-

The Nervous System: B. The Special Senses

658

Unit X

necting the two inferior colliculi.

tracts pass directly into the reticular activating system
of the brain stem.
upward in the brain stem and downward into the
spinal cord and activates the entire nervous system in

vermis of the cerebellum, which is also activated instan-

tained in the fiber tracts from the cochlea all the way

three spatial patterns for

termination of the different sound frequencies in the
cochlear nuclei,

in the inferior colliculi,

for discrete sound frequencies in

at least five other less precise

patterns in the auditory cortex and auditory associa-

Single

nerve fibers entering the cochlear nuclei from the audi-
tory nerve can fire at rates up to at least 1000 per

usually no longer synchronized with the sound fre-

demonstrate that the sound signals are not transmitted
unchanged directly from the ear to the higher levels of

begins to be dissected from the impulse traffic at levels

that the auditory cortex lies principally on the
supratemporal plane of the superior temporal gyrus but
also extends onto the
over much of the insular cortex
lateral portion of the parietal operculum.

primary auditory cortex and the auditory

association cortex (also called the secondary auditory
cortex

whereas the auditory association areas are excited
secondarily by impulses from the primary auditory
cortex as well as by some projections from thalamic
association areas adjacent to the medial geniculate

At least six tonotopic maps have been found in the
primary auditory cortex and auditory association

Primary
auditry
cortex

Medial
geniculate
nucleus

Inferior
colliculus

Nucleus of
the lateral
lemniscus

Superior
olivary
nuclei

Intermediate
acoustic
site

Medulla

Trapezoid
body

Midbrain

Pons

Dorsal

acoustic

stria

Cochlear

nuclei

N. VIlI

3rd ed. New York: Oxford University Press, 1981.)

system. In Neurological Anatomy in Relation to Clinical Medicine,

Auditory nervous pathways. (Modified from Brodal A: The auditory

Figure 52–10

Chapter 57.

repeat them. These functions of the auditory associa-

tory association cortex, often make it impossible for a

which is called Wernicke’s area and is part of the audi-

the posterior portion of the superior temporal gyrus,

of the sound heard. For instance, lesions in

However, he or she is often unable to interpret the

person’s ability to hear and differentiate sound tones

localize the source of a sound, because comparative

pathway. However, it does affect one’s ability to

reduces hearing in the opposite ear; it does not cause

for hearing. Destruction of one side only slightly

the human being greatly reduces one’s sensitivity

patterns.

Therefore, the auditory cortex is especially important

more, the animal cannot relearn this type of response.

ability when the auditory cortex is destroyed; further-

lowing the other in a particular pattern, loses this

ognize a combination or sequence of tones, one fol-

For instance, an animal that has been trained to rec-

even abolish the animal’s ability to discriminate dif-

However, it does greatly reduce or sometimes

sounds or reacting in a crude manner to sounds.

Discrimination of Sound “Patterns” by the Auditory Cortex.

sory area II, which could provide an easy opportunity

areas of the cortex. Indeed, the parietal portion of the

believed that these neurons “associate” different

only to specific sound frequencies in the ear. It is

sensations.

somesthetic images, visual images, and other types of

adjacent pathways. The same effect has been demon-

on both sides of this primary frequency; this is

cochlea at one frequency inhibits sound frequencies

ting information in nerves. That is, stimulation of the

nomenon of lateral inhibition, which is discussed

“sharpen” the frequency response. It is believed that

where along the pathway, processing mechanisms

cies rather than to a broad range. Therefore, some-

reached the cerebral cortex, most sound-responsive

tation is found. Yet, by the time the excitation has

cochlear nuclei, this same breadth of sound represen-

stimulated by sounds of all frequencies, and in the

relay nuclei. Referring back to Figure 52–6

The frequency range to which each individual

pure frequency sounds.

or perhaps special modulations, such as noise versus

special qualities, such as the sudden onset of sounds,

sound comes. Other auditory cortex areas detect

the psychic sensation of sound pitches. Another map

sounds. For instance, one of the large maps in the

maps? The answer, presumably, is that each of the sep-

the maps. The question one must ask is, Why does the

Chapter 52

The Sense of Hearing

659

auditory cortex have so many different tonotopic

arate areas dissects out some specific feature of the

primary auditory cortex almost certainly discriminates
the sound frequencies themselves and gives the person

is probably used to detect the direction from which the

neuron in the auditory cortex responds is much
narrower than that in the cochlear and brain stem

B, note that

the basilar membrane near the base of the cochlea is

neurons respond to only a narrow range of frequen-

this sharpening effect is caused mainly by the phe-

in Chapter 46 in relation to mechanisms for transmit-

caused by collateral fibers angling off the primary
signal pathway and exerting inhibitory influences on

strated to be important in sharpening patterns of

Many of the neurons in the auditory cortex, espe-

cially in the auditory association cortex, do not respond

sound frequencies with one another or associate sound
information with information from other sensory

auditory association cortex partly overlaps somatosen-

for the association of auditory information with
somatosensory information.

Complete bilateral removal of the auditory cortex
does not prevent a cat or monkey from detecting

ferent sound pitches and especially patterns of sound.

in the discrimination of tonal and sequential sound

Destruction of both primary auditory cortices in

deafness in the ear because of many crossover con-
nections from side to side in the auditory neural

signals in both cortices are required for the localiza-
tion function.

Lesions that affect the auditory association areas

but not the primary auditory cortex do not decrease a

or even to interpret at least simple patterns of sound.

meaning

person to interpret the meanings of words even though
he or she hears them perfectly well and can even

tion areas and their relation to the overall intellectual
functions of the brain are discussed in more detail in

Low

frequency

High
frequency

Primary

Association

Auditory cortex.

Figure 52–11

abilities, the “audiometer” is used. Simply an earphone

To determine the nature of hearing dis-

Audiometer.

applied to the skull over the ear.

ankylosed (“frozen” in place by fibrosis or calcification),

However, if the cochlea and nerve are still intact but the

destroyed, the person becomes permanently deaf.

deafness.”

itself to the cochlea, which is usually called “conduction

deafness,” and (2) that caused by impairment of the

the auditory nerve, which is usually classified as “nerve

Deafness is usually divided into two types: (1) that

Types of Deafness

Hearing Abnormalities

ties. This is readily demonstrated when one listens to a

bels. One can readily understand how this could allow

Corti, reducing their sound sensitivities 15 to 20 deci-

These retrograde fibers are inhibitory. Indeed, direct

the cochlea in the ear itself. The final pathway is mainly

Central Nervous System to Lower

Centrifugal Signals from the

“quality” of sound tones at the level of the superior

“quality” of sound direction is separated from the

different levels of neuronal activity. In this case, the

This mechanism for detection of sound direction

stimulated neurons. It is believed that all these signals

then transmitted to the auditory cortex, where sound

on opposite sides. This spatial orientation of signals is

develops in the medial superior olivary nucleus, with

lags. Thus, a spatial pattern of neuronal stimulation

lag; those in between respond to intermediate time

nucleus respond maximally to a short time lag, while

from the two ears. The neurons near one border of the

the left ear impinges on the left dendrite. The intensity

ear impinges on the right dendrite, and the signal from

other to the left. The acoustical signal from the right

major dendrites, one projecting to the right and the

This nucleus

acoustical signals entering the two ears.

however, has a

medial superior olivary nucleus,

The

the sound is coming, presumably by simply comparing

The lateral nucleus

lateral superior olivary nucleus.

tions: (1) the

The superior olivary nucleus is divided into two sec-

believed to be the following.

for interpretation of the signals. The mechanism is

in the brain stem, even though the neural pathways all

which sound comes. Yet the neural analyses for this

whether in human beings or in lower mammals, causes

ferent directions.

tion from which the sound comes. It does this by

of the sound entering the ear, depending on the direc-

two ears. The shape of the pinna changes the

behind the person or from above or below. This dis-

The two aforementioned mechanisms cannot tell

enter the brain ahead of those from the left ear.

than the left ear is, the sound signals from the right ear

instant, whereas if the right ear is closer to the sound

sound, the sound reaches both ears at exactly the same

exact interval of time between two acoustical signals.

The time lag mechanism discriminates direction much

head is a greater sound barrier at these frequencies.

below 3000 cycles per second, and the second mecha-

The first mechanism functions best at frequencies

between the intensities of the sounds in the two ears.

its entry into the opposite ear, and (2) the difference

which sound comes by two principal means: (1) the

Determination of the Direction from

The Nervous System: B. The Special Senses

660

Unit X

Which Sound Comes

A person determines the horizontal direction from

time lag between the entry of sound into one ear and

nism operates best at higher frequencies because the

more exactly than the intensity mechanism because it
does not depend on extraneous factors but only on the

If a person is looking straight toward the source of the

whether the sound is emanating from in front of or

crimination is achieved mainly by the pinnae of the

quality

emphasizing specific sound frequencies from the dif-

Neural Mechanisms for Detecting Sound Direction.

Destruc-

tion of the auditory cortex on both sides of the brain,

loss of almost all ability to detect the direction from

detection process begin in the superior olivary nuclei

the way from these nuclei to the cortex are required

medial superior olivary nucleus and (2)

the
is concerned with detecting the direction from which

the difference in intensities of the sound reaching the
two ears and sending an appropriate signal to the audi-
tory cortex to estimate the direction.

specific mechanism for detecting the time lag between

contains large numbers of neurons that have two

of excitation of each neuron is highly sensitive to a
specific time lag between the two acoustical signals

those near the opposite border respond to a long time

sound from directly in front of the head stimulating
one set of olivary neurons maximally and sounds from
different side angles stimulating other sets of neurons

direction is determined by the locus of the maximally

for determining sound direction are transmitted
through a different pathway and excite a different
locus in the cerebral cortex from the transmission
pathway and termination locus for tonal patterns of
sound.

indicates again how specific information in sensory
signals is dissected out as the signals pass through

olivary nuclei.

Auditory Centers

Retrograde pathways have been demonstrated at each
level of the auditory nervous system from the cortex to

from the superior olivary nucleus to the sound-receptor
hair cells in the organ of Corti.

stimulation of discrete points in the olivary nucleus has
been shown to inhibit specific areas of the organ of

a person to direct his or her attention to sounds of par-
ticular qualities while rejecting sounds of other quali-

single instrument in a symphony orchestra.

caused by impairment of the cochlea or impairment of

physical structures of the ear that conduct sound

If either the cochlea or the auditory nerve is

tympanum-ossicular system has been destroyed or

sound waves can still be conducted into the cochlea by
means of bone conduction from a sound generator

of complex sound: a way forward? Trends Neurosci 27:181,

Griffiths TD, Warren JD, Scott SK, et al: Cortical processing

motor. Annu Rev Physiol 66:521, 2004.

Gillespie PG, Cyr JL: Myosin-1c, the hair cell’s adaptation

Mammalian Ear. New York: Oxford University Press,

Geisler CD: From Sound to Synapse: Physiology of the

cells. Nat Rev Genet 5:489, 2004.

Frolenkov GI, Belyantseva IA, Friedman TB, Griffith AJ:

Opin Neurobiol 13:446, 2003.

Fettiplace R, Ricci AJ:Adaptation in auditory hair cells. Curr

Curr Biol 13:R767, 2003.

channels.

Eatock RA: Auditory physiology: listening with K

ing. New York: Plenum Press, 1998.

Brugge J: Central Auditory Processing and Neural Model-

sound from the incus to the oval window.

a minute Teflon or metal prosthesis that transmits the

this case, the person becomes totally deaf for ossicular

bone overgrowth to the edges of the oval window. In

the faceplate of the stapes becomes “ankylosed” by

frequencies. In some instances of conduction deafness,

greatly depressed at all frequencies, but more so at low

normal, but conduction through the ossicular system is

deafness.” In this case, bone conduction is essentially

gram from a person with “middle ear air conduction

brane to the oval window. Figure 52–13 shows an audio-

In either case, the sound waves cannot be transmitted

as streptomycin, kanamycin, and chloramphenicol.

Corti—in particular, sensitivity to some antibiotics such

aging to the organ of Corti, and (2) deafness for all

low-frequency sounds are usually louder and more dam-

sounds (a rock band or a jet airplane engine), because

follows: (1) deafness for low-frequency sounds caused

all older people.

This type of deafness occurs to some extent in almost

ness is mainly for high-frequency sound. Such deafness

ness is shown in Figure 52–12. In this figure, the deaf-

conduction. An audiogram depicting partial nerve deaf-

nerve, or the central nervous system circuits from the

which includes damage to the cochlea, the auditory

In nerve deafness—

by the ear, is equipped with a mechanical vibrator for

spectrum. The audiometer, in addition to being

plotted, as shown in Figures 52–12 and 52–13, depicting

these frequencies. Then the so-called

spectrum, and the hearing loss is determined for each of

In performing a hearing test using an audiometer, one

ular frequency.

bels above normal before it can be heard, the person is

zero level. If the loudness must be increased to 30 deci-

that can barely be heard by the normal ear. A calibrated

intensity-level sound at each frequency is the loudness

quencies, the instrument is calibrated so that zero-

Chapter 52

The Sense of Hearing

661

connected to an electronic oscillator capable of emitting
pure tones ranging from low frequencies to high fre-

volume control can increase the loudness above the

said to have a hearing loss of 30 decibels at that partic-

tests about 8 to 10 frequencies covering the auditory

audiogram is

hearing loss at each of the frequencies in the auditory

equipped with an earphone for testing air conduction

testing bone conduction from the mastoid process of the
skull into the cochlea.

Audiogram in Nerve Deafness.

ear—the person has decreased or total loss of ability to
hear sound as tested by both air conduction and bone

could be caused by damage to the base of the cochlea.

Other patterns of nerve deafness frequently occur as

by excessive and prolonged exposure to very loud

frequencies caused by drug sensitivity of the organ of

Audiogram for Middle Ear Conduction Deafness.

common type of deafness is caused by fibrosis in the
middle ear following repeated infection or by fibrosis
that occurs in the hereditary disease called otosclerosis.

easily through the ossicles from the tympanic mem-

conduction but can regain almost normal hearing by the
surgical removal of the stapes and its replacement with

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1000 2000 4000 8000

Loss in decibels

Frequency

100

Normal

Air conduction

Bone conduction

Figure 52–12

Audiogram of the old-age type of nerve deafness.

125

250

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1000 2000 4000 8000

Loss in decibels

Frequency

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Normal

Air conduction

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The Nervous System: B. The Special Senses

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