HUMAN BIOLOGY

272 Chapter 14

cochlea Coiled structure in

the inner ear that contains

the organ of hearing (organ

of Corti).

hair cells Mechanorecep-

tors that are the sensory

receptors for sound.

organ of Corti Organ in the

ear where sensory hair cells

are located.

tectorial membrane

Jellylike structure that bend-

ing hair cells press against

in response to pressure

waves in the cochlear fluid.

tympanic membrane

The eardrum.

Hearing: Detecting sound Waves

n The sense of hearing depends on structures in the ear that

trap and process sounds traveling through air.

Sounds are waves of compressed air. They are a form of

mechanical energy. If you clap your hands, you force out

air molecules, creating a low-pressure state in the area they

vacated. The pressure variations can be depicted as a wave

form, and the amplitude of its peaks corresponds to loud-

ness. The frequency of a sound is the number of wave cycles

per second. Each cycle extends from

the start of one wave to the start of

the next (Figure 14.8).

The sense of hearing starts with

vibration-sensitive mechanorecep-

tors deep in the ear. When sound

waves travel down the ear’s audi-

tory canal, they reach a membrane

and make it vibrate. The vibrations

cause a fluid inside the ear to move,

the way water in a waterbed sloshes.

In your ear, the moving fluid bends

the tips of hairs on mechanore-

ceptors. With enough bending, the

result will be action potentials sent

to the brain, where they are inter-

preted as sound.

The ear gathers “sound signals”

A human ear has three regions (Figure 14.9A), each with its

own role in hearing. The outer ear is a pathway for sound

waves to enter the ear, setting up vibrations. The vibrations

are amplified in the middle ear. The inner ear contains the

coiled cochlea (kahk-lee-uh; Figure 14.9B), where vibra-

tions of different sound frequencies are sorted out as they

stimulate different patches of receptors. The inner ear also

contains semicircular canals, which are involved in balance

(Section 14.6).

Sensory hair cells are the key to hearing

Hearing begins when the outer ear’s fleshy flaps collect and

channel sound waves through the auditory canal to the

tympanic membrane (the eardrum). Sound waves cause

the membrane to vibrate, which in turn causes vibrations in a

leverlike array of three tiny bones of the middle ear: the mal-

leus (“ham mer”), incus (“anvil”), and stirrup-shaped stapes.

The vibrating bones transmit their motion to the oval window,

an elastic membrane over the entrance to the cochlea. The

oval window is much smaller than the tympanic membrane.

So, as the middle-ear bones vibrate against its small surface

with the full energy that struck the tympanic membrane, the

force of the original vibrations is amplified.

Now the action shifts to the cochlea. If we could uncoil

the cochlea, we would see that a fluid-filled chamber folds

around an inner cochlear duct (Figure 14.9C). Each “arm” of

the outer chamber functions as a separate compartment (the

scala vestibuli and scala tympani, respectively; Figure 14.9D).

The amplified vibrations of the oval window create pres-

sure waves in the fluid within the chambers. These waves

are transmitted to the fluid in the cochlear duct. On the

floor of the cochlear duct is a basilar membrane, and resting

Figure 14.8 Sound travels in the form of a wave. (© Cengage Learning) on the basilar membrane is a specialized organ of Corti,

Low

note

High

note

Same loudness,

different pitch

Soft

Loud

Same frequency,

different amplitude

Amplitude

Frequency per

unit of time

one cycle

Middle ear bones:

stirrup

anvil

hammer

Eardrum

auditory

canal Cochlea

auditory nerve

round

window

oval window

(behind stirrup)

Middle ear

eardrum,

ear bones

vestibular

apparatus,

cochlea

Outer ear

pinna,

auditory

canal

A

B

Inner ear

Figure 14.9 Animated! The ear gathers sound waves and

converts them to nerve impulses. (© Cengage Learning)

Fabian Cevallos/Sygma/Corbis

14.5

Low

note

High

note

Same loudness,

different pitch

Soft

Loud

Same frequency,

different amplitude

Amplitude

Frequency per

unit of time

one cycle

Low

note

High

note

Same loudness,

different pitch

Soft

Loud

Same frequency,

different amplitude

Amplitude

Frequency per

unit of time

one cycle

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