HUMAN BIOLOGY

SenSory SyStemS 273

What is the sense of hearing?

• Hearing is the perception of sound waves that are detected by
  mechanoreceptors in the ears.


• Hair cells in the inner ear are the receptors for sound.


• Hair cells are attached to membranes inside the cochlea.


• Pressure waves generated by sound cause membrane vibrations
  that bend hair cells. The bending produces nerve impulses in
  neurons of the auditory nerve.

taKe-Home message

which includes hair cells. These cells are the mechanore-

ceptors that serve as the sensory receptors for sound.

Slender projections at the tips of hair cells rest against

an overhanging tectorial (“roof like”) membrane (Figure

14.9.E), which is not a membrane at all but a jellylike struc-

ture. When pressure waves in the cochlear fluid vibrate the

basilar membrane, its movements can press hair cell projec-

tions against the tectorial membrane so that the projections

bend like brush bristles. Affected hair cells release a neu-

rotransmitter. It triggers action potentials in neurons of the

auditory nerve, which carries them to the brain.

Different sound frequencies cause different parts of the

basilar membrane to vibrate—and, accordingly, to bend

different groups of hair cells. Apparently, the total num-

ber of hair cells stimulated in a given region determines

the loudness of a sound. The perceived tone or “pitch”

of a sound depends on the frequency of the vibrations

that excite different groups of hair cells. The higher the

frequency, the higher the pitch.

Eventually, pressure waves moving through the cochlea

push against the round window, a membrane at the far end of

the cochlea. As the round window bulges outward toward

the air-filled middle ear, it serves as a “release valve” for

the force of the waves. Air also moves through an opening

in the middle ear into the eustachian tube. This tube runs

from the middle ear to the throat (pharynx), permitting air

pressure in the middle ear to be equalized with the pres-

sure of outside air. When you change altitude (say, during

a plane trip), this equalizing process makes your ears pop.

Sounds such as amplified music and the thundering of

jet engines are so intense that long-term exposure to them

can permanently damage the inner ear (Section 14.7). Evo-

lution has not equipped hair cells of the human ear to cope

with such extremely loud, modern-day sounds.

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

scala vestibuli

cochlear duct

sensory scala tympani

neurons (to

the auditory

nerve)

organ of Corti

hair cells of organ of Corti

tectorial membrane basilar membrane

oval window scala vestibuli

eardrum

round window

cochlear duct scala tympani

waves

of fluid

pressure

waves

of air

pressure

the cochlea, “uncoiled” for clarity

C

D E

© Dr. Thomas R. Van DeWater, University of Miami Ear Institute

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