THE HUMAN EAR: READING AND QUESTIONS
MAINTENANCE EQUILIBRIUM (BALANCE)
The ears, like the eyes, are an extension of the human brain. You may think that the main function
of the ears is hearing, but the main function of the ear in all vertebrates is to maintain equilibrium
or balance. Some vertebrates, such as snakes, don’t even have a middle or outer ear like humans
but receive sound in the form of vibrations through other body structures, such as the jawbone of
a snake.
Later when you read about the middle ear, you will find out how ears keep humans
upright!
EAR ANATOMY
OUTER EAR
The ear is divided
into
three
main
parts: outer ear,
middle
ear
and
inner ear. The shell
shaped outermost
part of your ear
containing the ear
lobe is called the
pinna. The pinna is
composed
of
cartilage contained
in the uppermost
portion of the ear,
which is why the ear
lobe is fleshy and
dangles. The function of the pinna is to direct sound waves into the ear canal (also called the
auditory canal). The pinna acts as a funnel for sound waves and allows us to pinpoint the direction
from which a sound originated (both horizontal and vertical direction). Some animals, like coyotes,
dogs and cats, are able to move their pinna toward a source of sound, but muscles used to control
this movement are vestigial (no longer functional) in humans and do not allow us to do the same.
The opening in the outer ear is called the external auditory canal, and consists of a short, narrow
chamber that extends from the pinna to the tympanic membrane (ear drum). The structure of the
outer portion of the external auditory canal is supported by cartilage, but the remaining portion is
actually carved into the temporal bone of the skull. The entire canal is lined with skin containing
thousands of tiny hairs that act as dust traps. Sweat glands and wax-producing glands (modified
sweat glands) called ceruminous glands, are located in the skin that lines the auditory canal. The
sticky wax (cerumin) acts as a trap for foreign materials that may enter the ear, such
as dust and insects. The wax traps the foreign bodies, dries up and falls out naturally cleaning the
ear! The odor of earwax also repels insects.
Middle ear
The outer ear and middle ear are separated by the tympanic membrane. Vibrations that pass
through the tympanic membrane into the middle ear are conducted through three small bones, the
malleus (hammer), incus (anvil), and stapes (stirrup). These vibrations move to the inner ear
through the oval window, a membrane beneath the stapes.
The middle ear is actually connected to the lining of the throat in humans by the Eustachian tube.
When you get a sore throat the lining of the Eustachian tube can become inflamed which leads to
swelling which can affect your hearing. The Eustachian tube is normally flattened and closed, but
yawning can open it briefly to equalize the pressure in the middle ear cavity with that of the outer
ear. Ever been on an airplane and needed to pop your ears? The air pressure in your outer ear
adjusts to the new cabin pressure and is no longer in equilibrium with the pressure in your middle
ear. Before you equalize the pressure between the middle ear and the outer ear, which makes that
popping sound, your ability to hear is reduced. Unequal pressure can cause the tympanic membrane
to bulge in or out, depending on the pressure differences, which causes voices to sound far away
and can be slightly painful.
Inner Ear
The inner ear is encased within the hardest bone of the body and consists of the cochlea,
semicircular canals, the auditory/cochlear nerve, and the vestibular/balance nerve. The
cochlea of the inner ear is a spiral tube, coiled two and a half times, which resembles a snail’s shell.
The cochlea contains fluid to stimulate the mechanoreceptors. Mechanoreceptors are special
hairs that respond to pressure changes within the fluid caused by the sound waves. There are
approximately 24,000 mechanoreceptor hairs in the cochlea arranged in tiny rows on the organ of
corti the actual auditory receptor organ in the cochlea. This structure is responsible for the
highly developed sense of hearing found in birds and mammals.
The semicircular canals contain mechanoreceptors that respond to pressure changes within the
fluid of the canals. These mechanoreceptors are gravity receptors that help maintain balance. In
humans, the inner ear is filled with fluid the fluid of the inner ear sloshes around, stimulating the
gravity receptor hairs in the ear. When the hairs are simulated, they send a message to the brain
through the balance nerve that relays information about a body’s position in space. Have you ever
wondered why you get dizzy after spinning in circles? The fluid in the inner ear keeps moving after
you stop spinning, and continues to stimulate the gravity receptor hairs telling your brain that you
are spinning! The motion of a moving car or rocking boat stimulate the semicircular canals and can
cause symptoms such as nausea or vomiting as a result. When the hairs are destroyed, organisms
have trouble keeping balance. For example, when the gravity receptor hairs in the inner ear of a
pigeon are destroyed, the pigeon cannot fly for a period of time, until the brain learns to use input
from the eyes as a substitute.
AUDITORY RECEPTION: Putting it all together!
As sound enters the ear and passes through the external auditory canal in the form of sound
waves, it vibrates the tympanic membrane or eardrum. The tympanic membrane is the thin
membrane, which separates the outer ear from the middle ear. The vibrations from the tympanic
membrane are transmitted across the middle ear by three tiny bones: the malleus, incus and
stapes, which are more commonly known as the hammer, anvil and stirrup (in order) because of
their shapes. The vibrations are passed from the stirrup to the inner ear through the oval window,
and then into the fluid of the cochlea. The vibrations travel through the fluid, creating pressure,
which stimulates the mechanoreceptors located on the organ of Corti. The mechanoreceptors
located on the organ of the Corti send a message in the form of a nerve impulse through the
cochlear nerve to the brain, which interprets the impulse as a sound.
PITCH and LOUDNESS
The pitch of the sound heard depends on the frequency of the sound vibrations. Low frequencies
produce a low pitch and high frequencies produce high pitches. The human ear can detect sound
frequencies between about 20 to .01 cycles per second. Some animals such as dogs, can hear
sounds at much higher frequencies than we can hear. The barn owl has the best hearing out of all
animals for several reasons. First it has special feathers around is eyes that’s form a dish shape
that act to funnel sound into their ears, much like our pinna. They can control the shape of the
“dish” with the feathers to either flatten out to reduce incoming sounds, or by raising them to
improve hearing. Second, the position of their ears is asymmetrical. One ear is located near the
forehead and the other ear on the opposite side is located near the nostril. This asymmetrical ear
placement helps the barn owl locate even the faintest sound, such as that of a tiny mouse walking
on the grass hundreds feet away.
Loudness is dependent on the energy or amplification of the sound vibrations. The higher the
energy of a wave, the louder the sound will be. An amplifier, or “amp”, is used by musicians to
increase the energy in the sound wave that is released when a note is played. The pitch of the
note is the same because the frequency remains the same. The amplifier just pumps up the energy
of the waves making the sound much louder.
HEARING LOSS & DEAFNESS
Deafness can be caused by many different factors. For example, ear-wax build up can limit
hearing, bones in the middle ear may become fused after an infection, and sometimes a prolonged
high fever can lead to injury of auditory nerves, all resulting in hearing loss. Prolonged exposure to
loud noise, or being in the presence of a very intense sound can cause the mechanoreceptor hairs in
the cochlea to break which leads to hearing loss. Particularly people who are around loud music
often, such as members of rock bands (or rap, or hip hop-doesn’t matter what type of music,
really), or people who work around loud, high pitched noises over a period of years frequently
become deaf to high tones because of the damage done to the hairs of the organ of Corti.
Fact: 200,000 people in the United States are deaf, and 3 million people in the US have serious
hearing problems.
QUESTIONS: Answer the following questions using complete sentences on a separate sheet of
paper. Restate the question in your answer; do not use “it” or “they,” instead, use the correct noun.
1.
2.
3.
4.
5.
Describe how the ear helps maintain balance.
What causes some people to experience sea sickness and what are the symptoms?
What function of earwax and what produces it?
How come you can’t get an ear infection from having a sore throat? Explain.
When an airplane takes off the runway, why is it harder to hear the person next to you
even though they are the same distance from you as before take off? How can you adjust
your hearing so that you can hear them better?
6. List the parts of the ear sound passes through starting with the pinna and ending with the
brain. (Total of ten parts, including the pinna and the brain).
7. Name and describe the adaptations that allow the barn owl to have the bets hearing out of
all the animals.
8. List three things that can contribute to hearing loss.
BIBLIOGRAPHY:
Marieb, Elain N. 1992. Human analysis and physiology. California: the Benjamin/cummings
Publishing Company, Inc.
Villee, C., Solomon, E., Martin, C., Berg, L. and Davis, P.W 1989. Biology. Philadelphia: Saunders
College Publishing.