Download THE HUMAN EAR: READING AND QUESTIONS

THE HUMAN EAR: READING AND QUESTIONS – Use this reading to create the model of the ear. The
parts in bold should be included as information on your ear model.
MAINTENANCE EQUILIBRIUM (BALANCE)
The ears, like the eyes, are an extension of the human brain. You may think that the main function of the
ear 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. The inner
ear of most vertebrates contains special hairs that act as gravity receptors, homologous (similar in
evolutionary terms) to the lateral-line system of boney fish. These special hairs are found in semicircular
canals of the inner ear (see figure 1, next page).
In humans, the inner ear is filled with fluid. As the head is rotated or the angle of the body is
changed 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 that relays information about
balance. 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 adjusts and uses input from the eyes as a substitute. Aside from input from the
inner ear, humans can rely on visual and tactile (feel or touch) stimuli to control and maintain balance.
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 still spinning! The motion of a moving car or rocking boat stimulate the semicircular canals in
a strange way and can cause symptoms such as nausea or vomiting as a result.
EAR ANATOMY
The ear is divided into three main parts: outer ear, middle ear and inner ear (see the figure below).
The shell-shaped outermost part of your ear containing the ear lobe is called the pinna (or auricle). The
pinna is composed of cartilage covered with thin skin and some hair. The ear lobe lacks the supporting
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
the sound originated (both horizontal and vertical direction). Some animals, like coyotes, dogs and cats,
are able to move their pinnae (plural for pinna) toward a source of sound, but muscles used to control this
movement are vestigial in humans and do not allow us to do the same.
The opening in the outer ear is called the external auditory canal (or meatus), 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 (cerumen) 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 functions repelling insects.
The outer ear and middle ear are separated by the tympanic membrane. 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 because it so closely connected to the lining of you
throat, that bacteria can cause the tympanic membrane to become inflamed and bulge, also affecting your
hearing. Normally the Eustachian tube is flattened and closed, but swallowing or yawning can open it
briefly to equalize the pressure in the middle ear cavity with the pressure outside the ear. If the
pressure in the middle ear cavity is not equal to outside pressure, the tympanic membrane cannot vibrate
freely which can reduce hearing ability. Unequal pressure can cause the tympanic membrane to bulge in or
out (depending on pressure differences) which cause voices to sound faraway, and can cause an
uncomfortable sensation sometimes even an ache. When you feel your ears “popping” the pressure is
being equalized between the middle ear and the external environment.
AUDITORY RECEPTION
Auditory reception, hearing, is the process of detecting sound waves. Hearing is very important sense in
vertebrates. Both birds and mammals have a highly developed sense of hearing based on ear structure. The cochlea
of the inner ear is a spiral tube, coiled two and a half times, which resembles a snail’s shell. If you could stretch it
out into a straight line, it would consist of three canals separated by thin membranes. The cochlea is the
structure most responsible for the highly developed sense of hearing found in birds and mammals. It contains
mechanoreceptors, special hairs that respond to pressure changes caused by the sound waves. Those
pressure changes are sent as electro-chemical information to the brain through the cochlear nerve.
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 mechanoreceptors in the
cochlea respond to the changes in pressure created by sound waves and send that information to the brain
as electro-chemical signals through the cochlear nerve.
The pitch of the sound heard depends on the frequency of the sound vibrations. Loudness is dependent on the
intensity or strength of the vibrations. The higher the intensity the louder the sound. The human ear can detect
sound frequencies between about 20 to 20,000 cycles per second. Some animals such as dogs, can hear sounds
sound at much higher frequencies that we can’t hear. The barn owl has the best hearing out of all animals for
several reasons. First it has special feathers around its ears that form a dish shape that act to funnel sound into
their ears, much like our pinnae. They can control the shape of the “dish” by 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!
HEARING LOSS & DEAFNESS
Deafness can be caused by many different factors, such as injury or irregularity in the structures of the ear.
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. Even ear-wax build-up can limit hearing! 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 (see figure 3). People who are around loud music often, such as members of
rock bands 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 mechanoreceptors in their cochlea.