ears are paired sensory organs comprising the auditory system, involved
in the detection of sound, and the vestibular system, involved with
maintaining body balance/ equilibrium. The ear divides anatomically
and functionally into three regions: the external ear, the middle
ear, and the inner ear. All three regions
are involved in hearing. Only the inner ear functions in the vestibular
Anatomy of the Ear
ear (or pinna, the part you can see) serves to protect
the tympanic membrane (eardrum), as well to collect and direct sound
waves through the ear canal to the eardrum. About 1¼ inches
long, the canal contains modified sweat glands that secrete cerumen,
or earwax. Too much cerumen can block sound transmission.
on the image below for more information about each region.
ear, separated from the external ear by the eardrum, is an air-filled
cavity (tympanic cavity) carved out of the temporal bone. It connects
to the throat/nasopharynx via the Eustachian tube. This ear-throat
connection makes the ear susceptible to infection (otitis
media). The eustachian tube functions to equalize air pressure on both
sides of the eardrum. Normally the walls of the tube are collapsed. Swallowing
and chewing actions open the tube to allow air in or out, as needed for
equalization. Equalizing air pressure ensures that the eardrum vibrates
maximally when struck by sound waves.
|Adjoining the eardrum are
three linked, movable bones called "ossicles," which convert the
sound waves striking the eardrum into mechanical vibrations. The smallest
bones in the human body, the ossicles are named for their shape. The hammer
(malleus) joins the inside of the eardrum. The anvil (incus),
the middle bone, connects to the hammer and to the stirrup (stapes).
The base of the stirrup, the footplate, fills the oval window which
leads to the inner ear.
ear consists of a maze of fluid-filled
tubes, running through the temporal bone of the skull. The bony
tubes, the bony labyrinth, are filled with a fluid called perilymph.
Within this bony labyrinth is a second series of delicate cellular tubes,
called the membranous labyrinth, filled with the fluid called endolymph.
This membranous labyrinth contains the actual hearing cells, the hair
cells of the organ of Corti. There are three major sections of the
The inner ear has two membrane-covered
outlets into the air-filled middle ear - the oval window and the
window. The oval window sits immediately behind the stapes, the third
middle ear bone, and begins vibrating when "struck" by the stapes. This
sets the fluid of the inner ear sloshing back and forth. The round window
serves as a pressure valve, bulging outward as fluid pressure rises in
the inner ear. Nerve impulses generated in the inner ear travel along the
nerve (cranial nerve VIII), which leads to the brain. This is actually
two nerves, somewhat joined together, the cochlear nerve for hearing and
the vestibular nerve for equilibrium.
The front portion is the snail-shaped
which functions in hearing.
The rear part, the semicircular
canals, helps maintain balance.
Interconnecting the cochlea
and the semicircular canals is the
vestibule, containing the sense
organs responsible for balance, the utricle and saccule.
How We Hear - The Auditory System
the ear hears
All sounds (music, voice,
a mouse-click, etc.) send out vibrations, or sound waves. Sound waves do
not travel in a vacuum, but rather require a medium for sound transmission,
e.g. air or fluid. What actually travels are alternating successions of
increased pressure in the medium, followed by decreased pressure. These
vibrations occur at various frequencies, not all of which the human ear
can hear. Only those frequencies ranging from 20 to 20,000 Hz (Hz
= hertz = cycles/sec) can be perceived.
In hearing, air-borne sound
waves funnel down through the ear canal and strike the eardrum, causing
it to vibrate. The vibrations are passed to the small bones of the middle
ear (ossicles), which form a system of interlinked mechanical levers:
First, vibrations pass to the
malleus (hammer), which pushes the
(anvil), which pushes the stapes (stirrup). The base of the stapes
rocks in and out against the oval window - this is the entrance
for the vibrations. The stapes agitates the perilymph of the bony labyrinth.
At this point, the vibrations become fluid-borne. The perilymph, in turn,
transmits the vibrations to the endolymph of the membranous labyrinth and,
thence, to the hair
cells of the organ of Corti. It is the movement of these hair cells
which convert the vibrations into nerve impulses. The round window
dissipates the pressure generated by the fluid vibrations, thus serves
as the release valve: It can push out or expand as needed. The nerve
impulses travel over the cochlear nerve to the auditory cortex of the brain,
which interprets the impulses as sound.
How We Balance - The Vestibular
The semicircular canals
and vestibule function to sense movement (acceleration and deceleration)
and static position. The three semicircular canals lie perpendicular to
each other, one to sense movement in each of the 3 spatial planes. At the
base of the canals are movement hair cells, collectively called the crista
ampullaris. Depending on the plane of movement, the endolymph flowing
within the semicircular canals stimulates the appropriate movement
hair cells. Static head position is sensed by the vestibule, specifically,
its utricle and
saccule, which contain the position
hair cells. Different head positions produce different gravity effects
on these hair cells. Small calcium carbonate particles (otoliths) are the
ultimate stimulants for the position hair cells.
The hair cells for both position
and movement create nerve impulses. These impulses travel over the vestibular
nerve to synapse in the brain stem, cerebellum, and spinal cord. No definite
connections to the cerebral cortex exist. Instead, the impulses produce
actions to produce the corrective response. For example, a sudden loss
of balance creates endolymph movement in the semicircular canals that triggers
leg or arm reflex movements to restore balance.
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