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Special Senses Lecture
Hearing
Our ears actually serve two functions:
1) Allow us to hear
2) Maintain balance and equilibrium
Hearing and balance work with the help of mechanoreceptors in the ear.
These receptors are activated by movement caused by sound vibrations
we receive or by movement of the head and body.
Anatomically, the ear is divided into (pg. 285):
1) Outer ear (hearing)
2) Middle ear (hearing)
3) Inner ear (hearing and balance)
inner ear
outer ear
middle ear
Outer Ear:
1) Pinna (or auricle): outer, visible part of ear.
2) External auditory canal: canal that focuses sound waves into
middle ear. Ceruminous glands lie under skin and secrete earwax.
3) Tympanic membrane (ear drum): receives sound waves and
transmits them to bones of middle ear. Only vibrates freely when
pressure on both sides of eardrum are equal.
pinna
eardrum
External auditory canal
Homeostatic imbalance:
Otitis externa (“Swimmer’s Ear”):
Infection of the external auditory canal caused by bacteria or
fungus. Too much moisture in ear can irritate and break down
skin in canal, allowing bacteria or fungus to penetrate, leading to
infection.
bacteria or fungus in canal
Middle ear
(like a small box filled with air within the temporal bone of skull)
Three ossicles of the middle ear:
1) Hammer (or malleus): receives sound vibrations from
eardrum and transmits to anvil.
2) Anvil (or incus): receives vibrations from hammer and
transmits to stirrup.
3) Stirrup (or stapes): receives vibrations from anvil and
transmits to oval window.
hammer
anvil
stirrup
middle ear
The middle ear is connected to the throat (nasopharynx) via the
Eustachian tube. This tube serves to equalize air pressure on both
sides of the eardrum. Normally, it is closed or flattened but can
open through swallowing or chewing. (Think of how chewing
gum or swallowing can help relieve pressure in your ears when
flying on an airplane).
eustacian tube
Because of this ear/throat connection, the ear becomes more
vulnerable to infections as bacteria and viruses can enter the ear via
the throat.
For example, Otis media (inflammation of the middle ear) is often due
to a bacterial infection. As a result, fluid and pus fill the middle ear
cavity. If severe, a myringotomy (puncture in eardrum) may be
necessary.
Ouch!
On medial side of middle ear “box”, there are two membranous“windows”:
1) Oval window: larger, superior membrane. Bones of stirrup press
on oval window, which sets fluids of inner ear into motion.
2) Round window: smaller, inferior membrane. Serves as a pressure
valve for the inner ear. It dissipates pressure generated by inner ear
fluid vibrations.
Oval window
Round window
Inner ear:
•Consists of a maze of bony chambers called the bony labyrinth
( or osseous labyrinth). This is located deep in the temporal bone,
behind the eye socket.
•The bony labyrinth is filled with perilymph, a plasma-like fluid that
helps with sound transmission.
inner ear
Three subdivisions of the bony labyrinth:
1) Cochlea: helps with hearing. Within the cochlea is the
Organ of Corti which contains hearing receptors (hair
cells).
cochlea
hair cells
2) Semicircular canals: helps with balance and helps us respond
to angular or rotation movements, such as twirling motions.
Within canal are the crista ampullaris, a cluster of hair cells
that detect changes in our position & send impulses through
the vestibulocochlear nerve, to brain stem, cerebellum, and
spinal cord. This results in our ability to quickly correct our
body’s position/equilibrium with reflexes and quick
movements.
Semicircular canals
3) Vestibule: helps with balance & helps us determine “up”
from “down” with the help of otoliths (tiny stones made of
calcium salts embedded in a gel-like membrane).
vestibule
How sound is transmitted:
 Sound waves reach the cochlea (after traveling through
outer ear and middle ear). These vibrations set the cochlear
fluids in motion. The cochlear fluids then move the tectorial
membrane which then bends the hairs of the Organ of
Corti.
tectorial membrane bends
hair cells
This stimulates the hair cells, which then transmit an
electrical impulse through the cochlear nerve (part of the 8th
cranial nerve, the vestibulocochlear nerve) which travels to
the auditory cortex in the temporal lobe of the brain. This is
where the interpretation of sound occurs.
vestibulocochlear nerve
sends nerve impulse to brain
And, voila, sound!!