The ear is responsible for hearing and equilibrium/balance,
and has 3 parts:
1. OUTER EAR:
- external ear (PINNA) that collects and channels sound into the auditory
canal and towards the ear drum
2. MIDDLE EAR:
- begins at the eardrum or TYMPANIC MEMBRANE
- an air-filled area with 3 bones (OSSICLES)
- MALLEUS (hammer)
- INCUS (anvil)
- STAPES (stirrup)
- ends with another membrane called the OVAL WINDOW
- the EUSTACHIAN TUBE controls ear pressure by releasing air from the middle
ear into the mouth and nose "ears pop".
3. INNER EAR:
- contains the VESTIBULE for static equilibrium and the SEMICIRCULAR CANALS
for dynamic equilibrium
- the COCHLEA, a snail-shaped structure, contains the ORGAN OF CORTI which
is where the actual hearing occurs
BALANCE AND POSITION IN SPACE
- STATIC EQUILIBRIUM
At the entrance to the inner ear is the VESTIBULE which contains
two sacs of fluid called the UTRICLE and the SACCULE which are floating in
perilymph. They are sensory organs responsible for detecting and transmitting
information regarding balance, position in space, acceleration, and deceleration.
Suspended in the fluid are calcium carbonate particles called OTOLITHS. As
the head moves from one position to the other, these ear stones will move
too. The otoliths brush against sensory nerve hairs that line the utricle
and saccule, which depolarize and send a message to the brain.
- DYNAMIC EQUILIBRIUM
The SEMICIRCULAR CANALS also contain fluid and are responsible
for detecting changes in motion. There are three semicircular canals (all
at different angles), each responsible for detecting motion on a different
plane. Within the canals are nerve hairs which sense changes in the movement
of the perilymph fluid and depolarize, thus sending a message to the brain.
Ex: spinning in a circle
The events that take place to hear a sound are:
- Sound waves are collected by the pinna, travel down the auditory canal and
cause the tympanic membrane to vibrate.
- The tympanic membrane causes the malleus (hammer) to vibrate, which transfers
this energy to the incus (anvil) and then to the stapes (stirrup), amplifying
the sound each time as the bones get smaller each time. The stapes rests on
the oval window.
- The oval window vibrates and sets the perilymph of the inner ear in motion.
The pressure waves travel through the canals of the cochlea and cause the
basilar membrane to move.
- The hair cells resting on the basilar membrane vibrate against the overlying
tectorial membrane, causing them to fire (the organ of Corti).
- This message is relayed to the auditory nerve that carries it to the brain
- The energy of the pressure wave in the cochlea is released when the wave
hits the round window, which stretches out and absorbs the energy.
Conductive hearing loss occurs in the outer or middle ear and
can be repaired with hearing aids and surgery. Sensory hearing loss is in the
inner ear and cannot be repaired, hair cells cannot regenerate.