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The three parts of the ear are
the inner
outer
middle ear
The outer and middle ear are involved with
hearing.
The inner ear functions in
hearing
equilibrium
outer ear is composed of
auricle (pinna)
external auditory canal (a short, curved tube filled with ceruminous glands)
tympanic membrane (eardrum)
The tympanic membrane is a
boundary between the outer and middle ear
It vibrates in response to sound and transfers sound energy to the middle ear ossicles.
the middle ear consists of
eustachian tube
tympanic cavity
The tympanic cavity is a
small, air‐filled, mucosa‐lined cavity that lies medial to the eardrum and lateral to the oval and round windows.
The pharyngotympanic tube (eustachian tube)
connects the middle ear to the nasopharynx and equalizes pressure in the middle ear cavity with the external air pressure.
The tympanic cavity contains three small bones that transmit vibratory motion of the eardrum to the oval window they are
malleus
incus
stapes
The inner ear is found within the
bony labyrinth of the temporal bone
the inner ear contains
the vestibule
cochlea
semicircular canals
is filled with perilymph.
Within the bony labyrinth there are a series of
membranous sacs called the membranous labyrinth
The membranous labyrinth contains endolymph
The Cochlea is
a spiral, conical, bony chamber that contains the cochlear duct.
Hearing receptors (organ of Corti) are found there.
the cochlea is divided into three chambers
scala vestibuli
scala tympani
cochlear duct (scala media)
scala vestibuli
starts at the oval window, continues to the apex of the cochlea, and connects to the scala tympani via the helicotrema.
It carries perilymph to the apex of the cochlea.
scala tympani
helicotrema at the apex of the cochlea → base of the cochlea → ends at the round window.
It carries perilymph to the base of the cochlea.
cochlear duct (scala media)
located in between the scalae vestibuli and tympani, but is not connected to them.
It is filled with endolymph and contains the organ of Corti
sound is a
mechanical pressure wave
causes eardrum to vibrate
vibration is passed on
the malleus → incus → stapes
what amplifies sound?
ossicles
The stapes passes the vibration onto
the membrane that covers the oval window.
perilymph → scala vestibuli → apex of cochlea → helicotrema → scala tympani → round window (where its dampened)
The pressure wave is passed to
the scala media, endolymph moves, activating hair cells in the organ of Corti
The movement of hair cells stimulates
the cochlear branch of the vestibulocochlear cranial nerve (VIII) sending impulses to the medial geniculate nucleus in the brain
auditory pathway
carry sound information from medial geniculate nucleus → primary auditory cortex → noise sensed in primary auditory cortex → auditory information sent to auditory association cortex (where it’s perceived)
sound characteristics are
loudness
pitch
direction of sound
Loudness of sound is related to the
amplitude of the sound wave.
Amplitude is perceived by varying thresholds of the cochlear cells and the number of cells stimulated.
This results in a varying rate of action potentials in the auditory cortex.
Pitch is related to the
frequency of the sound wave.
High frequency sounds cause AP in hair cells at the base of the cochlea.
Low frequency sounds cause AP in hair cells at the apex of the cochlea.
direction of sound
If the sound occurs on the right side of the head, the right ear receives it before the left.
The brain compares the time delay between sound arrival to the right and the left side and establishes the location.
Equilibrium involves two components:
static equilibrium
dynamic equilibrium.
Static Equilibrium provides
information regarding the position of the head relative to gravity
information on linear acceleration.
The organ of static equilibrium is
the macula, located in the vestibule of the inner ear.
The macula consists of
sensory hair cells covered by a gel‐like cap with tiny crystals (otoliths) inside.
When the head is tilted, gravity causes
the crystals to slide to one side, pulling the gel and the sensory hairs.
This causes hair cells to trigger nerve impulses along the vestibular nerve to the brain.
Dynamic Equilibrium provides
information regarding linear and angular acceleration.
Receptors for dynamic equilibrium are located in
the crista ampullaris of the ampullae of each of the semilunar canals.
When the head accelerates in the plane of one of the canals,
fluid moves in the canal.
The hair cells are stretched generating AP.
Angular acceleration may affect one, two, or all of the canals.
AP from both the macula and crista ampullaris pass along
AP → the vestibular branch → the cranial nerve VIII → to the brain.
Most of the proprioceptive information is directed to the cerebellum and processed subconsciously.
There are three modes of input for balance and orientation:
Vestibular receptors
Visual receptors
Somatic receptors (proprioceptors)
These receptors allow our body to respond reflexively
