Download 02 Physiology of hearing. Vestibular analyzer

Physiology of hearing.
Vestibular analyzer
Main Components of the Hearing
Mechanism:
Divided into 4 parts (by function):
• Outer Ear
• Middle Ear
• Inner Ear
• Central Auditory Nervous System
Structures of the Outer Ear
Auricle (Pinna)
• Gathers
sound waves
• Aids in
localization
• Amplifies
sound
approx. 5-6
dB
External Auditory Canal:
• Approx. 1 inch long
• “S” shaped
• Outer 1/3 surrounded by
cartilage; inner 2/3 by
mastoid bone
• Allows air to warm before
reaching TM
• Isolates TM from physical
damage
• Cerumen glands
moisten/soften skin
• Presence of some
cerumen is normal
Tympanic Membrane
• Thin membrane
• Forms boundary
between outer and
middle ear
• Vibrates in response to
sound waves
• Changes acoustical
energy into mechanical
energy
(From Merck Manual)
Eustachian Tube (AKA: “The
Equalizer”)
• Mucous-lined, connects
middle ear cavity to
nasopharynx
• “Equalizes” air pressure in
middle ear
• Normally closed, opens under
certain conditions
• May allow a pathway for
infection
• Children “grow out of” most
middle ear problems as this
tube lengthens and becomes
more vertical
Stapedius Muscle
• Attaches to stapes
• Contracts in response to loud sounds; (the
Acoustic Reflex)
• Changes stapes mode of vibration; makes it
less efficient and reduce loudness perceived
• Built-in earplugs!
• Absent acoustic reflex could signal
conductive loss or marked sensorineural loss
Structures of the Inner Ear:
The Cochlea
• Snail shaped cavity within mastoid
bone
• 2 ½ turns, 3 fluid-filled chambers
• Scala Media contains Organ of Corti
Converts mechanical energy to
electrical energy
Central Auditory System
• VIIIth Cranial Nerve or “Auditory Nerve”
– Bundle of nerve fibers (25-30K)
– Travels from cochlea through internal auditory meatus to
skull cavity and brain stem
– Carry signals from cochlea to primary auditory cortex,
with continuous processing along the way
• Auditory Cortex
– Wernicke’s Area within Temporal Lobe of the brain
– Sounds interpreted based on experience/association
Summary: How Sound Travels
Through The Ear
Acoustic energy, in the form of sound waves, is
channeled into the ear canal by the pinna. Sound waves
hit the tympanic membrane and cause it to vibrate, like
a drum, changing it into mechanical energy. The
malleus, which is attached to the tympanic membrane,
starts the ossicles into motion. The stapes moves in and
out of the oval window of the cochlea creating a fluid
motion, or hydraulic energy. The fluid movement causes
membranes in the Organ of Corti to shear against the
hair cells. This creates an electrical signal which is sent
up the Auditory Nerve to the brain. The brain
interprets it as sound!
Organ Of Corti
• The end organ of hearing
–
–
–
–
Contains stereocilia & receptor hair cells
3 rows OHC, 1 row IHC
Tectorial and Basilar Membranes
Cochlear fluids
(From Augustana College, “Virtual Tour of the Ear”)
Vestibular Apparatus
• Vestibular apparatus
and cochlea form the
inner ear
• Vestibular apparatus –
provides sense of
equilibrium
– consists of otolith
organs (utricle and
saccule) and
semicircular canals
Vestibular Apparatus
• Sensory structures located within membranous labyrinth
– filled with endolymph and located within bony labyrinth
Vestibular Apparatus
• Utricle and saccule provide info about linear acceleration
• Semicircular canals, oriented in 3 planes, give sense of
angular acceleration
Vestibular Apparatus
• Hair cells – receptors for equilibrium
– Each contains 20-50 stereocilia (hair-like extensions)
• 1 of these is a kinocilium---a true cilium
Vestibular Apparatus
• Stereocilia bend toward kinocilium – hair cell depolarizes
– releases NT that stimulates CN VIII
• When bent away from kinocilium – hair cell hyperpolarizes
– In this way, frequency of APs in hair cells carries information about
movement
• When stereocilia are bend away from
kinocilium, hair cell is hyperpolarized, i.e.
inhibited. It occurs because acceleratory force
acts to flow of fluid in semicircular canals during
circular motion of the head or whole the body.
• Hair cells are located along crista ampularis and
protect their cilia in cupula. Hair cells are
secondary sensor cells, which synapse with
neurons. Axons of these nerve cells compose
vestibular nerve.
Utricle and Saccule
• Have a macula that
contains hair cells
– Hair cells embedded in
gelatinous otolithic
membrane
• contains calcium
carbonate crystals
(otoliths) that resist
change in movement
Utricle and Saccule
• Utricle sensitive to
horizontal acceleration
– Hairs pushed backward
during forward
acceleration
• Saccule sensitive to
vertical acceleration
– Hairs pushed upward
when person descends
Semicircular Canals
• Provide information
about rotational
acceleration
• Project in 3 different
planes
• Each contains a
semicircular duct
• Crista ampullaris –
where sensory hair
cells are located
10-42
Semicircular canal function
• Ampula is enlargement at
one end of semicircular
canal. It has a small crest
on top of which is a
gelatinous mass known as
cupula. Hair cells have
two kinds of cilia –
kinocilium and stereocilia.
• Kinocilium is large cilium
located at one end of hair
cell. Stereocilia are small.
When stereocilia are bent
towards kinocilium, hair
cell is depolarized, i.e.
stimulated.
Semicircular Canals
• Hair cell processes
embedded in cupula of
crista ampullaris
• When endolymph
moves cupula moves
– Sensory processes bend
in opposite direction of
angular acceleration
Neural Pathways for Equilibrium and Balance
Nystagmus and Vertigo
• Vestibular nystagmus – involuntary oscillations
of eyes
– occur when spinning person stops
– Eyes continue to move in direction opposite to spin,
then jerk rapidly back to midline
• Vertigo – loss of equilibrium
– Natural response of vestibular apparatus
– Pathologically, may be caused by anything that alters
firing rate of CN VIII
• Often caused by viral infection
Vestibular System
• Consists of three semi-circular
canals
• Monitors the position of the head in
space
• Controls balance
• Shares fluid with the cochlea
• Cochlea & Vestibular system
comprise the inner ear
Vestibular System
• Consists of three semi-circular
canals
• Monitors the position of the head in
space
• Controls balance
• Shares fluid with the cochlea
• Cochlea & Vestibular system
comprise the inner ear