Transcript AUDITION

AUDITION
The Human Ear
Divisions of the Ear
The ear is divided into three main areas:
• Outer (external) ear
• Middle ear
• Inner ear
Outer Ear
• The outer ear consists of:
– an outer, funnel-like structure called the
auricle(or pinna)
– an S-shaped tube, the external auditory meatus.
The shapes of the outer ear (the auricle) and the
external auditory canal can help collect sound
waves travelling through air, amplify them and
direct them to the middle ear.
Auricle or Pinna
• It is composed of irregular plate of elastic
cartilage covered with skin, and an occasional
hair.
• Many muscles are present, but most modern
humans cannot move or wiggle ears
Middle Ear
• The middle ear consists of:
– an air-filled space in the temporal bone called the
tympanic cavity,
– an eardrum or tympanum (tympanic membrane),
– and three small bones called auditory ossicles.
• Sound waves entering the external auditory canal
eventually hit the tympanic membrane, or
eardrum (tympanum = drum), the boundary
between the outer and middle ears.
Tympanic Membrane
• It is stretched across
the end of the
external auditory
canal
• It is cone-shaped,
with the apex of the
cone directed
inward. Its cone
shape is attached to
one of the auditory
ossicles (malleus).
• air molecules push
against the
membrane, causing
it to vibrate at the
same frequency as
the sound wave.
Eustachian Tube
It is NOT considered a part of the ear – it is part of the
nasopharynx
Air enters the middle ear through the pharyngotympanic tube
(Eustachian tube)to
– ensure that atmospheric pressure is maintained either side of the
tympanic membrane
– to equalize pressure for correct vibration
The Ossicles
The auditory ossicles are the smallest bones in the
body and are named according to their shape:
Footplate of stapes
1. malleus (hammer)
2. incus (anvil)
3. stapes (stirrup)
tiny ligaments suspend the
ossicles, and minisynovial joints
link them together into a chain that
spans the middle ear cavity.
The middle ear also
contains two small skeletal
muscles that are attached
to the auditory ossicles
Arrangement of the ossicles
• The handle of the malleus is secured to the eardrum
• the base of the stapes fits into the oval window.
Middle Ear Skeletal muscles
• The tensor tympani muscle is attached to the malleus
• The stapedius muscle attaches the Stapes
• These muscles are the effectors in the protective tympanic
reflex
– they contract in response to loud sounds that might damage the
hearing receptors.
• The ossicles transmit the vibratory motion of the eardrum to
the oval window
The Oval Window
• It is the intersection of the middle ear with the inner
ear, and is directly contacted by the stapes’ footplate
Inner Ear
• The transmission of the vibration to the oval
window by the footplate of the stapes, sets
the fluids of the inner ear into motion,
eventually exciting the hearing receptors.
• These launch an impulse that is sent to the
temporal lobe of the brain
• The inner ear is innervated by the eighth
cranial nerve in all vertebrates.
Inner Ear
Consists of a complex system of
• The cochlea, dedicating to hearing; converting
sound pressure impulses from the outer ear
into electrical impulses which are passed on to
the brain via the auditory nerve.
• The vestibular system, dedicated to balance
Together, they are called the inner ear labyrinth.
The Labyrinth
• There are two structures within this system
– the osseous labyrinth – bony canal in temporal
bone
– membranous labyrinth (lies inside the osseous
labyrinth)
• Between the osseous and membranous
labyrinths is a fluid, called perilymph, that is
secreted by cells in the wall of the bony canal.
• The membranous labyrinth contains another
fluid, called endolymph
Cranial Nerve 8 – Vestibulochochlear nerves
Cochlea
Vestibular System
calcium carbonate
crystals exist in the
succule and utricle of
the vastibular system
Hair cells present in the cochlea and semicircular canals
The Vestibular System
• Works with the eyes and visual cortex to keep
objects in focus when the head is moving.
• Joint and muscle receptors also help maintain
balance.
• The brain receives, interprets, and processes the
information from these systems to control balance.
• Its semicircular canals uses fluids and detection cells
called hair cells to detect rotational motion, angle,
etc.
• Its otoliths (Saccule and utricle) contain calcium
carbonate crystals to help detect horizontal motion
The Chochlea
• It is the auditory portion of the inner ear.
• It is a spiral-shaped cavity in the bony
labyrinth
• It has 2.5 turns around its axis
• The main part of the cochlea is the Organ of
Corti - the sensory organ of hearing
– it is distributed along the partition separating fluid
chambers in the coiled tapered tube of the
cochlea.
How does the cochlea help us hear?
• The cochlea is filled with a watery liquid, the perilymph
• perilymph moves in response to the vibrations coming from
the middle ear via the oval window.
• As the fluid moves, the organ of Corti moves
• thousands of hair cells sense the motion via their cilia, and
convert that motion to electrical signals
• These signals are communicated via neurotransmitters
(chemical signal)to many thousands of primary auditory
neurons.
• These neurons transform the signals into action potentials
(electrochemical signals), which travel along the auditory
nerve to structures in the brain for further processing.
Cochlear nerve fibers
Hair cells
THE END