Transcript
DR. MOHNAD R. ALWAN
KEY TERMS
Auditory periphery: The outer ear, middle ear, and inner ear, ending at the nerve fibers exiting the inner ear.
Auditory central nervous system: The ascending and descending auditory pathways, which centers in the brainstem and cortex.
Tonotopic organization: The systematic mapping of sound frequency to the place of maximum stimulation within the auditory system that begins in the cochlea and is preserved through the auditory cortex.
MAIN COMPONENTS OF THE HEARING MECHANISM:
Divided into 4 parts (by function):Outer EarMiddle EarInner EarCentral Auditory Nervous System
STRUCTURES OF THE OUTER EAR
Auricle (Pinna) Gathers
sound waves Aids in
localization Amplifies
sound approx. 5-6 dB
The outer ear serves a variety of functions:1. It protects the more delicate middle and inner ears
from foreign bodies. 2. It boosts or amplifies high-frequency sounds. 3. The outer ear provides the primary cue for the
determination of the elevation of a sound’s source. 4. The outer ear assists in distinguishing sounds that
arise from in front of the listener from those that arise from behind the listener.
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
TYMPANIC MEMBRANE
Thin membrane Forms boundary between
outer and middle ear Vibrates in response to
sound waves Changes acoustical
(sound) energy into mechanical energy
Middle Ear
The middle ear consists of a small air-filled cavity lined with a mucous membrane that forms the link between the air-filled outer ear and the fluid-filled inner ear.
This link is accomplished mechanically via three tiny bones, the ossicles:MalleusIncusStapes
The purpose of the elaborate link between the air-filled outer ear and the fluid-filled inner ear is to compensate for the loss of energy that would occur if sound waves struck the fluid-filled inner ear directly.
The middle ear compensates for this loss of sound energy through two primary mechanisms:1. The areal ratio of the tympanic membrane to the
footplate of the stapes2. The lever action of the ossicles
EUSTACHIAN TUBE ( “THE EQUALIZER”)
Mucous-lined (protection), 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
TRANSMISSION OF SOUND TO THE INNER EAR
The route of sound to the inner ear follows this pathway: Outer ear – pinna, auditory canal, eardrum Middle ear – malleus, incus, and stapes to the oval
window Inner ear – scalas vestibuli and tympani to the cochlear
duct Stimulation of the organ of Corti Generation of impulses in the cochlear nerve
ORGAN OF CORTI
The end organ of hearingContains stereocilia & receptor hair
cells(contains auditory sensory cells)3 rows OHC, 1 row IHCTectorial and Basilar MembranesCochlear fluids
(From Augustana College, “Virtual Tour of the Ear”)
HAIR CELLS Frequency specific
High pitches= base of cochlea Low pitches= apex of cochlea Fluid movement causes deflection
of nerve endings Nerve impulses (electrical energy)
are generated and sent to the brain
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
CENTRAL AUDITORY SYSTEM
VIII th Cranial Nerve or “Auditory Nerve”Bundle of nerve fibersTravels from cochlea through internal auditory
meatus to skull cavity and brain stemCarry signals from cochlea to primary auditory
cortex, with continuous processing along the way Auditory Cortex
Wernicke’s Area within Temporal Lobe of the brainSounds interpreted (analyze) based on
experience/association
MECHANISMS OF HEARING
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.
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The fluid movement causes membranes in the Organ of Corti to shear against the hair cells.
The neurotransmitter released by hair cells to stimulate the dendrites of afferent neurons
This creates an electrical signal which is sent up the Auditory Nerve to the brain.
The brain interprets it as sound!
Mechanisms of Hearing
Slide 8.29Figure 8.14
6 STEPS OF HEARING PROCESS
Figure 17–29
BALANCE AND ORIENTATION PATHWAYS
There are three modes of input for balance and orientationVestibular receptorsVisual receptorsSomatic receptors
These receptors allow our body to respond reflexively
MECHANISMS OF EQUILIBRIUM AND ORIENTATION
Vestibular apparatus – equilibrium receptors in the semicircular canals and vestibuleMaintains our orientation and balance in spaceVestibular receptors monitor static equilibriumSemicircular canal receptors monitor dynamic
equilibrium
EQUILIBRIUM: VESTIBULAR APPARATUS
Organs of Equilibrium
Slide 8.30a
Receptor cells are in two structures
Vestibule
Semicircular canals
Figure 8.16a, b
Organs of Equilibrium
Slide 8.30b
Equilibrium has two functional parts
Static equilibrium – sense of gravity at rest
Dynamic equilibrium – angular and rotary head movements
Figure 8.16a, b
Static Equilibrium - Rest
Slide 8.31
Maculae – receptors in the vestibule Report on the position of the head
Send information via the vestibular nerve
Anatomy of the maculae Hair cells are embedded in the otolithic
membrane
Otoliths (tiny stones) float in a gel around the hair cells
Movements cause otoliths to bend the hair cells
Function of Maculae
Slide 8.32
Figure 8.15
Dynamic Equilibrium - Movement
Slide 8.33a
Crista ampullaris – receptors in the semicircular canals
Tuft of hair cells
Cupula (gelatinous cap) covers the hair cells
Dynamic Equilibrium
Action of angular head movements
The cupula stimulates the hair cells
An impulse is sent via the vestibular nerve to the cerebellum
EFFECT OF GRAVITY ON UTRICULAR RECEPTOR CELLS
Figure 15.36
DON’T FORGET…
The vestibular apparatus DOES NOT automatically compensate for forces acting on the body…it sends warning signals to the CNS which initiates the appropriate “righting” compensations to keep your body balanced, weight distributed, & eyes focused on location.
AUDITORY PATHWAY TO THE BRAIN
Organ of Cortispiral ganglion
(in cochlear nerve)cochlear
nuclei of medullasuperior
olivary nucleus (pons/
medullary junction)along
the lateral lemniscal tract to
inferior colliculus
(midbrain)medial geniculate
body of thalamusauditory
cortex in temporal lobe
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