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Sense Of Hearing, Its Mechanism And Auditory Pathway.
Learning Objectives:
By the end of the lecture, the student should be able to :
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Describe the components and functions of the external, middle, and inner ear.
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The sound stimulus.
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The parts of the inner ear and their roles in equilibrium and hearing.
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Auditory Physiology.
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The pathways for the sensations of hearing.
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Auditory Pathology.
Hearing:
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Hearing is the sense by which we perceive sound.
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The receptors for the sensory sensations (hearing and equilibrium) are
located in the ear.
Hearing & Equilibrium: Introduction
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External ear, middle ear, and cochlea of
inner ear are concerned with hearing.
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Semicircular canals, utricle, and saccule of
inner ear are concerned with equilibrium.
The Sound Stimulus:
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Typically sounds are generated by vibrations of
air molecules that are transmitted to inner ear
through outer and middle ears
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compression waves of air pressure.
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Speed: about 340 m/s through air,
considerably faster through denser materials (e.g., liquids or solids)
Sounds cannot travel through a vacuum.
Sound waves = alternating compressions and decompressions of
air molecules
Properties
Pitch = frequency of waves arriving
“high” (treble) or “low” (bass) measured in hertz (Hz)
Loudness = intensity in decibels (dB)
Main Components of the Hearing Mechanism:
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Peripheral
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Outer ear
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Middle ear
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Inner ear
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Auditory nerve
Central
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Brainstem
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Midbrain
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Cerebral
The outer ear:
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PINNA:
Important for sound gathering and localization of sound.
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EAR CANAL or AUDITORY MEATUS: Important for sound
selection.
Protect middle and inner ear.
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EARDRUM or TYMPANIC MEMBRANE:
Vibrates in response to sound/pressure change.
External ear:
External auditory canal or ear canal:
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Provides communication between middle and inner ears by conducting sound
to the ear drum.
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S-shaped tube 2.5cm to 3cm long and 7mm wide.
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Lining of the lateral 1/3rd of canal has cilia and
glands.
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lined with skin and near entrance are fine hairs ,
sebaceous gland and ceruminous glands.
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external auditory canal ends at the tympanic
membrane.
Functions:
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Collect sound
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Localization
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Resonator
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Protection
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Sensitive (earlobe).
Mastoid Process of Temporal Bone:
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Bony ridge behind the auricle.
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Hardest bone in body, protects cochlea and vestibular
system.
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Provides support to the external ear and posterior wall of
the middle ear cavity.
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Contains air cavities which can be reservoir for infection.
Middle ear:
Middle ear- small air-filled cavity:
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tympanic membrane at junction of middle and outer ear
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semitransparent fibrous connective tissue
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hole in tympanic membrane is a perforated
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oval and round windows.
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mastoid sinus:
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separated from brain by only
thin bony partition
Tympanic Membrane:
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Thin membrane
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Forms boundary between outer and middle ear.
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Vibrates in response to sound waves
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Changes acoustical energy into mechanical energy
eardrum.
Middle Ear Bones
Malleus Or Hammer:
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connects to tympanic membrane.
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connects to oval window
b. Incus Or Anvil.
C. Stapes Or Stirrup:
Eustachian or auditory tube:
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4 cm long and opens into nasopharynx.
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infections may travel along this passageway.
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equalizes the pressure on either side of the eardrum, which is necessary for
optimal hearing.
Without this function, a difference between the static pressure in the middle ear and
the outside pressure may develop, causing the eardrum to displace inward or
outward.
Middle ear muscles:
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The movements of the ossicles is controlled by muscles attached to them:
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the tensor tympani and
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the stapedius.
Function:
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Help maintain ossicles in proper position
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Protect inner ear from excessive sound levels
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When ear exposed to sound levels above 70 dB, the muscles contract,
decreasing amount of energy transferred to inner ear
This protective reflex termed "acoustic reflex"
Ligaments of Middle Ear:
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Function
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restrict and confine the effect of ossicles to act as a lever.
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restrict movements to reduce the chance of damage to the inner ear .
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prevents distortion to sound.
Inner ear:
A. BONY LABYRINTH consist of:
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3 semicircular canals (lateral, posterior
and superior).
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It is lined with periosteum and filled
with perilymph.
B.MEMBRANOUS LABYRINTH CONSIST OF:
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series of ducts called
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scala vestibuli,
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scala tympani and
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scala media.
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The inner ear is fluid filled and the fluid in each duct has a different composition.
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The fluid in scala vestibuli and scala tympani is called perilymph, which is similar to ECF.
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The fluid in scala media is called endolymph, which has high K+ conc. and low Na+ conc. K+ ions
plays a role in the generation of auditory signals.
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The endolymph is unusual in that its composition is similar to that of ICF but technically ,it is
ECF.
The cochlea and the vestibule are formed from the bony and membranous labyrinths.
1. Cochlea:
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spiral shaped series of 3 tubular canals or ducts.
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contain organ of corti.
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Organ of corti. contains the receptor cells
and is site of auditory transduction.
Basilar membrane
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short, stiff fibers near windows
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vibrate at higher frequencies
long, limber fibers near helicotrema
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vibrate at lower frequencies
Organ of Corti:
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hair cells are receptors for auditory
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when portion of basilar membane vibrates, the sterocilia of
the hair cells are pressed against the tectorial membrane
and become distorted.
sensations.
2. Vestibule:
a. utricle
b. saccule.
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oval central portion of bony labyrinth
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contains a membranous labyrinth consisting of
two sacs
3. Semicircular Canals.
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Three - one in each plane
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One end of each canal enlarges into a swelling called the ampulla.
Physiology of Hearing:
1) Sound waves strike tympanic membrane causing it to vibrate. Membrane moves in with each
compression & out with each decompression.
2) Movement of membrane is transferred to the ossicles in order; malleus, incus, & stapes. Vibrations
are conducted across tympanic cavity and amplified about 22 times.
3) The stapes moves in and out at the oval window. Since liquids don’t compress, the pressure of the
perilymph in the scala vestibuli (vestibular duct ) goes up and down. The secondary tympanic
membrane over the round window bulges out with each increase in pressure.
4) Each increase in pressure in the scala vestibuli pushes the vestibular membrane down, which
increases the pressure of the endolymph in the cochlear duct, which pushes the basilar membrane
down.
5) Vibration of the basilar membrane causes vibration of hair cells against the tectorial membrane. This
causes movement of the stereocilia on the hair cells.
6) Information about the region of the cochlea (pitch) & intensity of the stimulus are relayed to CNS over
cochlear branch of vestibulocochlear cranial nerve.
Auditory Pathways:
1. Sound waves travel toward tympanic membrane, which vibrates
2. Auditory ossicles conduct the vibration into the inner ear
1. Tensor tympani and stapedius muscles contract to reduce the amount of movement
when loud sounds arrive
3. Movement at the oval window applies pressure to the perilymph of the cochlear duct
4. Pressure waves distort basilar membrane
5. Hair cells of the Organ of Corti are pushed against the tectorial membrane
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Ascending Pathways
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Cochlear Nerve
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Brain Stem
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Cochlear Nuclei
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Superior Olivary Nuclei
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Nuclei of Lateral Lemnisci
Midbrain
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Inferior Colliculi
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Medial Geniculate Nuclei on Thalamus
Temporal Cerebral Cortex
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Primary Auditory Receiving Area (A1)
Descending Pathways from brain stem nuclei to cochlea