Physiology of Auditory Physiology of Auditory SystemSystem

Dr.Sherif BugnahENT Resident

Components of the Hearing Mechanism Components of the Hearing Mechanism (by function(by function))

Outer Ear Outer Ear : Capture; Amplify frequencies

Middle EarMiddle Ear: Protection(by stapedius reflex) & Impedance match of large amplitude vibration in air into a small amplitude vibration Inner Ear &CNS : Frequency analysis Transduction

Structures of the Outer EarStructures of the Outer Ear

Auricle (Pinna)▫Collects sound▫Helps in sound localization▫Most efficient in directing high frequency sounds to TM▫Gathers sound waves▫Helps in localization▫Amplifies sound approx. 5-6 dB

External Auditory CanalExternal Auditory Canal

• Approximately 1¼ inch in length• “S” shaped• Lined with cerumen glands

(moisten skin)• Outer 1/3rd cartilage; inner 2/3rds

mastoid bone• Allows air to warm before

reaching TM• Isolates TM from physical damage• Increases sound pressure at the

tympanic membrane by as much as 5-6 dB (due to acoustic resonance)

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Mastoid ProcessMastoid Process

•Bony ridge behind the auricle

•protects cochlea and vestibular system

• Provides support to the external ear and posterior wall of the middle ear cavity

•Contains air cavities which can be reservoir for infection

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Tympanic MembraneTympanic Membrane

•Thin membrane•Forms boundary

between outer and middle ear

•Vibrates in response to sound

•Changes acoustical energy into mechanical energy

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The Ossicular ChainThe Ossicular Chain

A: Malleus (TM attaches at Umbo)

B: Incus (Connector function)C: Stapes

Smallest bone in the bodyFootplate inserted in oval window on medial wallFocus/amplify vibration of TM to smaller area, enables vibration of cochlear fluids

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Eustachian Tube Eustachian Tube

• Mucous-lined, connects middle ear cavity to nasopharynx

• “Equalizes” air pressure in middle ear

• Normally closed, opens under certain conditions(swallowing)

• Pathway for infection• Children “grow out of” most

middle ear problems as this tube lengthens and becomes more vertical

• Not a part of the hearing process

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Stapedius MuscleStapedius 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

•Absent acoustic reflex could signal conductive loss or marked sensorineural loss

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CochleaCochlea

CochleaCochlea

Oval Window – located at the footplate of the stapes; when the footplate vibrates, the cochlear fluid is set into motion (vibrations in the oval window are 20X larger than those in the eardrum)

Round Window – functions as the pressure relief port for the fluid set into motion initially by the movement of the stapes in the oval window

CochleaCochlea

CochleaCochlea

Three-chambered tube: Two chambers are separated by the basilar membrane, on which sits the organ of Corti. The arch in the middle of the organ of Corti separates the inner from the outer hair cells

Organ of CortiOrgan of Corti

On top of the hair cells sits the tectorial membrane which is attached only along its inner edge. The stereocilia (hairs) of the outer hair cells are embedded in the tectorial membrane, but those of the inner hair cells are moved by movement of the fluid in the space between the hair cells and the tectorial membrane.

The end organ of hearing Contains: . Stereocilia & receptor hair cells. 3 rows OHC . 1 row IHC. Tectorial and Basilar Membranes. Cochlear fluids

Cochlea cross-sectionCochlea cross-section

Organ of CortiOrgan of Corti16

When the pressure across the basilar membrane changes (through activity of the stapes), the membrane bends and fluid flows in this space, causing the inner hair cell stereocilia to move.

Hair CellHair Cellss

•When the hairs are bent towards the tallest stereocilium ,the cell's voltage is increased, more neurotransmitter is released and auditory nerves connected to the hair cell increase their activity.•When the hairs are bent away from the tallest stereocilium ,the cell's voltage is decreased. Less neurotransmitter is released and auditory nerves connected to the hair cell decrease their rate.

Hair CellHair Cellss

Inner hair cells are responsible for turning mechanical movement of the basilar membrane into neural signal of the auditory nerve.

Frequency-specific High pitch sounds = base of cochlea Low pitch sounds = apex of cochlea

Opens ion channels, allows K+ to enter = depolarization

AP wave travels length of basilar membrane Auditory nerves send AP to brain for analysis

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Hair CellHair Cellss

Inner vs Outer Hair CellsInner vs Outer Hair Cells

Inner Hair CellsInner Hair CellsOuter Hair CellsOuter Hair Cells

SensorySensoryMotorMotor

Afferent nervesAfferent nervesEfferent nervesEfferent nerves

Single rowSingle row3 rows3 rows

Vestibular System

• Consists of three semi-circular canals

• Monitors the position of the head in space

• Controls balance • Shares fluid with the

cochlea• No part in hearing

process

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Central Auditory SystemCentral 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

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Auditory nerve Innervation

OHC (2)IHC (1)

Physiology of Auditory Physiology of Auditory SystemSystem

The EndThe End