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ANATOMY AND PHYSIOLOGY OF THE EAR

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Main Components of the Hearing Mechanism

Outer Ear Middle Ear Inner Ear Central Auditory Nervous System

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Structures of the Outer Ear

Auricle (Pinna)– Collects sound– Helps in sound

localization– Most efficient in

directing high frequency sounds to the eardrum

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External Auditory Canal

Approximately 1¼ inch in length

“S” shaped Lined with cerumen glands Outer 1/3rd cartilage; inner

2/3rds mastoid bone Increases sound pressure at the

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

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Mastoid Process

Bony ridge behind the auricle

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

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Tympanic 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 Chain

A: Malleus B: Incus C: Stapes

– Ossicles are smallest bones in the body

– Act as a lever system– Footplate of stapes

enters oval window of the cochlea

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Eustachian Tube

Lined with mucous membrane; connects middle ear to back of the throat (nasopharynx)

Equalizes air pressure Normally closed except

during yawning or swallowing

Not a part of the hearing process

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Stapedius Muscle

Connects the stapes to the middle ear wall Contracts in response to loud sounds; known

as the Acoustic Reflex

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Structures of the Inner Ear

Cochlea - Snail-shaped organ with a series of fluid-filled tunnels; converts mechanical energy into electrical energy

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Oval Window – located at the footplate of the stapes; when the footplate vibrates, the cochlear fluid is set into motion

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

Structures of the Inner Ear (Cont.)

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Organ of Corti

The end organ of hearing; contains stereocilia and hair cells.

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Hair Cells Frequency-specific

High pitch sounds = base of cochleaLow pitch sounds = apex of cochlea

When the basilar membrane moves, a shearing action between the tectorial membrane and the organ of Corti causes hair cells to bend

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Vestibular System Consists of three semi-

circular canals Shares fluid with the

cochlea Controls balance No part in hearing process

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Central Auditory System

8th Cranial Nerve or “Auditory Nerve” carries signals from cochlea to brain

Fibers of the auditory nerve are present in the hair cells of the inner ear

Auditory Cortex: Temporal lobe of the brain where sound is perceived and analyzed

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How Sound Travels Through How Sound Travels Through The Ear...The Ear...

Acoustic energy, in the form of sound waves, is channeled into the ear canal by the pinna. Sound waves strike the tympanic membrane, causing it to vibrate like a drum, and changing it into mechanical energy. The malleus, which is attached to the tympanic membrane, starts the ossicles into motion. (The middle ear components mechanically amplify sound). The stapes moves in and out of the oval window of the cochlea creating a fluid motion. The fluid movement within the cochlea causes membranes in the Organ of Corti to shear against the hair cells. This creates an electrical signal which is sent via the Auditory Nerve to the brain, where sound is interpreted!

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