ANATOMY AND PHYSIOLOGY OF THE EAR

Main Components of the Hearing Mechanism Outer Ear Middle Ear Inner Ear Central Auditory Nervous System

Structures of the Outer Ear Auricle (Pinna) Collects sound Helps in sound localization Most efficient in directing high frequency sounds to the eardrum

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)

Mastoid Process Bony ridge behind the auricle Provides support to the external ear and posterior wall of the middle ear cavity

Tympanic Membrane Thin membrane Forms boundary between outer and middle ear Vibrates in response to sound Changes acoustical energy into mechanical energy

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

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

Stapedius Muscle Connects the stapes to the middle ear wall Contracts in response to loud sounds; known as the Acoustic Reflex

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

Structures of the Inner Ear (Cont.) 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

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

Hair Cells Frequency-specific High pitch sounds = base of cochlea Low 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

Vestibular System Consists of three semi-circular canals Shares fluid with the cochlea Controls balance No part in hearing process

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

How Sound Travels Through 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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