2 Main Components of the Hearing Mechanism Outer EarMiddle EarInner EarCentral Auditory Nervous System
3 Structures of the Outer Ear Auricle (Pinna)Collects soundHelps in sound localizationMost efficient in directing high frequency sounds to the eardrum
4 External Auditory Canal Approximately 1¼ inch in length“S” shapedLined with cerumen glandsOuter 1/3rd cartilage; inner 2/3rds mastoid boneIncreases sound pressure at the tympanic membrane by as much as 5-6 dB (due to acoustic resonance)
5 Mastoid Process Bony ridge behind the auricle Provides support to the external ear and posterior wall of the middle ear cavity
6 Tympanic Membrane Thin membrane Forms boundary between outer and middle earVibrates in response to soundChanges acoustical energy into mechanical energy
7 The Ossicular Chain A: Malleus B: Incus C: Stapes Ossicles are smallest bones in the bodyAct as a lever systemFootplate of stapes enters oval window of the cochlea
8 Eustachian TubeLined with mucous membrane; connects middle ear to back of the throat (nasopharynx)Equalizes air pressureNormally closed except during yawning or swallowingNot a part of the hearing process
9 Stapedius Muscle Connects the stapes to the middle ear wall Contracts in response to loud sounds; known as the Acoustic Reflex
10 Structures of the Inner Ear Cochlea - Snail-shaped organ with a series of fluid-filled tunnels; converts mechanical energy into electrical energy
11 Structures of the Inner Ear (Cont.) Oval Window – located at the footplateof the stapes; when the footplate vibrates,the cochlear fluid is set into motionRound Window – functions as the pressure relief port for the fluid set into motion initially by the movement of the stapes in the oval window
12 Organ of CortiThe end organ of hearing; contains stereocilia and hair cells.
13 Hair Cells Frequency-specific High pitch sounds = base of cochleaLow pitch sounds = apex of cochleaWhen the basilar membrane moves, a shearing action between the tectorial membrane and the organ of Corti causes hair cells to bend
14 Vestibular System Consists of three semi-circular canals Shares fluid with the cochleaControls balanceNo part in hearing process
15 Central Auditory System 8th Cranial Nerve or “Auditory Nerve” carries signals from cochlea to brainFibers of the auditory nerve are present in the hair cells of the inner earAuditory Cortex: Temporal lobe of the brain where sound is perceived and analyzed
16 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!