the ear. components of hearing mechanism - outer ear - middle ear - inner ear - central auditory...
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The EarComponents of hearing mechanism- Outer Ear- Middle Ear- Inner Ear- Central Auditory Nervous System
The EarAuricle (Pinna)Collects soundHelps in sound localizationMost efficient in directing high frequency sounds to the eardrum
The EarExternal Auditory CanalApproximately 1 inch in lengthS 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)
The EarMastoid ProcessBony ridge behind the auricleProvides support to the external ear and posterior wall of the middle ear cavity
The EarTympanic MembraneThin membraneForms boundary between outer and middle earVibrates in response to soundChanges acoustical energy into mechanical energy
The EarThe Ossicular ChainA: MalleusB: IncusC: StapesOssicles are smallest bones in the bodyAct as a lever systemFootplate of stapes enters oval window of the cochlea
The Ear 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
The EarStapedius MuscleConnects the stapes to the middle ear wallContracts in response to loud sounds; known as the Acoustic Reflex
The EarStructure of The Inner EarCochlea - Snail-shaped organ with a series of fluid-filled tunnels; converts mechanical energy into electrical energy
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.)
The EarOrgan of CortiThe end organ of hearing; contains stereocilia and hair cells.
The EarHair CellsFrequency-specificHigh 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
The EarVestibular SystemConsists of three semi-circular canalsShares fluid with the cochleaControls balanceNo part in hearing process
The EarCentral Auditory System8th 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
The EarHow 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!
The EarTransduction of sound into an auditory perceptionSound is a propagating pressure wave.Perception of sound involves the electrical activity of neurons in the auditory cortex of the brain.The transduction process is the means by which the pressure waves in air (a mechanical stimulus) is converted into neural activity (action potentials).This process involves a number of stages, some of which involve conduction and impedance matching.
The EarThe path of soundear canal vibrate tympanic membrane vibrate ossicles (3 bones: Malleus, Incus, Stapes) vibrate oval window of cochlea create waves in cochlea fluid create standing waves in basilar membrane movement of hair cells generates electrical activity through mechanicallygated ionic channels hair cells stimulate the auditory nerve series of action potentials up to the auditory cortex.
The EarCochlear Mechanics
Basilar membrane : The spectral analyser Basilar membrane (BM) is approx 33mm long in humans Apex of BM is wide and relatively loose Base of BM is thinner and more stiff Variations in length and stiffness provides BM with a continuum of resonant frequencies along its length: low frequencies at apex and high frequencies at base A wave with a particular frequency produces a maximum displacement at a particular portion of the basilar membrane: tonotopic organization BM is heavily damped beyond the resonant frequency Travelling wave velocity is in range 1-20m/sec and is frequency dependent (velocity is reduced apically for low frequencies) High frequency waves vibrate the basal part of the basilar membrane, dissipate energy and then die out. Lower frequency waves travel further towards apex before dying out.
The EarMechanism of Hearing by Organ of Coti Vibration of the basilar membrane produces shear forces that bend the stereocilia (hairs protruding from the hair cells) against the tectorial membrane Movement of the stereocilia either cause the hair cell to depolarise or hyperpolarise, depending upon the direction of movement Changes in the membrane potential of the hair cell generate an AP in the nerve fibre attached to the hair cell.
The EarInner Hair Cells