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The Ear The Ear Components of hearing mechanism - Outer Ear - Middle Ear - Inner Ear - Central Auditory Nervous System The Ear The Ear Auricle (Pinna) Collects sound Helps in sound localization Most efficient in directing high frequency sounds to the eardrum The Ear 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) The Ear Mastoid Process Bony ridge behind the auricle Provides support to the external ear and posterior wall of the middle ear cavity The Ear Tympanic Membrane Thin membrane Forms boundary between outer and middle ear Vibrates in response to sound Changes acoustical energy into mechanical energy The Ear 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 The Ear 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 The Ear Stapedius Muscle Connects the stapes to the middle ear wall Contracts in response to loud sounds; known as the Acoustic Reflex The Ear Structure 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 The Ear Organ of Corti The end organ of hearing; contains stereocilia and hair cells. The Ear 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 The Ear Vestibular System Consists of three semicircular canals Shares fluid with the cochlea Controls balance No part in hearing process The Ear 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 The Ear 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! The Ear Transduction of sound into an auditory perception Sound 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 Ear The path of sound ear 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 mechanically gated ionic channels → hair cells stimulate the auditory nerve → series of action potentials up to the auditory cortex. The Ear Cochlear 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 Ear The Ear The Ear Mechanism 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 Ear Inner Hair Cells