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Ch11
 

Ch11

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    Ch11 Ch11 Presentation Transcript

    • THE AUDITORY AND VESTIBULAR SYSTEMS
    • THE NATURE OF SOUNDSound is an audible variations in air pressure, defined by:1) frequency: Number of cycles (distance between successive compressed patches)per second expressed in units called Hertz (Hz). Human Range is btw 20 Hz to 20,000 Hz2) Intensity: Difference in pressure between compressed and rarefied patches of air. Itdetermines the loudness of the sound.Sounds propagate at a constant speed: 343 m/sec
    • THE AUDITORY SYSTEM
    • THE MIDDLE EARSound Force (pressure) is amplified by the Ossicles, producing greater pressure at oval window(smaller surface) than tympanic membrane, in order to move more efficiently the fluid inside thecochelaThe Attenuation Reflex: response where onset of loud sound causes tensor tympani andstapedius muscle contraction. It’s used to adapt ear to loud sounds, or understand speech betterin noisy environment (more attenuation of low sounds)
    • THE INNER EARPerilymph: Fluid in scala vestibuli and scala tympaniEndolymph: Fluid in scala mediaEndolymph has an electric potential 80 mV more positive than perilymph (Endocochlear potential)
    • THE INNER EARBasilar Membrane is wider at apex, stiffness decreases from base to apex
    • THE INNER EARPressure at oval window, pushes perilymph into scala vestibuli, round window membrane bulgesout. Endolymph movement bends basilar membrane near base, wave moves towards apex
    • THE INNER EARThe Organ of Corti and Associated Structures. Here the mechanical energy of thesound is transformed in electrical signal by the auditory receptor cells (hair cells).Each hair cells has around 100 stereocilia.Rods of corti provide structural support. Hair cells form synapses with bipolar neuronsthat have their body in the spiral ganglion. Their axons form the auditory nerve
    • THE INNER EARTransduction by Hair CellsWhen sound arrives, basilar membrane moves. According to the movement, stereociliabends on one or the other direction: i.e. Basilar membrane upward, reticular lamina upand stereocilia bends outward
    • THE AUDITORY PATHWAY Auditory cortex A1MGN MGN Superior olive Auditory nerve
    • INFORMATION ABOUT THE SOUNDInformation About Sound Intensity is encoded in 2 ways: Firing rates of neurons and number of active neuronsStimulus Frequency Frequency sensitivity: in Basilar membrane is Highest at base, lowest at cochlea apex. This coding is kept separate along the auditory pathways (tonotopy)Phase Locking is another way to code for frequency Consistent firing of cell at same sound wave phase. Only for frequency below 4kHz
    • SOUND LOCALIZATION: HORIZONTAL PLANEInteraural time delay: Time taken for Interaural intensity difference: Sound atsound to reach from ear to ear high frequency from one side of ear Sound Sound Sound shadow waves waves Sound waves Sound waves Sound shadow Sound shadow Duplex theory of sound localization: Interaural time delay: 20-2000 Hz Interaural intensity difference: 2000-20000 Hz
    • SOUND LOCALIZATION: VERTICAL PLANE pinna Path 2, direct sound Path 2, reflected sound Path 2, direct sound Path 2, reflected sound Path 3, direct sound Path 3, reflected soundBased on reflections from the pinna
    • THE AUDITORY CORTEX: BA 41Axons leaving MGN project to auditory cortex viainternal capsule in an array called AcousticRadiation Primary auditory cortex Secondary auditory cortex
    • THE VESTIBULAR SYSTEM Importance of Vestibular System Balance, equilibrium, posture, head position, eye movementThe Vestibular Labyrinth
    • THE VESTIBULAR SYSTEMThe Otolith Organs (saccule and utricle). Detect force of gravity (linear acceleration)and tilts (change of angle) of the head.Saccule is vertically oriented and utricle horizontally oriented Crystals of calcium carbonate Bending of the hairs toward kinocilium: depolarization
    • THE VESTIBULAR SYSTEMThe Semicircular Canals. Detect rotation of the head and angular acceleration Crista: Sheet of cells where hair cells of semicircular canals clustered Ampulla: Bulge along canal, contains crista Cilia: Project into gelatinous cupula Kinocili oriented in same direction so all excited or inhibited together Filled with endolymph Three semicircular canals on one side helps sense all possible head- rotation angles Each Canal paired with another on opposite side of head Rotation causes excitation on one side, inhibition on the other endolymph
    • CENTRAL VESTIBULAR PATHWAY S1/M1 Face area
    • VESTIBULO-OCULAR REFLEX (VOR) Motion of the headFunction: Line of sight fixed on Motion of the eyesvisual targetMechanism: Senses rotations ofhead, commands compensatorymovement of eyes in oppositedirection.Connections from semicircularcanals, to vestibular nucleus, tocranial nerve nuclei exciteextraocular muscles