17Su11Lec5 - CS17 Neurobiology of Cognition Summer 2011...

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CS17 Neurobiology of Cognition Summer 2011 Lecture 5: Perception: Audition ANATOMY OF AUDITORY SYSTEM Pinna - Outer ear; individually distinct; We learn to use echoes in localization; Most mammals can orient pinna to sound Auditory Canal - Air channel leading to. . . Tympanic Membane (Eardrum) Ossicles - 3 tiniest bones in body ( Malleus /Hammer , Incus /Anvil , Stapes /Stirrup ), jointed into a lever system that converts vibration of large Tympanic Membrane into high intensity vibration of Oval Window = a membrane at the Base of the upper chamber of the … Cochlea = snail-shaped (coiled) tube w/3 fluid-filled chambers : top ( Scala Vestibuli ), mid ( S. Media ), bottom ( S. Tympani ) - Fluid is vibrated by oscillation of Stapes against Oval Window - Vibration travels from Base of top chamber to Apex (tip), around to Round Window at Base of bottom chamber, and back - This sets up a vibration in the Scala Media, where Transduction (conversion of env energy into neural signal) occurs ORGAN OF CORTI - The floor of the central Scala Media (or “Cochlear Canal”) is the Basilar Membrane , its ceiling is the Tectorial Membrane - The Auditory Receptor Cells - the HAIR CELLS - are located between these two membranes - This chamber is filled with a viscous, non-compressible, Potassium (K+) rich fluid called Endolymph - Vibration causes Basilar Membrane to move up and down and Tectorial Membrane to move left and right - Cilia of Hair Cells (arranged short to long) are bent between these two membranes - When bent toward longest cilium, K+ gates open, K+ enters cell (not Na+, unlike in prototypical neuron!) - Influx of K+ decreases polarity from –60mV (i.e. Same effect as Na+ in prototypical cell, just different mechanism) - The farther the cilia are bent, the greater the impact on the polarity and thus the on the release of NT => Chain reaction involving Second Messengers allows Ca++ to enter cell ==> release excitatory NT ( Glutamate ) - When cilia bent toward shortest cilium, K+ leaves cell (per gradients) and Ca++ is actively pumped out , restoring polarity - Typically, Hair Cell potential oscillates (polarized/depolarized/polarized/etc), repeatedly releasing at least some NT - If Cilia bent as little as 10 -10 m = 0.1 nm = diameter of an atom, can trigger a response! - So, Hair Cells show graded hypo-polarization and thus graded release of NT , depending on how far & often Cilia are bent => to Spiral Ganglions ( = ANFs = Auditory Nerve Fibers) which may then fire Action Potentials along axons to Brain CODING FOR FRQUENCY – Auditory Systems codes the freq of a sound in two ways:
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This note was uploaded on 09/07/2011 for the course COGS 17 taught by Professor Pineda during the Spring '06 term at UCSD.

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17Su11Lec5 - CS17 Neurobiology of Cognition Summer 2011...

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