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PHGY 209 Nerves and synapses.doc

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McGill University
PHGY 209
Ann Wechsler

NerveSynapsenot a single atom that is in your body today was there when that event took placeSteve Grand computer scientistspinal cordhighway of processing information relaying up to brain and down to periphery of body and mediates automated activities ex walkingafferent fibers or sensory neurons bring information from periphery to spinal cord ex skin sensationsefferent fibers or motor neurons allow movementnervous system comprises about 100 billion neuronsneurons use electricity to propagate information from a place to anotherthere is probably from a hundred trillion to a quadrillion of synapses in human nervsystsoma cell body containing nucleus where metabolic and synthetic activities take place maintaining neuron in living statedendritesantennas of neuron for information to flow inaxon unique for information to flow out initial segment inserts in cell soma presynaptic terminalswelling at axons end Resting membrane potential the inside of a neuron has a slightly negative voltageelectrical pressure compared to the outside 60mV to 70mV tiny fraction of unpaired negatives charges less than 1 Neurons use it as a starting point for propagating electrical signalsalmost all cells have a resting membrane potentialat rest neurons still have great permeability to potassium ions because of leak potassium channels but Caothers cant flow in nor outnegative voltage of neurons is then due to high concentration of potassium ionWhen the chemical and electrical gradients are equal the system is at equilibriumquickly attained The membrane potential at equilibrium is described by the Nernst equationNernst equation tells us exactly what the voltage is going to beW the Nernst equation we find ENa50mV and ECl70mVThe resting membrane potential is a bit more positive than E because there is a small inward leak of Na which Kpushes the membrane slightly toward ENaThe membrane potential is determined by concentration gradients and relative permeabilities of membrane to different physiological ionsThe dominant permeability makes greatest contribution to the membrane potentialAt rest the dominant permeability is to potassium so the membrane potential is close to EK
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