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University of Toronto Scarborough
Janelle Leboutillier

Ch 4The Action Potential AP Action potentialconveys info Over long distances in the CNSAt rest cytosol is negative The frequency and pattern of action potentials constitute the code used by neurons to transfer info from one location to anotherUps and Downs At rest neuronal membrane is 65 mV Action potentialbecomes positive brieflyOscilloscope studies the action potential And records voltage as it changes over time First part of APrising phase a rapid depolarization of the membrane continues until vm reaches 40 mVpeak Overshoot where inside of neuron is positively charged with respect to the outsideFalling phaserapid repolarization until membrane become more neg than the resting potential Undershoot after hyper polarization is the last part of falling phase until gradual restoration of resting potential All of this lasts 2 ms The Generation of an APChain of events of an AP 1 Thumbtack enters skin 2 Membrane of nerve fibers in skin is stretched 3 Na permeable channels open and due to large conc gradient andcytosol Na enters through channels Na depolarizes the membrane cytosol becomes less negative This is generator potential If GP achieves critical level membrane goes through AP Critical level of depolarization that must be crossed is thresholdTherefore AP are caused by depolarization of membrane beyond threshold Interneurons depolarization is usually caused by Na entry through channels that are sensitive to NT released by other neuronsThe Generation of Multiple AP If we pass continuous depolarizing current into a neuron through a microelectrode we will generate many AP in succession Depends on magnitude of depolarizing currentfiring frequency Although there is a limit to the rate at which a neuron can generate AP 1000Hzmax once AP initiated cannot initiate anotherabsolute refractory period Relative refractory periodamount of current required to depolarize the neuron to AP threshold is elevated above normalAP in Theory Ideal neuron NaK pump K channel Na channelK conc 20fold inside cell Na conc 10fold outside cell
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