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Chapter 4

Chapter 4 - The AP

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University of Toronto Scarborough
Janelle Leboutillier

Chapter 4The Action PotentialIntroductionAction potential generated by a cell are all similar in size and durationDo no diminish as they are conducted down the axonFrequency and pattern of APs constitute the code used by neurons to transfer info form 1 location to anotherProperties of the Action PotentialThe Ups and Downs of an Action PotentialMembrane Potential V can be determine by insetting a mmicroelectrode in the cell Voltmeter is used o measure electrical potential difference bw tip of intracellular microelectrode and another placed outside cellPotential difference of about 65 MvThe action potential has certain identifiable parts 1 Rising Phase Rapid depolarization of the membrane continue until V reaches a peak value of 40 mV mOvershoot Part of AP where inside of neuron is charged outside 2 Falling Phase Rapid repolarization until the membrane is actually morethan resting potential 3 Afterhyperpolarizationundershoot Last part of falling phase 4 Gradual restoration of resting potential LASTS2 MSECThreshold Critical level of depolarization that must be crossed in order to trigger an APThe Generation of an Action PotentialPerception of sharp pain when thumbtack enters good is caused by generation of APs in certain nerve fibers in Depolarization that causes AP comes from different way in different neurons 1 Depolarization was cause by entry of Na through specialized ion channels that were sensitive to membrane stretching 2 Interneurons depolarization usually cause by Na entry through channels that are sensitive to NTs released by other neurons Could also be depolarized by injecting electrical current through microelectrodeThe Generation of Multiple Action PotentialsRate of AP generation depends on magnitude of continuous depolarizing currentFiring frequency of AP reflects the magnitude of depolarizing currentthis is 1 way that stimulation intensity is encoded in NSThere is a limit to rate at which neuron can generate AP MAX1000 HzAbsolute Refractory Period Period of time where it is impossible to initiate 1 msecRelative Refractory Period Amount of current required to depolarize the neuron to AP threshold is elevated above normal The Action Potential in TheoryThe AP is a dramatic redistribution of electrical charge across membraneDepolarization fo cell during te AP is caused by the iflux of Na ions across membrane and repolarization is caused by the efflux of K ions
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