Chapter 4

Chapter 4 - Action Potentials Properties of the Action...

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Action Potentials Properties of the Action Potential o Oscilloscope used to measure action potentials because it is 100x faster than a blink of an eye o Stages of an action potential Rising phase Rapid depolarization of the membrane Continues until Vm reaches a peak of 40mV Overshoot Part where the inside of the neuron is positively charged with respect to the outside Falling Phase Rapid repolarization until the membrane is actually more negative than the resting potential Undershoot or After-hyperpolarization Gradual restoration of the resting potential. Lasts about 2 milliseconds o Generation of an Action Potential 1) thumbtack enters skins 2) membrane of the nerve fibers in the skin is stretched 3) Na+ permeable channels open. Since there is a large concentration gradient and the negative charge of the cytosol, Na+ enters the fiber through these channels Na+ depolarizes the membrane (the cytoplasmic (inside) surface of the membrane becomes less negative) o This is the generator potential and if it reaches a critical level, the membrane will generate an action potential. The critical level of depolarization that must be crossed in order to trigger an action potential is called a threshold Depolarizations that cause action potentials arises in different ways in different neurons. In this example, it is through the Na+ channels o Generation of Multiple Action Potentials Firing frequency of action potentials reflects the magnitude of the depolarizing current. Maximum firing frequency is about 1000Hz (1000 impulses per second) After an action potential is initiated, it is impossible to initiate another for about 1 msec This is called the absolute refractory period
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o It can also be hard to initiate another action potential for several milliseconds after the end of the absolute refractory period. This is called the relative refractory period, when the current required to depolarize the neuron to action potential threshold is elevated above normal. o Action Potential, in theory Depolarization is caused by the influx of sodium ions across the membrane, and repolarization is caused by the efflux of potassium ions Membrane currents and conductances o Assume K+ is 20X inside the cell and Na+ is 10X outside the cell Assume both channels are closed and Vm is
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This note was uploaded on 04/17/2008 for the course NEURO 101 taught by Professor Mosh. during the Fall '08 term at NYU.

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Chapter 4 - Action Potentials Properties of the Action...

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