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BIO203 - Chapter 4 Summary

BIO203 - Chapter 4 Summary - CHAPTER 4 SUMMARY Introduction...

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CHAPTER 4 SUMMARY Introduction n Nerve and muscle cells are excitable because they can rapidly alter their membrane permeabilities and thus undergo transient membrane potential changes when excited. n Compared to resting potential, a membrane becomes depolarized when its potential is reduced and hyperpolarized when its potential is increased. n Changes in potential are brought about by triggering events that alter membrane permeability, in turn leading to changes in ion movement across the membrane. n There are two kinds of potential change: (1) graded potentials, which serve as short- distance signals that quickly die off within a close range of the small patch of membrane where they are first triggered, and (2) action potentials, the long-distance signals. Graded Potentials n Graded potentials occur in a small specialized region of an excitable cell membrane. n The magnitude of a graded potential varies directly with the magnitude of the triggering event. n Graded potentials passively spread decrementally by local current flow and die out over a short distance. Action Potentials n During an action potential, depolarization of the membrane to threshold potential triggers sequential changes in permeability caused by conformational changes in voltage- gated channels. These permeability changes bring about a brief reversal of membrane
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potential, with Na + influx being responsible for the rising phase (from -70 mV to -30 mV), followed by K + efflux during the falling phase (from peak back to resting potential). n Before an action potential returns to resting, it regenerates an identical new action potential in the area next to it by means of current flow that brings the previously inactive area to threshold. This self-perpetuating cycle continues until the action potential has spread throughout the cell membrane in undiminished fashion.
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