NeurobioIogyII - Box 7-1 The action potential Threshold...

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Box 7-1
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The action potential Threshold overshoot peak of AP repolarization after hyperpolarization refractory period All-or-none (I) Refractory period 1 msec.
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If the membrane were solely permeable to K + , the membrane potential would be: E K = 58 log [K out ]/ [K in ] or 58 log(20/400) = -75 mV If the membrane were solely permeable to Na + , the membrane potential would be: E Na = 58 log [Na out ]/ [Na in ] or 58 log (440/50) or 58 (0) = +55 mV Intra- and extracellular Na + and K + concentrations for squid axon: Intra (mM) Extra (mM) Na + 50 440 K + 400 20
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The difference in intra- and extracellular ion concentrations is due to the Na + -K + pump that pumps Na + out of and K + into the cell. K + K + K + K + K + K + K + K + K + K + K + K + K + K + Na + Na + Na + Na + Na + Na + Na + Na + Na + Na + Na + Na + Na + Na + Cl - Cl - Cl - Cl - Cl - Cl - Cl - Cl - Cl - Cl - Cl - Cl - Cl - Cl - Cl - Cl - Cl - Cl - Cl - Cl - Cl - Cl - Cl - Cl - Cl - Cl - Cl - Cl -
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At rest, the K + leak channel (not voltage-dependent) is open and K + ions run down their concentration gradient (until opposed by the developing electrical gradient) setting V m near E K . K + K + K + K + K + K + K + K + K + K + K + K + K + K + Na + Na + Na + Na + Na + Na + Na + Na + Na + Na + Na + Na + Na + Na + Cl - Cl - Cl - Cl - Cl - Cl - Cl - Cl - Cl - Cl - Cl - Cl - Cl - Cl - Cl - Cl - Cl - Cl - Cl - Cl - Cl - Cl - Cl - Cl - Cl - Cl - Cl - Cl -
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When a neuron is depolarized, voltage-dependent Na + channels open. Na + rushes in until Na + channels inactivate (next slide) or V m reaches E Na . K + K + K + K + K + K + K + K + K + K + K + K + K + K + Na + Na + Na + Na + Na + Na + Na + Na + Na + Na + Na + Na + Na + Na + Cl - Cl - Cl - Cl - Cl - Cl - Cl - Cl - Cl - Cl - Cl - Cl - Cl - Cl - Cl - Cl - Cl - Cl - Cl - Cl - Cl - Cl - Cl - Cl - Cl - Cl - Cl - Cl -
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Voltage-dependent Na + channels inactivate (no more Na + enters). Voltage-dependent K + channels open, K + leaves (flows down its electrical gradient because V m is near E Na ). This continues until V m approaches E K . K + K + K + K + K + K + K + K + K + K + K + K + K + K + Na + Na + Na + Na + Na + Na + Na + Na + Na + Na + Na + Na + Na + Na + Cl - Cl - Cl - Cl - Cl - Cl - Cl - Cl - Cl - Cl - Cl - Cl - Cl - Cl - Cl - Cl - Cl - Cl - Cl - Cl - Cl - Cl - Cl - Cl - Cl - Cl - Cl - Cl -
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Action potential Resting potential (cell is negative). Only leak K + channels open. Na + channels open, Na + enters (now cell is positive). Na + channel close, voltage-gated K + channels open (membrane repolarizes, now cell returns to negative).
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1937--Cole & Curtis record from squid giant axons, show conductance increase during AP . K.S. “Kasey” Cole AP Membrane conductance
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intracellularly and discover the AP overshoots 0 mV. A. Hodgkin
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This note was uploaded on 03/28/2012 for the course BIO 49593 taught by Professor Wanser during the Spring '10 term at University of Texas at Austin.

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NeurobioIogyII - Box 7-1 The action potential Threshold...

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