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Action potential

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Physiology 3120
Tom Stavraky

Human Physiology Wednesday, September 30, 2009 Action Potential • Depolarization  Stimulate the nerve  Membrane depolarizes (from -70mV to -55mV)  Voltage-gated sodium channels open (very rapidly), and lots of sodium flows into the cell  Action potential is initiated  Membrane depolarizes rapidly to +45mV (inside of cell has positive charge), but never reaches ENa • Peak of the action potential  Potassium voltage-gated channels begin opening while sodium channels become inactivated  Little bit of overlap between channel openings • Repolarization  Eventually all K channels are open & all Na channels are inactivated  Potassium rushes out of the cell  Membrane repolarizes (from +45mV to -70mV) • Hyperpolarization  K continues to leave the cell (channels close slowly)  Membrane hyperpolarizes (E K -90mV)  Potassium channels close  Membrane potential returns to normal (-70mV), and channels return to resting configuration • Voltage-gated ion channels  Grayanotoxin (found in rhododendrons)  Reversibly binds to sodium voltage-gated channels, and prevents inactivation  Cells are in a constant state of depolarization  In the area of Trabzon on the Black Sea • Changes in sodium & potassium conductance  At rest. . .  Sodium conductance very low  Potassium conductance slightly higher  After stimulation  Sodium conductance rises very rapidly (not all sodium channels open at once [stepwise])  Potassium conductance rises after, and is more gradual (not all these channels open at once, either)  Overlap of sodium & potassium conductance gives AP a rounded peak • Facts about the action potential  Very few ions move through membrane during an action potential  It takes 50 million ions to change the
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