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Action potential

Course Code
Tom Stavraky

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Human Physiology
Wednesday, September 30, 2009
Action Potential
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
Peak of the action potential
Potassium voltage-gated channels begin opening while sodium channels become inactivated
Little bit of overlap between channel openings
Eventually all K channels are open & all Na channels are inactivated
Potassium rushes out of the cell
Membrane repolarizes (from +45mV to -70mV)
K continues to leave the cell (channels close slowly)
Membrane hyperpolarizes (EK= -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 membrane potential by 100mV
1 mol = 6.02 x 1023; [Na]0 = 140 mOsmoles/kg water = 8.4 x 1022 outside the cell
So an action potential won’t affect concentration gradients dramatically
Duration is very short
1-2ms in nerves & muscle
300ms in cardiac muscle
Refractory periods
Absolute refractory period
No stimulus will excite the nerve
Occurs during increased Na conductance
Caused by inactivated sodium channels
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