Action potential

29 views2 pages
Published on 26 Nov 2011
School
Western University
Department
Physiology
Course
Physiology 3120
Professor
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 (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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Document Summary

Voltage-gated sodium channels open (very rapidly), and lots of sodium flows into the cell. Membrane depolarizes rapidly to +45mv (inside of cell has positive charge), but never reaches. Potassium voltage-gated channels begin opening while sodium channels become inactivated. Little bit of overlap between channel openings: repolarization, hyperpolarization. Eventually all k channels are open & all na channels are inactivated. K continues to leave the cell (channels close slowly) Membrane potential returns to normal (-70mv), and channels return to resting configuration: voltage-gated ion channels. 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. 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.

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