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Generation of Action Potential



  • Action potential is a rapid change in membrane potential that travels rapidly along the membrane.

  • Different cells show different duration and shapes of action potential depending on their special properties and functions. Here action potential in neurons is explained as an example.

Resting Stage

  • Inside of the cell is electronegative.

  • e.g. resting membrane potential of neuron is about -90 mV.

  • Na and K channels are closed.

Depolarization

Stimulation of the cell

e.g. acetylcholine opens ligand-gated Na channels

โ†“

Influx of some Na ions

โ†“

As Na is positively charged ion, its entry moves the voltage inside the cell in positive direction

โ†“

Potential reach the threshold

โ†“

Opening of voltage-gated Na channels

โ†“ Massive influx of Na

โ†“

Rapid rise in potential

โ†“

Potential approaches zero

โ†“

Thus the membrane is depolarized


Repolarization

Na channels start closing โ†’ inflow of Na stops.

K channels start opening โ†’ K moves out of the cell.

โ†“

Net loss of positive charge from inside to outside the cell

โ†“ Voltage moves in the negative direction

โ†“ Reaches resting membrane potential

โ†“

This is called repolarization

โ†“

K efflux continues for some time even after this

โ†“

Voltage goes below resting membrane potential

โ†“

This is called afterhyperpolarization

โ†“

K channels start closing

โ†“

K efflux stops

โ†“

Potential reaches resting membrane potential


Restoration of Na and K concentration

During action potential, there is a net movement of Na from outside to insideand a net movement of K from inside to outside the cell

โ†“

Slight rise in intracellular Na concentration & slight fall in intracellular K concentration

โ†“

This is corrected by ongoing activity of Na-K ATPase pump which moves Na out and K in.


Properties of Action Potential

  • Its all or none phenomenon

    • If the stimulus depolarizes the cell up to threshold โ†’ action potential occurs fully.

    • If the stimulus cannot depolarize the cell up to threshold โ†’ no action potential at all.

  • It occurs very fast: Duration of depolarization to repolarization is 1-2 milliseconds.

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