QS Study

Action Potential is produced and maintained in the Cell

An action potential is the series of membrane potential changes that allows a suprathreshold stimulus followed by a return to the resting membrane potential and results in cell excitation.

Stage of Action Potential

  • Resting stage

This is the resting membrane potential before the action potential begins. The membrane is said to be ‘polarized’ during this stage because of very large negative resting membrane potential e.g. RMP of nerve fiber -90 mv.

  • Threshold potential

It is the voltage at which a tremendous number of Na+ gates are opened and depolarization stage begins sharply.

  • Depolarization stage

Rapid rising of potential in the positive direction due to an influx of Na+ is called depolarization.

  • Repolarization stage

Rapid return to die normal negative resting membrane potential due to rapid diffusion of K+ to the exterior within a few 10,000ths of a second is called repolarization of the membrane.

  • Overshoot

The part of the action potential above ‘0’ (Zero) level is called overshoot.

  • Spike potential

The sharp rise of depolarization wave and rapid fall of repolarization wave is called spike potential. It is typical in smooth muscle.

  • Negative After potential

Failing of membrane potential to reach the normal resting stage is called negative after potential. It causes when the conc. of K+ outside the membrane increases thus prevents a flow of K+ from inside to outside.

  • Positive after potential

Sometimes the negativity of membrane potential become more than its normal level. This is called positive afterpotential or hyperpolarization. It is due to the excess permeability of the membrane to the K+ at the end of spike potential. Positive after potential is a physiological misnomer.

Properties of action potential: It occurs in several types of animal cells, called excitable cells, which include neurons, muscle cells, endocrine cells, and in some plant cells.

  • It propagates with undiminished amplitude.
  • It obeys ‘all-or-none law’.
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