The action potential is caused by a sequence of changes or events occurring in the membrane permeability to Na+ and K+ ions. When an excitable cell is stimulated the cell membrane permits the inflow of Na+ ions into the cell to generate the action potential.
Action potential is the rapid changes in the membrane potential from its normal negativity to positive potential inside the cell membrane which last for few milliseconds and then returns back to its original resting negative potential level.
Polarised membrane is the resting cell membrane with a normal negative resting membrane potential.
Depolarization stage:
It is the first event of action potential and is characterized by rapid increase in the permeability to Na+ ions (5000 folds) to interior of the cell generating more positive electrical potential inside of the cell.
This is followed by a gradual inactivation of Na+ channels (closure) that occur within another few milliseconds and the membrane becomes impermeable to Na+ ions. It is associated with gradual opening of voltage gated K+ channels to allow K ions outflow to the exterior of the cell membrane. The potential inside the cell is re-established to its normal resting level (- 90mV). This stage is called as the repolarisation stage
Higher concentration of K+ ions in the exterior of the cell towards the end of the action potential continues for a short period creates more negativity inside referred to as hyper-polarised state. At this state, re-excitation of the cell will not occur.
The final event is characterized by electrogenic pump mechanism, which aids in the transport of three Na+ ions to the exterior for every two K+ ions to interior of the cell and create the normal resting potential (- 90 mV) on the inside of the cell membrane.
