Action Potential

  • All cells maintain a potential difference across the membrane.
  • Though all cells have a membrane potential, only a few have the ability to change in reponse to a stimulus.
  • These cells include cells of the nervous system and muscles.
  • Such cells can change the membrane potential by changing the permeability of the membrane to specific ions.
  • The ability of these tissues to change their membrane potential is termed excitability or irritability.

  • The sodium channels in the excitable tissues have voltage regulated gates, in fact they have two gates that resond to different voltage changes. In the resting cell, one gate (the most external one) is closed while the other is open.
  • In the case of the potassium channels, there are in fact two channels. On type lack gates, and are always open while the others have voltage regulated gates.
  • Thus, in the resting cell, the membrane is more permeable to potassium and thus has a negative potential.
  • Once the cell membrane is adequately stimulated and the voltage has reached a certain level (Threshold Level, e.g. -55mV), the external gates open up. and an inflow of sodium ensues. This causes the membrane potential to become more positive and more sodium gates are opened.
  • This results in an explosive influx of sodium, and the mebrane potential at that point, becomes nearer to the equilibrium potential for sodium (+35mV). This is called the Depolarisation Phase - the membrane potential is changing polarity.
  • At this voltage, the internal sodium gate closes up stopping the inflow of sodium. At the same time the voltage gated potassium channels open up and an eflux of potassium follows. This results in a change of potential to a more negative one. This phase is called the Repolarisation Phase.
  • Due to the opening up of the potasium channels, the negative membrane potentail can overshoot the resting membrane potential. This part is called the Hyperpolarisation.
  • The internal sodium gates, open up only after the external gates have closed down (below -55mV). This difference in the closing and opening voltage for the internal gates, ensures that the sodium channels are not open during the repolarisation phase.