What is the first event of an action potential

Last update: Dec 2nd, 2017

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• At rest, the axon membrane is slightly
  
  polarized to about -70mV,
  
  meaning the intracellular fluid (ICF) is relatively negative to the extracellular fluid (ECF).

• An action potential occurs when a portion of the membrane rapidly depolarizes and then repolarizes again to the original resting state.
• The process is initiated by a threshold level
  
  stimulus, such as a nearby
  
  change in membrane potential (threshold potential, local potential).

• At threhsold (about -55mV), many
  
  Na+ voltage-gated channels open.

• Na+ ions entering the cell make the membrane potential less negative. More Na+ channels open as result and a cycle of depolarization develops.

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• When the membrane potential reaches about +30mV (reverse polarization), the timed
  
  Na+ channels close due to inactivation and the Na+ influx stops.

• The K+ channels are slow to close, so the membrane briefly hyperpolarizes.
• As the K+ channels close, Na+/K+
  
  pumps actively transport Na+ ions out of the cell and K+ ions into the cell.

• The ion exchange helps
  
  re-establish the ion diffusion gradients and resting membrane potential.

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