Action Potential

An action potential happens when the membrane potential of a specific neuron rapidly rises and falls.

Action Potential

There are two refractory periods:

  • Absolute refractory period (ARP), when sodium voltage-gated channels become inactivated, they cannot be immediately opened again.
  • Relative refractory period (RRP): both sodium and potassium channels are open. Requires a stronger than usual stimulus to start a new action potential.

The action potential can be described in multiple phases.

  • Resting phase
    • Membrane potential of -64 mV
    • Voltage-gated $\ce{Na+}$ and $\ce{K+}$ channels closed
    • $\ce{Na+}$ and $\ce{K+}$ conductance is low
  • Rising phase
    • Membrane potential rises after threshold is reached
    • Voltage-gated $\ce{Na+}$ channels are quick to open
    • $\ce{Na+}$ conductance is high
    • $\ce{Na+}$ flows into the cell
    • $\ce{K+}$ channels are slow to open
    • $\ce{K+}$ conductance remains low
  • Falling phase
    • Membrane potential falls
    • Voltage-gated $\ce{K+}$ channels are opened
    • $\ce{K+}$ conductance is high
    • $\ce{K+}$ flows out of the cell
    • Voltage-gated $\ce{Na+}$ channels become inactivated
    • $\ce{Na+}$ conductance drops
    • Flow of $\ce{Na+}$ into the cell stops
  • Undershot phase
    • Membrane potential becomes more negative than at rest
    • Voltage-gated $\ce{K+}$ channels begin to close
    • $\ce{K+}$ conductance falls
    • Flow of $\ce{K+}$ out of the cell begins to stop
    • Voltage gated $\ce{Na+}$ remain inactivated
    • $\ce{Na+}$ conductance is low
  • Recovery
    • Voltage-gated $\ce{Na+}$ and $\ce{K+}$ channels are closed
    • $\ce{Na+}$ inactivation gate opens
    • $\ce{Na+}$ and $\ce{K+}$ conductance are low
    • Membrane potential gradually returns to baseline