# Action Potential

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

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