GLOSSARY

An action potential is a rapid, transient change in membrane voltage in excitable cells (neurons and muscle fibres), in which the inside of the cell becomes positive for about 1–2 milliseconds (in a neuron) and then returns to rest; the action potential is the basic language of neural communication.

Phases

• Reaching threshold: about −55 millivolts.
• Depolarisation: voltage-gated sodium channels open and Na⁺ rushes in; voltage rises to about +30 millivolts.
• Repolarisation: sodium channels inactivate and potassium channels open; K⁺ flows out.
• Hyperpolarisation: slightly more negative than rest, because potassium channels close slowly.
• Return to rest: gradients maintained by the sodium–potassium pump.

Refractory Period