The Human Brain : From Neurone to Nervous System


  1. The Action Potential (Nerve Impulse) is a transient electrical event in the axonal membrane that is conducted at speed along the axon.

  2. At rest the axon is polarised, with a transmembrane potential of ~-70mv (axoplasm negative to the outside).

  3. Action potentials are initiated by depolarisation of the resting membrane from ~-70mV to a threshold membrane potential of about -55 mV,

  4. During the action potential there are rapid transient changes in the transmembrane potential: at its peak the membrane potential reaches nearly +40 mV (the 'spike') then returns quickly to the initial membrane potential. The duration of the spike is around 1 msec in mammalian myelinated axons.

  5. After the rapid decline of the spike potential, the membrane potential can fall below the normal resting potential and achieve a transmembrane potential of around -90mv (a phase known as the afterpotential) before slowly returning to normal.

  6. The threshold potential at which action potentials are generated is the voltage at which voltage-gated sodium channels open and allow sodium ions to enter the axoplasm.

  7. Key Words: Resting Potential, Action potential, 'Spike', Afterpotential, Threshold