action potential

An action potential is the brief, all-or-nothing reversal of a neuron's membrane voltage that carries a signal along its axon. At rest the inside of the cell is negative relative to the outside; a stimulus opens sodium channels, sodium rushes in, and the membrane depolarizes. Potassium channels then open, potassium leaves, and the membrane repolarizes toward rest. The response does not grade with the size of the stimulus: past the threshold it fires at full size, and below it nothing travels, so a stronger stimulus is encoded as more impulses rather than larger ones. On a myelinated axon the impulse does not travel along every stretch of membrane — it jumps between the gaps in the sheath, the nodes of Ranvier, and that saltatory conduction is many times faster. A local anesthetic works here: it blocks voltage-gated sodium channels, so the reversal cannot start and the signal never leaves the area.