neural impulse

Meaning: A neural impulse is the action potential, a brief reversal of membrane potential that travels along an axon. At rest a neuron sits near −70 mV. A stimulus that depolarizes the membrane to threshold (about −55 mV) opens voltage-gated Na⁺ channels, and Na⁺ rushes in. Na⁺ channels then inactivate, voltage-gated K⁺ channels open, and K⁺ leaves, repolarizing the membrane, often with a brief hyperpolarization. The refractory period keeps the impulse moving in one direction, away from the cell body.

On the MCAT: Expect a graph of membrane potential versus time to label with ion movements, or questions on what happens if Na⁺ or K⁺ channels are blocked. Myelin speeds conduction because the impulse jumps between nodes of Ranvier (saltatory conduction). Stimulus intensity is coded by firing rate and by how many neurons fire, not by spike size.

Common confusion: A stronger stimulus does not make a bigger impulse. Once threshold is reached the spike has the same size, and below threshold no spike occurs. Also, the impulse is electrical along the axon, but signaling between neurons is usually chemical, through neurotransmitters released at the synapse.