A neural impulse; a brief electrical charge that travels down an axon is called

• A. Resting potential
• B. EPSP
• C. Graded potential
• D. Action potential

Answer: (D)

The neural impulse is the action potential, a brief, all-or-none electrical event that travels along the axon. When a neuron at rest is stimulated to threshold, voltage-gated sodium channels open and sodium rushes in, causing rapid depolarization. The sodium channels then inactivate and potassium channels open, allowing potassium to exit and the membrane to repolarize (briefly hyperpolarizing before returning to resting potential). This depolarization wave propagates along the axon as each segment depolarizes the next, ensuring the signal travels quickly and in one direction due to the refractory period. This differs from resting potential, which is just the steady negative voltage when the neuron isn’t firing, and from EPSP or graded potentials, which are local, variable-amplitude changes that may influence firing but do not themselves travel the length of the axon as a full impulse.