How would you best describe an action potential in neurons?

• A. A rapid, transient change in the membrane potential that travels along the axon
• B. The resting potential of a neuron
• C. Diffusion of neurotransmitters across the synapse
• D. Passive leakage of ions through channels

Answer: (C)

An action potential is a rapid, transient change in the neuron's membrane potential that travels along the axon as an electrical signal. It begins when a stimulus depolarizes the membrane to a threshold, opening voltage-gated sodium channels and allowing a quick influx of Na+. This causes a sharp depolarization, bringing the inside of the cell briefly to a positive charge. Then voltage-gated potassium channels open, potassium leaves the cell, and the membrane potential returns toward the resting level, often dipping below it briefly before stabilizing. This sequence—the depolarization, peak, repolarization, and brief hyperpolarization—constitutes the spike that propagates down the axon. The signal is all-or-none: once threshold is reached, the spike has a consistent size, regardless of further stimulus strength, and it travels along the axon by local currents that depolarize the neighboring segment in turn. Myelination increases speed through saltatory conduction, where the action potential effectively “jumps” between nodes of Ranvier. After the spike passes, ion gradients are restored by pumps and exchangers.

This description contrasts with resting potential, which is just the steady baseline state; diffusion of neurotransmitters across the synapse refers to chemical signaling between neurons, not the electrical spike along the axon; and passive leakage of ions through channels is a slow, continuous process that does not produce the rapid, propagating event of an action potential.