What is an action potential?

• A. Chemical signal between neurons
• B. Resting state of neuron
• C. Propagation of neurotransmitters across synapses
• D. Rapid electrical signal along neuron membrane

Answer: (D)

An action potential is a rapid electrical signal that travels along the membrane of a neuron, typically along the axon. It starts when the membrane depolarizes past a threshold, triggering voltage-gated sodium channels to open and sodium ions to rush in, causing a quick spike in membrane potential. This depolarization then opens more channels in a cascading way, reaching a peak around +30 mV. Next, voltage-gated potassium channels open, potassium exits the cell, and the membrane repolarizes back toward the resting level, often briefly undershooting before the resting state is restored. The event travels as a wave along the membrane in an all-or-nothing fashion, with each segment triggering the next; in myelinated fibers, the signal jumps between gaps in myelin (nodes of Ranvier) to speed up conduction. A refractory period follows, preventing backward movement and giving the neuron time to reset. This electrical phenomenon is distinct from chemical signaling across synapses or the neuron’s resting state.