Which description best characterizes the action potential phase in neurons?

Which description best characterizes the action potential phase in neurons?

• A. A brief hyperpolarization following the spike.
• B. A rapid reversal of the resting potential caused by changes in membrane permeability to Na+ and K+.
• C. A gradual depolarization that does not reach threshold.
• D. A plateau phase due to calcium influx.

Answer: (B)

The action potential is defined by a rapid, all-or-none reversal of the neuron's resting membrane potential. This happens because voltage-gated Na+ channels open in response to depolarization, dramatically increasing Na+ permeability and driving the membrane from about -70 mV toward positive values. As the spike reaches its peak, these Na+ channels inactivate and voltage-gated K+ channels open, boosting K+ permeability and repolarizing the membrane back toward the resting potential, often with a brief hyperpolarization. This sequence of rapid depolarization due to Na+ influx followed by repolarization due to K+ efflux best describes the action potential phase. The hyperpolarization after the spike is a separate aftereffect, a subthreshold depolarization that follows the spike, while a gradual depolarization that doesn’t reach threshold or a plateau from Ca2+ influx describes other, non-representative scenarios for the typical neuronal action potential.

The action potential is defined by a rapid, all-or-none reversal of the neuron's resting membrane potential. This happens because voltage-gated Na+ channels open in response to depolarization, dramatically increasing Na+ permeability and driving the membrane from about -70 mV toward positive values. As the spike reaches its peak, these Na+ channels inactivate and voltage-gated K+ channels open, boosting K+ permeability and repolarizing the membrane back toward the resting potential, often with a brief hyperpolarization. This sequence of rapid depolarization due to Na+ influx followed by repolarization due to K+ efflux best describes the action potential phase. The hyperpolarization after the spike is a separate aftereffect, a subthreshold depolarization that follows the spike, while a gradual depolarization that doesn’t reach threshold or a plateau from Ca2+ influx describes other, non-representative scenarios for the typical neuronal action potential.