What physical process occurs during an action potential?

The correct answer is the change in electrical charge across the neuron membrane. During an action potential, the neuron's membrane undergoes a rapid change due to the movement of ions, primarily sodium and potassium, across its membrane. This process begins when a neuron reaches a threshold level of depolarization, causing voltage-gated sodium channels to open. Sodium ions rush into the neuron, making the interior more positive compared to the outside of the membrane.

This change in electrical charge is crucial for the propagation of the action potential along the axon, allowing for the transmission of signals in the nervous system. Following the depolarization phase, potassium channels open, allowing potassium ions to exit the neuron and restore the resting membrane potential, leading to repolarization.

The other options reflect related but distinct processes. The release of neurotransmitters happens at the synaptic cleft after an action potential has traveled down the axon, while the formation of the myelin sheath involves processes that occur during the development of neurons rather than during an action potential. The influx of calcium ions plays a role in different processes, such as neurotransmitter release, but is not the primary event occurring during the action potential itself.