Final answer:
The term that indicates a nerve impulse is being transmitted is an action potential. It is a rapid change in electrical charge across a neuron's membrane that propagates down the axon, carrying the nerve impulse.
Step-by-step explanation:
The term that indicates a nerve impulse has been initiated and is being transmitted is an action potential. An action potential is a sudden reversal of the electrical charge across the membrane of a resting neuron. This event begins when a neuron receives a chemical signal from another cell, causing gates in sodium ion channels to open and allow positive sodium ions (Na+) to flow into the cell, making the inside of the cell positively charged relative to the outside. This change in charge propagates rapidly down the axon, functioning as an electrical current and transmitting the nerve impulse.
Other terms provided in the options, such as depolarization and repolarization, are steps within the action potential sequence. Depolarization refers to the specific phase during which there is an influx of Na+ ions into the neuron, making the inside of the cell membrane more positive. Repolarization occurs thereafter, by making the membrane impermeable to Na+ and allowing potassium ions (K+) to move out, restoring the negative membrane potential. Graded potentials and local depolarization are changes in membrane potential that can lead up to the generation of an action potential if they reach threshold levels.