Final answer:
The concept that an action potential will continue at the same speed to the very end of the axon is known as the all-or-none law. It involves a wave-like movement of depolarization and repolarization that travels down the axon after initiation at the neuron's membrane due to sodium influx. The correct option is D.
Step-by-step explanation:
When an action potential starts at the beginning of an axon, it will indeed continue at the same speed to the very end of the axon without diminishing in strength. This principle is described by the all-or-none law. The action potential is generated when the neuron's membrane depolarizes due to sodium ions flowing into the cell, creating a positive charge within the cell.
Following depolarization, potassium ions flow out to repolarize the membrane, bringing the cell back to its resting state. This wave-like movement of depolarization and repolarization travels down the axon towards the synapse.
The speed of this transmission can vary depending on whether an axon is myelinated or unmyelinated. Myelinated axons feature nodes of Ranvier, where saltatory conduction occurs as the action potential 'jumps' from node to node, allowing faster transmission. Larger axon diameters can also facilitate faster propagation of the action potential.
Notably, once an action potential is triggered and reaches the threshold of excitation, there is no stopping it—it is an all-or-none event. The action potential moves in only one direction because the sodium channels become temporarily inactivated after depolarization, preventing the reversal of direction of the propagation. The refractory period that follows ensures that each action potential is a separate and distinct event.