Final answer:
When a neuron's action potential occurs, sodium ions are the primary ions that rush into the axon, which causes depolarization of the neuron's membrane and initiates the action potential.
Step-by-step explanation:
When the neuron's action potential occurs, sodium ions are rushing into the axon through openings on the membrane. The first steps in the generation of an action potential involve the opening of sodium ion channels, which allow sodium ions (Na+) to enter the cell rapidly, causing a reversal of the electrical charge across the neuron's membrane from negative to positive. This is known as depolarization. Following this influx of Na+, potassium ion channels open, allowing potassium ions (K+) to flow out, which helps to end the action potential by repolarizing the membrane and returning it to a negative state internally.
During a neuron's action potential, the rapid influx of Na+ ions is the defining event that triggers the subsequent processes leading to the transmission of electrical impulses along the neuron's axon to another neuron or to a muscle cell. This electrical phenomenon is critical to the functioning of the nervous system and is a fundamental concept in the study of biology and neurophysiology.