Mastering A&P Neurophysiology Practice Test

The depolarization phase of an action potential is primarily caused by the influx of sodium ions into the neuron. This process begins when a neuron's membrane potential reaches a certain threshold due to local depolarization, often triggered by the binding of neurotransmitters that open sodium channels.

As these voltage-gated sodium channels open, sodium ions, which are in higher concentration outside the cell compared to the inside, rush into the neuron. This rapid influx of positively charged sodium ions reduces the negative charge inside the cell, moving the membrane potential toward a more positive value, which is the hallmark of the depolarization phase.

This process is crucial because it ultimately leads to the propagation of the action potential along the axon, allowing for the transmission of electrical signals in the nervous system. The subsequent phases of the action potential involve the efflux of potassium ions and typically the influence of calcium ions, but they are not the primary drivers of the initial depolarization.