Final answer:
The rising phase of an action potential is caused by sodium ions (Na+) entering the neuron, as voltage-gated Na+ channels open, leading to the depolarization of the cell membrane.
Step-by-step explanation:
The rising phase of the action potential is due to Na+ flow into the cell.
During the rising phase of an action potential, the membrane permeability to sodium (Na+) ions increases as voltage-gated Na+ channels open. This causes a rapid influx of Na+ ions into the neuron, thereby reducing the membrane potential from its resting state to zero or even making the cytoplasm more positive than the extracellular fluid. This process, known as depolarization, is crucial for the propagation of the action potential along the neuron.
After reaching the peak of the action potential, the Na+ channels close and voltage-gated K+ (potassium) channels open. The resulting efflux of K+ ions out of the cell leads to repolarization and eventually hyperpolarization of the neuron, restoring the resting membrane potential.