Final answer:
The inward Na+ current results in the upstroke of the action potential in Purkinje fibers (B), which is the rapid depolarization phase crucial for the propagation of electrical signals in the heart's conduction system.
Step-by-step explanation:
The inward Na+ current is primarily associated with the upstroke of the action potential in excitable cells. In cardiac physiology, these currents play crucial roles depending on the type of cardiac cell in question.
The correct answer to the student's question is (B) Upstroke of the action potential in Purkinje fibers. The rapid depolarization phase of the action potential in Purkinje fibers, as well as other myocardial cells, is characterized by a rapid influx of Na+ ions into the cell. This phase raises the membrane potential up to approximately +30 mV causing the cell to depolarize. This depolarization phase lasts about 3-5 ms and is critical for the propagation of the electrical signal through the heart's conduction system.
For the SA node (option A) and ventricular muscle cells (options C and D), the inward Na+ current does not cause the upstroke of the action potential in the SA node nor does it cause the plateau or repolarization phases in ventricular muscle cells. The plateau phase is associated with Ca2+ influx, not Na+ influx, and repolarization is due to K+ outflow.