Neuronal Depolarization and Ion Transport

In order to maintain homeostasis, cells require a mechanism to regulate the transport of ions across their membranes. This process is critical for maintaining the electrochemical gradient essential for cellular functions, including muscle contraction and neurotransmission. One such mechanism involves active transporters that use ATP to move ions against their concentration gradients. Given this context, consider the following ions: sodium (Na⁺), potassium (K⁺), calcium (Ca²⁺), and chloride (Cl^-). Which of the following ions is most directly involved in depolarizing the neuronal membrane during an action potential in excitable cells?