Sodium ions have various significant roles in the body. They are cations which are essential for the generation of action potentials in the nervous and cardiac tissue. When one nerve cell needs to communicate with another cell, it opens specialized channels in the cell membrane to permit sodium to flood inside. This movement of sodium into the cell signals the nerve cell to fire an action potential, and sends an electrical signal down the axon. The signal is then carried from nerve cell to nerve cell until it reaches the brain or a muscle for contraction. Sodium is also an essential nutrient involved in the maintenance of cellular homeostasis and in regulation of fluid, electrolyte balance, and blood pressure. The kidneys maintain a balance of electrolyte by altering sodium levels as the body requires. The opening of the primary cardiac voltage-gated sodium channel induces depolarization and propagation of an action potential which promotes the coordinated contraction of the heart. Sodium is also vital for maintaining extracellular fluid (ECF) volume due to its osmotic action. Studies have also shown that sodium can be a significant signaling factor involved in the up-regulation of N-methyl-D-aspartate receptors (NMDARs), which are involved in the development of various diseases correlated with injuries to the central nervous system (CNS).