In biology, depolarization refers to the event a cell undergoes when its membrane potential grows more positive with respect to the extracellular solution. Depolarization typically results from the influx of positively charged ions (such as sodium or calcium) into the cell. Alternatively, depolarization can also happen if potassium channels are closed. Compare to hyperpolarization. Depolarization of membrane potentials is important for other cell types, particularly muscle cells.