An action potential is a short-lasting event in which the electrical membrane potential of a cell rapidly rises and falls, following a stereotyped trajectory. Action potentials occur in several types of excitable cells, including neurons, muscle cells, and endocrine cells. In neurons, they play a central role in cell-to-cell communication. In other types of cells, their main function is to activate intracellular processes.
Membrane potential (or transmembrane potential) is the voltage difference between the interior and exterior of a cell. All animal cells are surrounded by a plasma membrane composed of a lipid bilayer with many diverse protein assemblages embedded in it. The fluid on both sides of the membrane contains high concentrations of mobile ions, of which sodium (Na), potassium (K), chloride (Cl), and calcium (Ca) are the most important.
The threshold potential is the membrane potential to which a membrane must be depolarized to initiate an action potential. It often can be between −40 and -55 mV, but it can vary based upon several factors.