When does an action potential occur in a cell membrane?
An action potential occurs when an electrical signal disrupts the original balance of sodium and potassium ions within a cell membrane, briefly depolarising the concentration of each.
Nearly all cell membranes in animals, plants and fungi maintain a voltage difference between the exterior and interior of the cell called the membrane potential. In most types of cells the membrane potential usually stays fairly constant. However in some cells (e.g. neurons and muscle cells) the voltage fluctuates over time. In such electrically active cells, voltage fluctuations take the form of rapid upward spikes followed by a rapid fall. These up and down cycles are known as action potentials.
Action potential occurs in several types of excitable cells including neurons, muscle cells, endocrine cells as well as some plant cells.
The main function of action potential is to activate intra cellular processes. For example in muscle cells an action potential is the first step in the chain of events leading to contraction. In beta cells of pancreas it induces the release of insulin. In neurons an action potential generates a spike train.
Action potential results due to special voltage gated ion channels present in the cell membranes. These channels are capable of producing action potential because they can give rise to positive feed back loops. Action potentials are triggered when enough depolarisation accumulates to bring the membrane potential up to a threshold.