Trigger Mechanism

in physiology and biology, an act or stimulus that causes the abrupt shifting of a cell, organ, or entire organism from one functional state to another. For example, the trigger action of a peripheral nerve causes a muscle to contract. In this case, the synaptic potential, that is, the minute electromotive force that arises at the site of contact between a nerve and muscle fiber, functions as a spontaneous trigger mechanism. All the processes characteristic of reflexes, such as excitation of receptors and transmission of excitation along the peripheral nerves and from neuron to neuron, may be regarded as a consecutive series of events since the threshold phenomenon occurs in all of these processes—that is, there is an abrupt shift from one state to another. Trigger mechanisms cause major changes in an organism, such as the change from the egg to the larva, from the larva to the pupa, and from the pupa to the adult organism. Trigger mechanisms are also responsible for the diurnal and seasonal activity of animals.

The new state produced by a trigger mechanism may be either maintained or gradually lost, causing a return to the original state. Most biological trigger mechanisms are automatic and are renewed by metabolic energy. The study of trigger mechanisms helps reveal the true causes of the automatic and spontaneous processes whose course is not determined by external influences.