Threshold of Stimulation

in the physiology of nerve and muscle cells, the minimum force of a stimulus (usually an electric current) capable of causing spreading action potential; a measure of cell excitability.

Within certain limits the threshold of stimulation is inversely related to the duration (t) of action of the stimulus and the steepness of increase in its force. The threshold of stimulation drops with increasing t. Only upon some critical increase in t, or “useful time,” does the threshold of stimulation remain at a constant level— the rheobase. The minimum t at a voltage twice the rheobase is called the chronaxie. At a given t, the threshold of stimulation of a single cell or fiber depends both on its “passive” properties (resistance and capacitance of the membrane and resistance per unit fiber length) and on the active properties of the membrane (the condition of the system of ionic canals— specifically, their sensitivity to depolarization and the rate at which they can be activated, or opened, in response to depolarization).