chronaxie

Chronaxie is a parameter used in electrophysiology to characterize the excitability of nerves and muscles. It is defined as the minimum time required for an electrical stimulus, applied at a specific amplitude, to excite the tissue. More precisely, chronaxie is the pulse duration at which the threshold current equals twice the rheobase—the minimum current amplitude that can elicit a response when the stimulus duration is sufficiently long. Both chronaxie and rheobase are derived from the strength–duration curve, which describes how threshold current varies with pulse width.

The rheobase represents the asymptotic minimum current necessary when the stimulus is very long in duration. The chronaxie is measured by applying stimuli of varying widths and currents, finding the rheobase, and identifying the pulse duration at which the required current is exactly twice that rheobase. In practice, square (rectangular) pulses are commonly used for these measurements, and the resulting curve provides a comparative measure of tissue excitability.

Values of chronaxie vary by tissue and conditions. For human peripheral nerves, chronaxie typically lies on the order of about 0.1 to 0.5 milliseconds, while skeletal muscle often shows longer values, roughly 1 to 2 milliseconds. Chronaxie is influenced by factors such as temperature, fiber type, myelination, electrode geometry, and tissue impedance; disease and aging can also alter excitability.

Applications of chronaxie include guiding electrical stimulation parameters in research and clinical settings, such as neuromuscular stimulation, transcutaneous nerve stimulation, and other electrotherapy modalities. It provides a comparative metric of excitability across tissues or conditions, though it is not a universal constant and must be interpreted within its context.