Rheobase

Rheobase is a fundamental parameter of excitable tissue that describes the minimum current amplitude, with effectively infinite stimulus duration, required to elicit an action potential. The term combines Greek roots meaning flow and base, reflecting its association with electrical stimulation thresholds.

In practice, rheobase is estimated from the strength–duration curve, which plots the least stimulus current necessary to provoke a response as a function of pulse duration. The rheobase corresponds to the extrapolated intercept on the current axis as the duration tends toward infinity, representing the lowest possible current capable of triggering excitation given a sufficiently long pulse.

A common way to relate rheobase to pulse duration is through the Weiss–Lapicque equation: I = I_r (1 + t_c / t), where I is the stimulus current, t is the pulse duration, I_r is the rheobase, and t_c is the chronaxie. The chronaxie is the duration required for activation at twice the rheobase and serves as a characteristic time constant for the tissue’s excitability. A shorter chronaxie generally indicates higher excitability.

Historically, the concept was introduced by Louis Lapicque in 1909 as part of strength–duration theory in neurophysiology. Rheobase remains a standard descriptor of tissue excitability and is used to guide stimulation parameters in research and clinical electrophysiology, including nerve and muscle stimulation techniques.

Notes: actual rheobase values vary with tissue type, electrode geometry, temperature, and experimental conditions, so rheobase is treated as a relative measure of excitability rather than an absolute constant.