WeissLapicque

Weiss-Lapicque refers to a historical threshold model in neurophysiology that describes how the strength and duration of an electrical stimulus relate to the excitation of nerves and muscles. The model is attributed to researchers Weiss and Lapicque and is primarily used to explain the strength-duration relationship observed in neural and muscular stimulation.

A central feature of the Weiss-Lapicque model is the strength-duration law, which expresses the stimulus current threshold I as a function of pulse duration t. The commonly cited form is I = I_r (1 + t_c / t), where I_r is the rheobase—the minimum current required for long-duration pulses—and t_c is the chronaxie—the pulse duration at which the threshold current equals twice the rheobase. This formulation implies that shorter pulses demand higher currents to elicit an action potential, producing a hyperbolic curve when threshold is plotted against duration.

Rheobase and chronaxie provide simple, interpretable measures of tissue excitability and have been used in clinical electrodiagnosis, nerve-conduction testing, and the planning of therapeutic electrical stimulation protocols. The model offers a first-order, phenomenological description that captures essential trends with relatively few parameters.

Limitations include its assumption of rectangular pulses, constant tissue properties, and a linear, time-invariant threshold. It does not account for detailed membrane dynamics, frequency effects, or non-linearities seen in some tissues or stimulation modalities. Nevertheless, the Weiss-Lapicque framework remains a foundational reference for understanding the basic trade-off between stimulus strength and duration in electrical excitability.