Fig. 3.

Strength–duration curves of the somatic membrane of Neuron 1 for stimulation with square pulses. Chronaxie defined as the pulse duration needed for twice the rheobase I₀ (horizontal arrows) is different for the space clamp condition (central group of three curves) and the case of current flowing into dendrites and the axon (upper curves). Injected anodic current amplitudes (upper and central groups) are shown as current densities for 1 cm² membrane. Extracellular stimulation (lower curves) is with cathodic currents for a point source 50 μm above soma center. Approximation of the strength–duration curves according to the classical formulas of Lapicque (1907): I_el = I₀/(1 − 2^−k) (dashed lines) and Weiss (1901): I_el = I₀(1 + 1/k) (dotted lines) where k is pulse duration/chronaxie.