Strength/Duration Curves. Strength / duration (S/D) curves for cardiac muscle cells (filled circles) and smooth muscle cells (unfilled circles) (5-cell chains) when Rgj was ∞ (0 channels) (A) or when Rgj was 10 MΩ(1000 channels) (B). The S / D curves are rectangular hyperbolas. The time (pulse duration) it takes for a current intensity of twice the rheobasic intensity to produce the all-or-none repolarization is the chronaxie (σ) The rheobase is the asymptote of the data points extrapolated back to the ordinate, as shown. The chronaxie was about 1.0 ms, in both panels A and B. But the absolute current intensity required was about 8–10-fold greater in panel B compared to panel A. The membrane time constant (τm) is related to the chronaxie (σ) by the equation shown in panel A.