Figure 1. Relationship between force response and electric field strength in skinned EDL fibres.

A, force response elicited by a 1 ms pulse at the indicated field strength in a skinned fibre with Cl⁻ present in the t-system and 3 mm Cl⁻ in the ‘intracellular’ solution bathing the fibre, both before and after addition of 100 μm 9-AC. B, responses to similar pulses in another skinned EDL fibre in the absence of any Cl⁻. Addition of 9-AC, a Cl⁻ channel blocker, reduced the field strength needed to elicit force in the presence of Cl⁻ (A), but had little if any effect on responses in the absence of Cl⁻ (B).