Figure 5. Effect on twitch responses of stopping the t-system Na⁺–K⁺ pump.

A, in a skinned fibre without Cl⁻ present, the twitch response to successive stimuli, applied 10 s apart, declined to less than 10% of the control level within 30 s after stopping the Na⁺–K⁺ pump; the pump was stopped by substitution of a bathing solution with no Na⁺ (force artefact produced by this solution change). Twitch and tetanic response returned to close to initial levels after return of Na⁺. Stimulus timing shown beneath force trace. B, data of the relative size of the twitch response after stopping the Na⁺–K⁺ pump in fibres with (Δ) and without (•) Cl⁻ present.