A large birefringence signal preceding contraction in single twitch fibres of the frog.

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Abstract
When a single muscle fiber [frog] was externally [electrically] stimulated to give a propagated action potential, a large decrease in light intensity was measured with the fiber positioned between crossed-polarizers oriented at .+-. 45.degree. with respect to the fiber axis. This large optical signal began just after stimulation and its main rising phase preceded the development of positive tension. The peak (or plateau) amplitude of the signal, expressed as the peak (plateau) change in light intensity, .DELTA.I, divided by the resting light intensity, I, was typically (minus) 1-3 .times. 10-3 and the time-to-peak (plateau) was 4-6 ms (20.degree. C). When mechanical activity was minimized by stretch or Ringer replacement with D2O or hypertonic solution, thepeak tension response was reduced in far greater proportion than the peak optical response, suggesting that the early optical signal is not due to changes in tension or gross fiber movement. The magnitude of the optical response was increased by nitrate and double-shock stimulation, procedures which potentiated the twitch response. The optical signal was propagated along the fiber length with a conduction velocity appropriate to the surface action potential. The large, early birefringence signal reflected a step or steps in the sequence of events leading to contractile activation.

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