Birefringence signals from surface and t‐system membranes of frog single muscle fibres.

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Abstract
When the tonicity of Ringer was increased above 2.5 times normal and a single fiber [electrically] stimulated externally, the large, early birefringence signal preceding twitch tension was sufficiently reduced and delayed so as to reveal a small but distinct signal (1st component) preceding it. For an average-sized fiber, the .DELTA.I/I [change in light intensity/resting light intensity] of the 1st component was (minus) 1-2 .times. 10-5. The time course of the 1st component superimposed with the surface action potential was simultaneously recorded by an internal micro-electrode. The polarity of the 1st component was reversed with compensation. The 1st component arose from a small change in optical retardation of the surface membrane due to the action potential. When a fiber was impaled with 2 micro-electrodes, retardation changes accompanying small hyperpolarizing and depolarizing current steps were detected. In some cases, the polarity of the observed signal was opposite in sign to that expected for a retardation change only from the surface membrane. Because the anatomical orientation of the T-system was primarily transverse rather than longitudinal, these signals of opposite polarity are probably, on balance, due to retardation changes from the membranes of the T-system. The possible origin of the large birefringence signal preceding contraction is discussed.