Abstract
WHEN a skeletal muscle is stimulated to twitch, small changes in the optical properties of the fibres can be detected which may give information about excitation–contraction coupling. An example is provided by the experiments of Bezanilla and Horowicz1,2. After treatment of muscles with Nile Blue A, fluorescence changes were found which, it was suggested, arise from changes in potential across the membranes of the sarcoplasmic reticulum (SR). Such potential changes might reflect the movement of calcium ions from inside the SR into the myoplasm, a step in the normal activation of contraction. Baylor and Oetliker3,4 measured changes in optical retardation, which rely on the inherent birefringence of a muscle fibre rather than the addition of an extrinsic probe. The large second component of the signal was suggested to arise from a change in SR membrane potential. These explanations are plausible since small changes in both extrinsic fluorescence and optical retardation are known to accompany voltage changes across neuronal membranes5–8. An important test of these hypotheses is to compare the fluorescence and retardation signals from the same fibre, to see if they follow the same time course.
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