Birefringence of Isolated Muscle Fibres in Twitch and Tetanus

The photoelectric measurement of changes in birefringence in single frog muscle fibres during isometric contraction is described. Birefringence, mechanical tension and intracellular action potential were recorded simultaneously. During a twitch birefringence decreased before tension was developed and reached minimum before tension reached maximum. The decrease in birefringence was 9 per cent at equilibrium length; at greater lengths the change in birefringence was less, as was twitch tension, but even when the sarcomeres were stretched to 3.6 and 3.8 μ a decrease in birefringence occurred. Double‐peak changes in phase difference, which have been described in whole muscle during isometric twitches, were not observed unless the fibre twisted in the light path. The decrease in birefringence was the same during twitch and tetanus. The decrease in birefringence during contraction may be due to an increase in the number of cross‐links between minute structural elements, or to a displacement of water within the fibre.