Inward spread of activation in vertebrate muscle fibres

1. A method for detecting the activation of individual myofibrils or groups of myofibrils within an isolated muscle fibre is described. It consists in making all the myofibrils wavy by setting the fibre in gelatine and compressing it longitudinally; active shortening of myofibrils can then be recognized by the straightening out of the waves.2. The time course of this straightening during a twitch was found by high-speed ciné micrography.3. There is a delay of activation between the superficial and central myofibrils, from which the velocity of inward spread of activation can be found.4. This velocity has a Q(10) of 2, and is about 7 cm/sec at 20 degrees C. The mechanism of the inward spread of activation is discussed.5. On relaxation the waves reappear, showing that there is a spontaneous elongation of the myofibrils.