Inward spread of activation in frog muscle fibres investigated by means of high-speed microcinematography.

1. Single fast muscle fibres of the frog were locally activated by applying current pulses to a pipette whose tip was in contact with the fibre surface, and the resulting local contractions were recorded with a high-speed ciné-camera at 1000-3000 frames/sec.2. In some but not all of the fibres examined, a moderate membrane depolarization of 20-30 mV initiated a phasic type of local contraction, which showed a definite threshold and relaxed spontaneously while the depolarization still continued.3. The phasic contraction was sometimes followed by a smaller steady contraction, which lasted as long as the depolarization went on and was regarded to be indentical with the graded type of local contraction.4. With pipettes of 20-40 mum diameter, the phasic contraction was first initiated at the depolarized fibre surface, and spread to some extent inwards with a velocity of 0.7-2 cm/sec at 18-26 degrees C. The velocity of inward spread of contraction had a Q(10) of about 2.3.5. With larger pipettes of more than 50-60 mum diameter, the phasic response was first initiated at the inner part definitely distant from the fibre surface, and the extent of contraction was greater at this part than at the superficial part during the course of the response. Furthermore, with depolarizations of nearly the threshold value, the steady contraction following the phasic one was seen only at the inner part, suggesting the reversal of the gradient of depolarization along the T tubules.6. The phasic contraction spread inwards or transversely with a considerable decrement. The response spreading across the whole diameter of the fibre could be observed only in some cases.7. The phasic response was not always sensitive to tetrodotoxin or to the removal of external sodium ions.8. These results not only give information about the nature of the regenerative mechanism within the T system, but also suggest that the organization of the T tubule network is such that the electrotonic depolarization becomes maximum at the T tubules located distant from the fibre surface when a sufficiently large area of surface membrane is depolarized.