Abstract
1. Single muscle fibres isolated from the claw adductor of the crayfish were stimulated directly by transversely applied rectangular current pulses and the resulting contractions were studied.
2. The height of tension response increased with the increase in the intensity of the current at constant duration or with the increase in the duration of the current at constant intensity, and reached a plateau in both cases.
3. The behaviour of suspended fibres in response to a brief pulse and the algebraical summation of tension response by two brief pulses of opposite sign have shown that the contraction elicited by a brief pulse is confined to the cathodal side of the fibre.
4. On the other hand, the whole fibre could be made to contract uniformly by a prolonged current pulse or a series of repetitive brief ones of sufficient intensities. This may be taken to indicate that the membrane depolarization around the whole fibre surface may not be necessary to bring the whole fibre to contract uniformly.
5. Using two pulses of same sign, it was possible to examine the summation of contraction up to the shortest interval without involving the summation of depolarization.
6. From these results it is concluded that the increase in the number of activated myofilaments may be involved in the process of summation by two pulses of same sign. The possible mechanism of the gradation of contraction in crustacean muscles was also discussed in this connection.
