Contractions of frog tonus fibers and their modification by length changes

Isometric and isotonic contractions of the tonus fibers of the frog were recorded using anodal block of the nerve fibers of the twitch fibers. Repetitive stimulation produced a contraction with a very slow rising phase because the individual responses were very weak. The first two or three stimuli usually did not give a visible response at all. However, if the twitch fibers were also stimulated, the responses of the tonus fibers were many times stronger and faster, but only under isotonic conditions. This indicates that the large increase in the responses of the tonus fibers was produced by the passive shortening caused by the contraction of the twitch fibers. A strong and fast response of the tonus fibers was also obtained if during stimulation of the tonus fibers the muscle was made to shorten by diminishing the load. It is suggested that the enormous effect of shortening is due to the regenerating action of shortening previously demonstrated.