Active relaxation of unstriated muscle

Abstract

Ramsey and Street, working on single muscle fibres, concluded that relaxation in skeletal muscle was active. Prof. A. V. Hill has conclusively shown that relaxation in skeletal muscle is passive. We stimulated the frog's rectus abdominis with potassium salts and acetylcholine and did not find any active relaxation. In unstriated muscle relaxation is active under some conditions and passive under others. In unstriated muscle there would appear to be two cycles: in one, the energy for contraction is derived from chemical stores; in the other, the energy for contraction is derived from energy previously stored in the structure. Striated muscle appears to possess the former cycle only. This, we think, is the fundamental difference between the two kinds of muscle.