Structural information is presented for three muscle systems—mammalian smooth muscle at rest and partially active, living toad striated muscle at rest and contracting, and glycerinated rabbit psoas muscle under various conditions of pH and ionic environment. In the smooth muscle no evidence of organized myosin filaments has been found. In the striated muscle the myosin-to-actin distance can vary widely, according to sarcomere length and to muscle treatment, both at rest and during contraction. In the discussion it is suggested that muscle should be considered as a colloidal system and that there need not necessarily be any chemical bonding (cross-linking) involved in the contractile process.

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