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Studies of ultrastructural changes of glycerinated rabbit psoas muscle allowed to relax in an ATP-EDTA solution following ATP-induced contraction as well as of rigid fibers plasticized in the same solution have shown that the contraction bands found in tissue fixed during contraction are completely reversible if more than 1 minute is allowed for relaxation. With times less than 1 minute, contraction band disappearance occurs only in those sarcomeres which have been significantly reextended. It also was observed that fibers which were relaxed for as long as 15 minutes were able to recontract to as great an extent as those relaxed for 30 seconds, that is to a length less than or equal to that obtained in the first contraction. The ultrastructural pattern of recontracted sarcomeres is the same as that seen in tissue fixed during the first contraction, with contraction bands reappearing at the A—I-boundary or Z-line. The above observations further support the conclusion that the presence of contraction bands is the only ultrastructural feature peculiarly diagnostic of contracted tissue, while varying degrees of interdigitation can be found in sarcomeres fixed in all the physiological states: rigid, plastic, contracted, and relaxed.

Studies of ultrastructural changes of glycerinated rabbit psoas muscle during contraction induced by ATP have shown that the sarcomere patterns of striated muscle seen in electron micrographs can be directly correlated with the physiological state of the tissue at the time of fixation. The relationships of band and sarcomere lengths of rigid control fibers agree well with those upon which the postulated sliding model is based. This is not true for tissue fixed during contraction. Sarcomeres fixed during ATP-induced contraction are characterized by the appearance of contraction bands at the A—I-boundary of sarcomeres with patent I-bands. The contraction band formation usually is accompanied by a slight decrease in thick filament length with no increase in filament diameter. In fibers which were held at extended length during glycerination, the ATP-contracted sarcomeres often have thin filaments which are shorter than those of rigid controls; and the contraction bands usually are extremely dense. Some of these sarcomeres which are rather short still have wide H-bands. Thus, although filament sliding usually accompanies contraction when the fibers contract from rest length, the contraction can occur with no sliding in connection with shortening of stretched muscle fibers with a narrow zone of filament overlap. Contraction band formation, on the other hand, is a feature which clearly distinguishes fibers fixed during contraction from those which have not been contracted.