The I segment length has been measured and the number of thin filament periods per I segment has been counted in electron micrographs of Fthesus monkey and human leg muscle fibers. The number of I segment periods in the monkey fiber (60) and in the human fiber (66) multiplied by axial periodicity (385 A ) in thin filaments of living fibers (Huxley and Brown, '67) plus 1,000 .A allowed for Z line width in living fibers, provided a formula for calculation of I segment lengths in living fibers. The calculated I segment lengths are 2.41 P for the monkey and 2.64 , u for the human fiber. The ratio of calculated I segment length over an assumed constant thick filament length (1.6 p ) among vertebrates is in close agreement with the ratio of measured I segment length over measured thick filament I!ength in micrographs of frog, rat, monkey and human leg muscle fibers. The calculated I segment length in the frog (1.95 ru) is compared with that of the human (2.64 P ) to show the relation of I segment length to the length-developed tension diagram and to the series elastic components of the skeletal muscle fiber.Extensive electron microscope studies on thin filament length in frog muscle fibers have been made by two groups of investigators. Carlsen et al. ('61) fixed unstimulated fibers in OsOc and reported the thin filament segments (I segments), which included the Z line and the thin filaments on both sides of the Z line, to be about 1.76 p in length. Page and Huxley ('63), using the same procedures, reported I segment length to be about 1.80 p. On the other hand, if they stimulated frog fibers tetanically and held them at a fixed length during fixation and dehydration the I segments measured 2.00 to 2.05 p . It was concluded that OsOI fixation shortens thin filaments in unstimulated fibers and that formation of cross linkages between thick and thin filaments during tetanic stimulation inhibits Os04-induced shortening. They also reported that the I segment of frog fibers contains 48 periods with a mean value for corrected axial periodicity of 406 A. But, it was necessary for them to multiply the number of periods (48) by 410 A and add 800 A for the measured width of the Z line to attain the 2.05 p AN&*, REC., 178: 63-82.I segment length. Nevertheless, they concluded that the I segment length in the living frog is about 2.05 p and that the periodicity in the thin filaments is about 406 A. The 410 A periodicity used by Page and Hudey in '63 for calculation of I segment length in frog muscle fibers was based upon the x-ray diffraction studies of glycerol-extracted rabbit psoas muscle fibers (Worthington, '59).Confluent evidence from more recent studies of x-ray diffraction patterns, optical diffraction patterns and thin filament structure now indicate that the axial periodicity of thin filaments is somewhat less than 410 A. Huxley and Brown ('67) found in living unstimulated frog sartorius muscle a meridional reflection in the x-ray diffraction pattern that varied from 380 to 390 A wj th an average of 385 A. They s...
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The effects of temperature on the basic characteristics of isometric contraction were studied in the triceps surae of white albino rats. The influences of tetanic fusion and Wedensky inhibition on tension development were analyzed. The results showed linear increase of maximal isometric tetanic tension and increase in the rate of rise and fall of twitch and tetanic isometric contractions with increase in temperature. Indirect stimulation produced somewhat lower tetanic tensions than direct stimulation. The optimal frequency of stimulation for maximal tetanic tension development was usually lower than the critical frequency of tetanic fusion. Reduction of tension by excessively high frequencies of stimulation, due to the Wedensky effect, was observed with indirect stimulation, and occurred at a lower frequency than the critical frequency of fusion.
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