From observations of fluorescent antibody staining and antibody staining in electron microscopy, evidence is presented for the following: (a) Direct contact of the actin and myosin filaments occurs at all stages of contraction. This results in inhibition of antibody staining of the H-meromyosin portion of the myosin molecule in the region of overlap of the thin and thick filaments. (b) Small structural changes occur in the thick filaments during contraction. This leads to exposure of antigenic sites of the L-meromyosin portion of the myosin molecule. The accessibility of these antigenic sites is dependent upon the sarcomere length. (c) The M line is composed of a protein which is weakly bound to the center of the thick filament and is not actin, myosin, or tropomyosin. (d) Tropomyosin as well as actin is present in the I band. (e) If actin or tropomyosin is present in the Z line, it is masked and unavailable for staining with antibody.

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