Myofibril tension fluctuations and molecular mechanisms of contraction

Recent tension fluctuation experiments that were performed on single myofibrils of cardiac and skeletal muscles established firmly that the fluctuations, if exist, must be below 0.1 ng/square root Hz. This value is about 100 times below the levels that were predicted by various models of cross-bridge mechanical cycling during isometric contraction. Similar measurements with slow stretch and shortening to promote cross-bridge cycling did not produce detectable increase of fluctuations either. Moreover, measurements of elastic transfer function using small length perturbations with white noise clearly demonstrated conduction of vibrations and increased stiffness during contraction of the myofibril; therefore, vibration attenuation within the sarcomere cannot be responsible for remarkable quietness of the tension. Electrostatic mechanism of muscle contraction advanced by Iwazumi gives physically straightforward explanations for the quietness. The ATPase cycling certainly produces fluctuations in the number of surface charges that constitute the dipole moment thus resulting in the field strength fluctuations. However, the magnitude of the fluctuations is only a small fraction of the mean strength due to large number of charges involved in the dipole. In addition, the field strength fluctuations do not couple effectively with the axial force acting on the thin filament bundle. This is due to the combined effects of three factors: 1. Three dimensional three-phase distribution of electrostatic energy density along the thin filament. This structural arrangement smoothes out the forces of three adjacent thin filaments due to complementary nature of the distribution. 2. Characteristic square mesh structure of the Z-disc results in very high shear compliance between adjacent thin filaments yet provides very low parallel compliance. 3. Electrostatic induction.