An instanteneous View of Contracting Muscle Captured by Rapid Freezing.

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A new freezing system using liquid helium was developed. Glycerinated rabbit psoas muscle fiber was rapidly frozen under isotonic contraction as well as in rigor, relaxation and isometric contraction states. Using this system, freeze-substituted images and freeze-etch replica images of contracting muscle were compared with those of rigor and relaxed muscle. The results lead us to conclude that, during muscle contraction, most of the myosin heads are arranged axially along thin filaments with a spacing of 143 A and that this arrangement of myosin heads may be required for force generation during contraction. These facts also mean a new contraction mechanism shoud be considered, so we propose here a electrostatic linear motor model for muscle contraction.