Functional morphology of force transmission in skeletal muscle. A brief review

The work done by the contractile proteins of muscle in accelerating, decelerating, or maintaining the positions of skeletal elements requires the efficient transmission of tension across the surface membranes of the fibers. The most widely studied sites of tension transmission are the ends of muscle fibers where they contact either connective or epithelial tissues. In most animals, regardless of phylum, muscle fiber ends are characteristically folded, producing a junctional interface that significantly reduces the absolute value of stress applied to the cell membrane, insures that the principle stress vector at the cell membrane is shear rather than tension, and minimizes stress concentrations. The morphological and molecular similarities of muscle-tendon junctions (MTJs) in different animals suggest that the problem of creating a strong adhesive joint between a muscle fiber and a tissue of dissimilar physical properties is essentially the same for all muscles, and that the solution arose early in evolution. In addition to those muscle fiber ends that occur where fibers contact dissimilar tissues, there are intramuscular fiber terminations that consist either of folded cell-cell junctions similar to the fasciae adherentes of cardiac muscle, or of gradually tapering fiber ends. Both sorts of intramuscular ends occur in those vertebrate muscles in which the individual muscle fibers are too short to reach from the tendon of origin to the tendon of insertion. In series-fibered muscles in which the fiber ends are tapered, tension is transmitted from contractile proteins to endomysial collagen fibrils across the fiber membranes. The endomysium of such muscles is an essential series-elastic element. The existing evidence suggests that tension transmission is a general property of muscle cell surfaces, and that specific junctional morphologies are the results of dynamic interactions between muscle cells and the tissues to which they adhere.