Force transmission across muscle cell membranes.

Myotendinous junctions (MTJs) display both morphological and molecular specializations for force transmission from contractile, cytoskeletal proteins to extracellular, structural proteins. MTJ membrane folding may be a mechanically important feature in junction structure in that it reduces membrane stress and situates the junction for loading primarily under shear. Force is likely to be transmitted, at least in part, by a chain of proteins including vinculin, talin, integrin, fibronectin and collagen. However, the concentration at MTJs of other structural proteins and of proteins involved in cell adhesion indicate that additional, force transmitting mechanisms also exist. Myonexin and dystrophin, muscle-specific proteins found at MTJs, may also be associated with MTJ force transmission. Periodic structures at non-MTJ membrane, called costameres, have molecular compositions similar to MTJs and may therefore also be involved in force transmission across the muscle cell membrane. Muscle tears occurring during muscle use following periods of disuse occur at or near MTJs. Disuse atrophy is associated with decreased MTJ folding and, therefore, an increase in MTJ stress during loading. This decrease in membrane folding may be the basis of increased tears in atrophied muscle.