Myotendinous junction: morphological changes and mechanical failure associated with muscle cell atrophy.

The ultrastructures of the myotendinous junctions of healthy and atrophied twitch fibers from frog semitendinosus muscle are compared. The degree of plasma membrane folding at the junction of these cell types are quantified. The junctional surface area of healthy cells is increased 13.2 times by membrane folding. Calculations indicate that the junctional plasma membrane of these cells bears an average load of 2.6 X 10(4) N X m-2 during maximum isometric tension. The junctional surface area of atrophied cells is similarly increased by a factor of 6.3. The degree of junctional membrane folding does not vary with degree of cell contraction. Ultrastructural findings indicate that mechanical failure of the junction occurs just external to the junctional plasma membrane in atrophied cells. Healthy cells do not exhibit mechanical failure at the junction. These findings suggest that increased stress at the junction is associated with muscle atrophy and may cause failure at the myotendinous junction.