Modifications in myotendinous junction structure following denervation.

Changes in the structure of myotendinous junctions in response to peripheral nerve lesions are examined by transmission electron microscopy and morphometric analysis. Modifications in the folding of the plasma membrane at myotendinous junctions relative to the cross-sectional area of myofibrils terminating at the membrane are evaluated quantitatively using a morphometric analysis in which the muscle cell processes at the myotendinous junction are modeled as circular paraboloids. Denervated frog semitendinosus muscles were analyzed at 2, 4 and 8 weeks following denervation and compared to innervated, contralateral controls. No significant differences were found in relative folding of junctional plasma membranes between any two data sets, although myofibril diameter decreased over time following denervation. This shows that junctional plasma membrane and associated junctional structures, such as basement membrane, are removed from the myotendinous junction at a rate similar to that of myofibril thinning, thereby keeping constant the ratio between junction area and myofibril cross-sectional area. Electron microscopic observations indicate that 4 weeks post-denervation is the most active stage of junction remodeling of the time points sampled.