Myotendinous junctions of tonic muscle cells: structure and loading.

Regions within frog semitendinosus muscle that are rich in tonic muscle cells were identified histochemically by myosin adenosine triphosphatase- and succinic dehydrogenase-staining procedures. Bundles of cells still attached to tendinous insertions were removed from those sites, prepared for electron microscopy and sectioned longitudinally through their myotendinous junctions. Tonic cells were identified by electron-microscopic criteria and their myotendinous junctions' morphology evaluated by morphometry. Although junctional components appear identical to those in twitch cells, the degree of membrane folding increases tonic junction area by a factor of 50.2 whereas twitch cells' junctional area is increased 22.2 times by folding relative to cells terminating as right circular cylinders. Calculations show that the tonic cell junction bears average loads of 3.4 X 10(3) N X m-2 during maximum force generation and that nearly all of the load is borne as shear stress at the junction. The junctions of twitch cells bear average loads of 1.6 X 10(4) N X m-2 during peak tension. The findings indicate that the magnitude of loading does not alone determine the degree of junctional membrane folding. Interpretation of the data in view of viscoelastic behavior of membranes indicates that duration of loading may be a functionally important correlate to degree of membrane folding at myotendinous junctions.