Rat extraocular muscle. 3. Histochemical variability along the length of multiply-innervated fibers of the orbital surface layer.

Multiply-innervated fibers from rat superior oblique extraocular muscle were followed in sequential serial sections using histochemistry. Sudan black and adenosine-triphosphatase (ATPase) histochemical staining reactions were used to identify these fibers in the muscle's global layer and orbital surface layer. Regional differences in ATPase staining occurred along the length of multiply-innervated fibers from the orbital surface layer. In their middle third where these fibers appear 'morphologically-fast' and contain endplatelike endings, they were found to exhibit a dual ATPase activity. In their distal third where they appear 'morphologically-slow' and contain simple, superficial endings, they had an alkaline-labile and acid-stabile ATPase activity profile. In contrast, the multiply-innervated fibers of the global layer exhibited a homogeneous ATPase activity, i.e., alkaline-labile and acid-stabile pattern. This histochemical homogeneity parallels their uniform morphologically-slow profile. These fibers contain only multiple superficial endings. It would appear that the histochemical and ultrastructural profile of a fiber is dependent upon the type and location of the motor innervation.