Rat extraocular muscle. 2. Histochemical fibre types.

A battery of histochemical stains was used to differentiate the component fibre populations in rat superior oblique muscle. Seven histochemical fibre types were identified and found to be comparable to those observed in a prior light and electron microscopic study of the muscle. The global region of the muscle contained three singly innervated fibre populations (pale, intermediate and red) and one of multiply innervated fibres. THe global singly innervated fibre populations produced a spectrum of fibre types with trichrome, succinate dehydrogenase and Sudan black. The overlying orbital surface layer contained two populations of singly innervated fibres and an additional population of multiply innervated fibres. All the singly innervated fibres (both global and orbital) exhibited a high mATPase activity at pH 10.4 and a low mATPase activity at pH 4.6. The global multiply innervated fibres had low mATPase activity at pH 10.4 and high mATPase activity at pH 4.6. The multiply innervated fibres of the orbital surface layer exhibited high mATPase activity at pH 10.4 and at pH 4.6. The global multiply innervated fibres are similar to the tonic fibres found in amphibian muscle, both histochemically and ultrastructurally. Based upon their histochemical reactivities with the various stains, the multiply innervated fibres of the orbital surface layer appear to be faster acting than the global multiply innervated fibres. Furthermore, it is postulated that the former have polyneuronal innervation and are capable of dual mATPase synthesis.