Limitations to the neuroregulation of enzymes in mammalian skeletal muscle.

Muscle fibers of the sternomastoid and the tongue of the rat were characterized histochemically according to mitochondrial distribution (succinic dehydrogenase), as well as reactivity for the alkali- and acid-stabile "myofibrillar" adenosine triphosphatases. The principal fiber types of the sternomastoid was the large, "white" AalphabetaM fibers and the smaller, "intermediate" BbetaD and "red CalphaL fibers (figs. 1, 2, 3). The unusual musculature of the tongue was populated by diminutive AalphaM and CalphaM fibers, and variants thereof; all with relatively high mitochondrial content (figs. 4, 5, 6). Reinnervation of the sternomastoid muscle by the hypoglossal nerve caused most fibers of the sternomastoid to assume histochemical profiles reminiscent of those commonly observed in the tongue. However, the BbetaD fibers of the sternomastoid persisted in near usual numbers and disposition (fig. 17), despite their reinnervation by a nerve normally destined for a muscle lacking that particular fiber type. Thus, there are as yet unrecognized factors, possibly of neural origin, though more likely of muscle origin (genetic), that impose restrictions on the metabolism-regulating functions of substitute motoneurons.