The effects of inactivity, programmed stimulation, and denervation on the histochemistry of skeletal muscle fiber types

Abstract

The role of nerve impulse pattern in determining the histochemical characteristics of muscle fibers was investigated in cats. Motor neuron activity was eliminated by transecting the spinal cord between segments S2 and S3 and cutting all dorsal rootlets caudal to this level. In some of these cats, caudal nerves to the tail muscles were stimulated supramaximally for 1 mo in patterns approximating the output of either phasic or tonic motor neurons. Intertransverse muscles of the tail were examined histochemically for activities of a mitochondrial oxidative enzyme, a glycolytic enzyme, and myofibrillar ATPase. Three fiber types, red, white, and intermediate, were defined in control muscles on the basis of histochemical staining intensities and myofibril size and shape. Red fibers have high oxidative, low glycolytic, and low ATPase activities and wide, polygonal fibrils. White fibers are at the opposite extreme in all of these features and have more narrow, rectangular fibrils. Intermediate fibers are intermediate in all of these features except that their ATPase activity is high like that of the white fibers, but is more sensitive to high pH and formaldehyde. Specific arrangement of these fiber types within muscle fasciculi facilitated their identification in experimental muscles. The red fibers tend to be in the deeper parts of fasciculi; white fibers tend to be at the periphery; and intermediate fibers are found in both locations. In the nonstimulated muscles, prolonged disuse brought about a progressive decrease of oxidative and glycolytic enzymatic activities. Tonic stimulation increased oxidative activity and decreased glycolytic activity. Phasic stimulation resulted in high glycolytic activity, while oxidative activity was decreased. Following prolonged disuse and both programs of stimulation, myofibrillar ATPase activity remained unchanged in white and red fibers but decreased some-what in varying numbers of intermediate fibers. There was preferential atrophy of white fibers in the disused muscles but they achieved nearly control size following tonic or phasic stimulation. Thus, the patterns of impulse activation exert a major influence on the oxidative and glycolytic metabolic properties of muscle fibers and fiber cross-sectional area. However, since myofibrillar ATPase activity remained relatively unaltered, this characteristic may be controlled by some mechanism other than impulse transmission.