Coordination of electromechanical and metabolic properties of cat soleus motor units.

Motor units were studied in the soleus muscle of normal adult cats and adult cats that had undergone complete spinal cord transection approximately 4 mo earlier. Intracellular recording and stimulation techniques were used to study selected electrical properties of the motoneuron and isometric contractile properties of the muscle unit. Motor unit fibers were depleted of their glycogen through repetitive stimulation of the motoneuron and identified by a quantitative histochemical determination of glycogen. A sample of muscle fibers from the glycogen-depleted unit and from fibers not depleted of glycogen were analyzed for cross-sectional area, succinate dehydrogenase (SDH), alpha-glycerolphosphate dehydrogenase (GPD), and alkaline myofibrillar adenosine triphosphatase. It was observed that the fiber-to-fiber variability in cross-sectional area and SDH and GPD activity within units of normal and transected cats was significantly larger than that measured in repeated samples from a single fiber. Additionally, for each of these properties, the range found among fibers within a unit was similar to that found among nondepleted fibers of the same myosin type. The influence of spinal cord transection on some muscle fibers seemed to result in a metabolic shift from the generalized category of slow-oxidative toward fast-oxidative glycolytic. This shift in metabolic properties appeared to be coupled with a similar shift in the physiological properties of the muscle unit and motoneuron from slow to fast.