Activity-independent neural influences on cat soleus motor unit phenotypes

The physiological and phenotypic properties of motor units in the cat soleus muscle were studied after 4 months of inactivity induced by spinal cord isolation (SI). The soleus of some SI cats were stimulated for 30 min/day during an isometric (SI-I), shortening (SI-S), or lengthening (SI-L) phase of a simulated step cycle. Mean maximum tetanic tensions were approximately 15, 26, 32, and 51% of the control in the SI, SI-S, SI-L, and SI-I groups. Mean time-to-peak tension was approximately 50% shorter than the control in all SI groups. One motor unit was glycogen-depleted in each muscle via repetitive stimulation. Eighteen physiologically slow and 9 fast motor units from the spinal cord-isolated groups consisted of fibers that contained only slow myosin heavy chain (MHC) and sarco(endo)plasmic reticulum calcium-adenotriphosphatase (SERCA) isoforms. Two motor units (physiologically fast) consisted primarily of fibers that contained both fast and slow MHC and SERCA. These data reflect a dissociation between isometric speed-related properties and MHC and SERCA isoforms following inactivity. The predominance of fibers containing both fast and slow MHC and SERCA isoforms in 2 motor units demonstrates a strong motoneuronal influence on the muscle-fiber phenotype even when the motoneurons are silent.