Activity‐independent neural influences on cat soleus motor unit phenotypes

Publisher Website
Google Scholar
Abstract
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. © 2002 Wiley Periodicals, Inc. Muscle Nerve 26: 252–264, 2002