AJP - Cell Physiology, Vol 255, Issue 5 C684-C693, Copyright © 1988 by American Physiological Society
Coordination of electromechanical and metabolic properties of cat soleus motor units
T. P. Martin, S. Bodine-Fowler and V. R. Edgerton
Department of Kinesiology, University of California, Los Angeles 90024-1568.
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.
