MOTONEURONS INNERVATING 2 REGIONS OF RAT MEDIAL GASTROCNEMIUS-MUSCLE WITH DIFFERING CONTRACTILE AND HISTOCHEMICAL PROPERTIES

The medial gastrocnemius (MG) muscle, which receives its innervation b y two extramuscular nerve branches, is representative of muscles which show a particular form of muscle compartmentalization (i.e. a regiona l specialization of muscle fibers) in which there is a 'deep' oxidativ e region and a 'superficial' low-oxidative region. Differential recrui tment of motor units from these two regions of the MG has been reporte d for different functional tasks. Our goal was to determine if the org anization of the MG motoneuron pool-muscle complex with its two extram uscular nerve branches could account for the phenomenon of regional sp ecialization of muscle fibers. The two extramuscular nerve branches in nervated muscle subvolumes which differed in contractile properties an d fiber type percentages. The MG proximal nerve branch (NBr) innervate d mostly high-oxidative and slow fibers, but with some low-oxidative f ast fibers. The distal NBr innervated mostly low-oxidative fibers, but also a small proportion of high-oxidative and slow fibers. These resu lts suggest that the two nerve branches do not strictly define a super ficial/deep organization of fiber types in the MG. The number and soma size characteristics of motoneurons supplying the two extramuscular n erve branches showed that the motoneurons innervating the deep more ox idative muscle region, supplied by the proximal NBr, were smaller than those innervating the superficial, primarily low-oxidative, region su pplied by the distal NBr. Our findings indicate that the MG motoneuron pool-muscle complex of the Sprague-Dawley rat will lend itself to stu dies of how the various motor unit types within a given spinal motor c omplex adapt to different conditions (e.g. aging, disease, injury, exe rcise).