Motoneurons innervating two regions of rat medial gastrocnemius muscle with differing contractile and histochemical properties.

The medial gastrocnemius (MG) muscle, which receives its innervation by two extramuscular nerve branches, is representative of muscles which show a particular form of muscle compartmentalization (i.e. a regional specialization of muscle fibers) in which there is a 'deep' oxidative region and a 'superficial' low-oxidative region. Differential recruitment of motor units from these two regions of the MG has been reported for different functional tasks. Our goal was to determine if the organization of the MG motoneuron pool-muscle complex with its two extramuscular nerve branches could account for the phenomenon of regional specialization of muscle fibers. The two extramuscular nerve branches innervated muscle subvolumes which differed in contractile properties and fiber type percentages. The MG proximal nerve branch (NBr) innervated mostly high-oxidative and slow fibers, but with some low-oxidative fast fibers. The distal NBr innervated mostly low-oxidative fibers, but also a small proportion of high-oxidative and slow fibers. These results suggest that the two nerve branches do not strictly define a superficial/deep organization of fiber types in the MG. The number and soma size characteristics of motoneurons supplying the two extramuscular nerve branches showed that the motoneurons innervating the deep more oxidative muscle region, supplied by the proximal NBr, were smaller than those innervating the superficial, primarily low-oxidative, region supplied by the distal NBr. Our findings indicate that the MG motoneuron pool-muscle complex of the Sprague-Dawley rat will lend itself to studies of how the various motor unit types within a given spinal motor complex adapt to different conditions (e.g. aging, disease, injury, exercise).