Physiological characteristics of identified motor units in the mouse extensor digitorum longus muscle: an in vitro approach.

Physiological, histochemical, and morphometric properties of fast-twitch single motor units were studied in mouse extensor digitorum longus muscles in an in vitro ventral root-nerve-muscle preparation. Single motor units were functionally isolated by microdissection of the ventral root, and the glycogen depletion technique was used to demonstrate the component muscle fibers. Monoclonal antibodies were used to identify their myosin heavy chain composition. The technique allows one to correlate physiological characteristics of single motor units with fiber type but is less useful for morphological assessment of motor unit size as a result of failure to deplete glycogen from all fibers of motor units containing fibers with high oxidative capacity.