Organization of skeletal muscle in the urodele Triturus cristatus : muscle fibre types and motor units

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Abstract
Four muscle fiber types are described in the biceps and extensor digitorum communis muscles of the newt forelimb. The histological criteria forming the basis for the distinctions include differential staining with p-phenylenediamine and succinate dehydrogenase histochemistry and EM. Three distinctive motor unit types are described for the biceps muscle. These are fast units, slow units and intermediate units. The structure of muscle fiber and the physiological characteristics of muscle fibers belonging to each motor unit, were correlated by using iontophoretic passage of Lucifer yellow into muscle fibers belonging to physiologically characterized motor units and their subsequent histological identification by the succinate dehydrogenase reaction. The 3 motor unit types correspond to slow muscle fibers, intermediate muscle fibers and 2 classes of fast muscle fibers.