Muscles of the male rat perineum attaching to the penis are the ischiocavernosus, the medial bulbocavernosus (BC), and the dorsal BC, also known as the levator ani (LA). The BC and the LA muscles are innervated by a morphologically, neurochemically, and physiologically homogeneous pool of motoneurons, the spinal nucleus of BC (SNB). The purpose of the present study was to determine whether BC and LA muscle fibers constitute histochemically and biochemically homogeneous populations, reflecting the homogeneity of the innervating motoneuron pool. Histochemical fiber type was based on the pH lability of myofibrillar adenosinetriphosphatase. Activity ofthe mitochondrial enzyme succinate dehydrogenase (SDH) was determined by using a quantitative histochemical procedure. A nonsexually dimorphic, androgen-insensitive muscle, the medial gastrocnemius (MG), was used as control. The superficial regions of the MG, BC, and LA muscles were composed exclusively of type IIb muscle fibers, whereas the fiber type composition of the deep region of the MG was mixed: 28.3% type I, 20.6% type IIa, 40.1% type IIx, and 11.0% type IIb. The SDH activities of type IIb fibers in the deep region of the MG ranged from 1.20 to 9.00 (mean 3.72 +/- 0.40) mmol fumarate . liter tissue(-1) . min(-1). Fiber SDH activities in the superficial region of the MG ranged from 0.04 to 2.70 (mean 1.20 +/- 0.21) mmol fumarate liter tissue(-1) . min(-1). In the BC muscle, the SDH activities of the type IIb fibers ranged from undetectable to 1.80 with a mean of 0.62 +/- -0.05 mmol fumarate . liter tissue(-1) . min(-1). In the LA muscle, the SDH activities of type IIb fibers ranged from undetectable to 2.10 with a mean of 0.73 +/- 0.15 mmol fumarate . liter tissue(-1) . min(-1). The interquartile range for fiber SDH activity, a measure of variance, in the superficial region of the MG, the BC, and LA muscles was similar to that previously observed along the longitudinal axis of individual fibers, indicating that, in these muscles, the type IIb fibers constitute a metabolically homogeneous population. This study demonstrates that the motor units of BC and LB muscles are a metabolically homogeneous population, which may be a reflection of the physiological, morphological, and neurochemical homogeneity of the innervating SNB motoneurons.
