Perineal muscles and their innervation: metabolic and functional significance of the motor unit

Cross-sectional areas and succinate dehydrogenase (SDH) activities of muscle fibers in the rat levator ani (LA) and bulbocavernosus (BC) were determined and compared with those of the soleus (SOL) and superficial (TAs) and deep (TAd) portions of the tibialis anterior (TA). In addition, cell body sizes and SDH activities of spinal motoneurons innervating the LA and BC were examined. Histochemical myofibrillar adenosine triphosphatase (mATPase) staining reactions following alkaline and acid preincubations revealed that all the muscle fibers in the LA and BC were type IIB. Gel electrophoresis, however, showed that the LA and BC contained 2.9 and 2.4% type IIx myosin heavy chain (MHC) isoform, respectively. Immunohistochemical analyses using MHC antibodies showed that the muscle fibers in the LA and BC had types IIx / IIa (approximately 3%) or type IIb MHC isoforms. The mean fiber cross-sectional areas in the LA and BC were significantly smaller than those in the SOL, TAs, or TAd. The mean fiber SDH activities in the LA and BC were significantly lower than those in the SOL or TAd, and similar to TAs. The population of alpha motoneurons innervating the LA and BC had similar SDH activities, irrespective of their cell body sizes. These data indicate that the LA and BC are comprised of a relatively homogeneous population of small, fast and low oxidative fibers innervated by a relatively homogeneous population of spinal motoneurons. These characteristics of the muscle fibers and motoneurons are consistent with their function in short, high-intensity activities.