The levator ani muscle (LAM), unlike other striated muscles has resting myoelectric activity, the cause of which is not precisely known. In a recent study this activity was suggested to be related to the presence of smooth muscle bundles in the LAM. The present study investigated this point in 25 cadaveric specimens (10 neonates, 15 adults). Histologic examination of the LAM was performed in 12 specimens, three to six slices being taken from the lateral to the medial side of each, processed and stained with H&E and Masson's muscle. trichrome stain. The remaining 13 specimens were studied by direct dissection and photographed. Microscopic examination of specimens from adult cadavers showed that the lateral part of the LAM consisted of purely skeletal muscle bundles, mostly of the small-caliber type and few of the large or intermediate type. As we proceeded medially, smooth muscle bundles started to appear. Examination of slices medial to the midportion of the LAM showed that the muscle began to be separated into two layers: a deep (pelvic) one formed of smooth fibers and a superficial (perineal) one of skeletal fibers, separated by a neurovascular plane. Microscopic examination of specimens from neonates showed that the whole LAM consisted of skeletal fibers; no smooth fibers or separation into two layers were identified. By direct dissection of the adult specimens the medial part of the LAM consisted of two layers, superficial and deep, which were separated by fascia containing vessels and nerves. The muscle was thicker at its medial part and tapered laterally, with loss of demarcation, into two layers. Specimens from neonates showed no differentiation of the LAM, into two layers. It is suggested that the deep layer of the LAM, which was formed of smooth fibers, acts involuntarily, supporting the pelvic viscera by its tone and responding to variations in intra-adominal pressure by adaptation of this tone. The Superficial layer of the LAM, made of skeletal fibers, appears to represent the functional mobile part that acts Voluntarily during Urination or defecation. The presence of the smooth fibers in the adult LAM and not in neonates seems to be adaptational, caused by the action of intra-abdominal pressure and visceral weight. The skeletal fibers pass into various histologic stages before they transform to smooth fibers.
