Etiology of the Resting Myoelectric Activity of the Levator Ani Muscle: Physioanatomic Study with a New Theory

Abstract

Of all the striated muscles in the bodies of mammals, only the pelvic floor muscles, which include the levator ani (LA), have resting electric activity. The cause and function of this resting myoelectric activity are not exactly known. The current study investigated the effect of intraabdominal pressure (IAP) and visceral weight on the electromyographic (EMG) activity of the LA, seeking to elucidate its cause and function. A series of 18 subjects (12 women, 6 men, mean age 38.6 years) were subjected to laparoscopic cholecystectomy for calcular cholecystitis. Prior to cholecystectomy, the resting LA EMG and IAP were recorded with the patient in the recumbent and erect positions. During laparoscopic cholecystectomy, the IAP was elevated by CO2 insufflation in increments of 5 cm of H₂O, and the LA EMG activity was recorded for the recumbent and vertical positions during inflation and after deflation at the end of the operation. In 5/18 patients in whom laparoscopic cholecystectomy was extended to open cholecystectomy, the IAP and LA EMG were also registered. The study also included histologic examination of the LA muscle from 15 cadavers (7 adults, 8 neonates). Levator ani EMG increased (p < 0.05) on standing. At operation, IAP elevation was associated with a significant increase of LA EMG activity. On deflation, the IAP and LA EMG activity level returned to the pre-insufflation state. In open cholecystectomy, the IAP was zero and the LA EMG recorded no activity for the recumbent position, but there was an activity for the vertical position. Histologically, the lateral part of the LA in the adult cadavers consisted solely of skeletal muscle fibers. Proceeding medially, smooth muscle fibers started to appear and gradually increase until, at the midportion, the LA split into two layers, a deep layer consisting of smooth muscle fibers and a superficial layer consisting of skeletal fibers. In neonates, the LA was composed of purely skeletal muscle fibers. The LA EMG activity seems to be related to both the IAP and the visceral weight. It is probably attributable to the presence of smooth muscle bundles in the LA muscle. The LA EMG activity increased with the elevation of the IAP and visceral weight, which resulted in increased muscle tone to oppose the augmented pressure or weight. This effect seems to be mediated through the straining-levator reflex. A chronic increase of IAP or visceral overload is suggested to affect muscle integrity and function.