Length-tension relationship of the external anal sphincter muscle: implications for the anal canal function

Abstract

Rajasekaran MR, Jiang Y, Bhargava V, Littlefield R, Lee A, Lieber RL, Mittal RK. Length-tension relationship of the external anal sphincter muscle: implications for the anal canal function. Am J Physiol Gastrointest Liver Physiol 295: 367-373, 2008. First published July 3, 2008 doi:10.1152/ajpgi.00033.2008The length at which a muscle operates in vivo (operational length) and the length at which it generates maximal force (optimal length) may be quite different. We studied active and passive length-tension characteristics of external anal sphincter (EAS) in vivo and in vitro to determine the optimal and operational length of rabbit EAS. For the in vitro studies, rings of EAS (n ϭ 4) were prepared and studied in a muscle bath under isometric conditions. For in vivo studies, female rabbits (n ϭ 19) were anesthetized and anal canal pressure was recorded by use of a sleeve sensor placed in the custom-designed catheter holders of 4.5-, 6-, and 9-mm diameters. Measurements were obtained at rest and during EAS electrical stimulation. Sarcomere length of EAS muscle was measured by laser diffraction technique with no probe and three probes in the anal canal. In vitro studies revealed 2,054 mN/cm 2 active tension at optimal length. In vivo studies revealed a probe size-dependent increase in anal canal pressure and tension. Maximal increase in anal canal tension with stimulation was recorded with the 9-mm probe. Increases in anal canal tension with increase in probe size were completely abolished by pancuronium bromide. EAS muscle sarcomere length without and with 9-mm probe in the anal canal were 2.11 Ϯ 0.08 and 2.99 Ϯ 0.07 m, respectively. Optimal sarcomere length, based on the thin filament length measured by thin filament analysis, is 2.44 Ϯ 0.10 m. These data show that the operational length of EAS is significantly shorter than its optimal length. Our findings provide insight into EAS function and we propose the possibility of increasing anal canal pressure by surgical manipulation of the EAS sarcomere length. anal continence; fecal incontinence; sarcomere length SMOOTH MUSCLE OF THE internal anal sphincter (IAS) and striated muscle of the external anal sphincter (EAS) and puborectalis muscle play essential roles in the fecal continence mechanism. Current understanding is that the resting anal canal pressure is mostly related to the IAS and the increase in anal canal pressure with voluntary squeeze is related to contraction of striated EAS muscle (5,8,25). Pressure recording of the anal canal using side-hole manometry and station pull-through technique show that, both at rest and during voluntary squeeze, the highest pressure is located in the part of the anal canal that is surrounded by the EAS, suggesting that EAS is most likely the strongest component of the anal continence mechanism (22). The EAS is also the most commonly affected muscle in women with fecal incontinence because it frequently gets damaged during childbirth-related trauma and episiotomy (29).The EAS muscle in humans shows both resting or sustained s...

Myoarchitectural and functional alterations in rabbit external anal sphincter muscle following experimental surgical trauma

“…1A). The anal canal pressures were recorded with the pressuresensing surface of the sleeve sensor facing the posterior midline direction (19,21,22). A pulse generator (model S48; Grass Technol-ogies, West Warwick, RI) connected to a constant current unit (model CCU1A; Grass Technologies) was used for electrical stimulation of the EAS muscle.…”

Length Tension Function of Puborectalis Muscle: Implications for the Treatment of Fecal Incontinence and Pelvic Floor Disorders

“…Increase in vaginal probe size should result in an increase in PRM sarcomere length, similar to what we described in our EAS muscle study. 13 Therefore, based on aforementioned arguments, we propose that probe size dependent increase in the vaginal HPZ pressure results from the increase in PRM sarcomere length and the latter is only possible if the PRM were to operate in vivo at a short sarcomere length. On the other hand, if the PRM were to operate at optimal or supra-optimal length in vivo; one would expect a decrease in the vaginal HPZ pressure with the increase in vaginal probe size.…”

“…13 According to the sliding filament theory, optimal muscle length is the one that generates maximal force/tension for the maximal neural input. 10 In rabbit studies, we used a probe system, similar to the one in the current study and observed that the increase in probe size increases EAS muscle sarcomere length.…”

“…Based on the above measurements, operational and optimal EAS muscle lengths in rabbit were measured to be approximately 2.1 and 2.6 m respectively. 13 In other words, EAS muscle length operates at approximately 20% shorter than its optimal length in vivo. Measurement of sarcomere length by laser diffraction technique and thin filament (actin) length by immunohistochemistry requires excision/harvesting of muscle, which is not practical for the human study.…”

“…10 Central drive or neural input to a muscle is measured by electromyographic (EMG) activity of muscle and increase in vaginal pressure with voluntary squeeze is a surrogate marker of the force/stress generated by the PRM. 6,9 Recent studies from our laboratory show that EAS muscle operates at a short sarcomere length in rabbits and humans, [11][12][13][14] i.e., increasing its in vivo length increase its contraction pressure. Whether the same is true for PRM in humans is not known and was the purpose of our present investigation.…”