Length tension relationship of the external anal sphincter muscle: Implications for the anal canal function.

Rajasekaran MR, Jiang Y, Bhargava V, Littlefield R, Lee A, Lieber RL, Mittal RK.

Am J Physiol Gastrointest Liver Physiol, 2008 295(2):G367-73.

Abstract:

The 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=4) were prepared and studied in a muscle bath under isometric conditions. For in-vivo studies, female rabbits (n=19) were anesthetized and anal canal pressure was recorded using 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 using laser diffraction technique with no probe and three probes in the anal canal. In-vitro studies revealed 2054 mN/cm² 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 sacromere length without and with 9 mm probe in the anal canal were 2.11 ± 0.08 and 2.99 ± 0.07µm respectively. 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.

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