Length-tension relationship of striated muscle of cat external anal sphincter.

Abstract

The active and passive length-tension curves of small strips of cat external anal sphincter (EAS) were examined in vitro. The striated muscle fibers were arranged perpendicular to the long axis of the longitudinal smooth muscle cells of the longitudinal layer of the anal canal. Histological examination indicated that the strips were comprised of striated muscle fibers oriented in the long axis of the strip. Electrical field stimulation elicited twitch and tetanus responses that were not altered by the administration of gallamine triethiodide (10(-6) to 10(-4) M), a neuromuscular blocking agent. At 37 degrees C the time from the onset of the twitch contraction to the development of peak force ranged from 30 to 37 ms, the time from peak force to one-half relaxation ranged from 20 to 25 ms. The maximum active twitch and tetanus tension (Po) averaged 0.23 and 0.90 kg/cm2, respectively. Active tension could be developed over a range of 0.6-1.4 optimum length (Lo). The passive length-tension curve showed that the muscle had significant passive tension at lengths below the Lo for tension development and a high passive tension at Lo (average of 44% of active isometric twitch tension, 12.2% of active isometric tetanus tension) and above Lo. We conclude that the passive length-tension curve for the EAS is stiffer than that of typical mammalian skeletal muscles. We suggest that this difference is related to the adaptation of the EAS to its sphincteric function as a component of a hollow organ.