Effects of rectal distension on the sphincter ani externus and levator ani muscles in cats

The response characteristics of levator ani (LA) muscles to rectal distension and its rectified and integrated EMG activity was recorded and compared with the responses of the sphincter ani externus (SAE) to the same stimuli. Experiments were performed in 42 cats anesthetized with chloralose (70 mg/kg) dissolved in a 25% urethan-saline solution. Fast (about 1-2 s) rectal distension (10-40 ml of air were injected into a rubber balloon in the rectum, reaching pressures of up to 80 mmHg) produced a reflex contraction of both LA and SAE muscles. Sustained distension elicited a sustained discharge from the LA muscles that endured for the duration of the distension, up to 2-3 min. These changes were observed in 75% of 210 trials. EMG activity increased from 30 to over 600% of base-line levels, with the majority, 65% of trials, showing greater than 100% increase. In contrast to these excitatory responses of the LA muscles, SAE responded to sustained rectal inflation with inhibition of ongoing activity, an inhibition that could reach muscular silence. Fast intermittent distension, one every 2-3 s, however, produced an increase in the level of activity of the SAE. Analysis of the EMG activity preceding defecation revealed an increase of LA activity 1-2 min before actual expelling. We believe this result to be consistent with the hypothesis that these muscles may contribute to anorectal evacuation. In the anesthetised animal, during the passage of solid material through the anus, the SAE showed increased activity. We believe this to be the result of a local phenomenon, the development of a tonic stretch reflex in the SAE.(ABSTRACT TRUNCATED AT 250 WORDS)