Elementary components of the recto-anal reflex observed in decerebrate dogs and cats

Abstract

The electrical activities of three peri-anal muscles; internal anal sphincters, external anal sphincters and anal longitudinal muscles in decerebrate dogs and cats, were recorded by single, bipolar, macro-electrodes with identical methods, as those described by Ihara & Takahira (1984) and by Tamura & Takahira (1985). The overwhelming activity could vary in different experiments, however, the activity of internal sphincters-slow spikes, was mostly revealing in majority of experiments. The waxing and waning amplitudes of internal sphincter-slow spikes were seen in almost all records, but their waxing cycles were not synchronized between different recording foci in single animals. Further, such alteration of spike size did not directly correlate with detectable change of anal canal pressure. Rather, the waxing and waning of slow spikes seem to be due to a 'beating' of pacemaker waves, originated from multiple generators in internal sphincters. Intravenous injection of 'somatic' neuro-muscular blocking agent; suxamethonium chloride, did not cause any alteration of pressure level and of internal sphincter-slow spike rate, indicating that the anal canal pressure was primarily dependent on sustained activity of internal sphincters, not on tonic discharge of external sphincters. The graded increase of distension volume in rectum brought about progressive lengthening of inhibition of internal sphincters, in turn, causing prolongation of falling phase of pressure in the recto-anal reflex. Although the tonic discharge of external sphincters in decerebrate animals was also completely inhibited by rectal distension, this silence did not induce any significant change in anal canal pressure. It is concluded that the external sphincter is not a tonic pressure-yielding element except a transient squeeze of anal canal, instead, it is a resistive element against complete relaxation of tonic pressure-yielding internal sphincter.