Functional coordination of motor activity in colonic and recto-anal smooth muscles in rat experimental model

Spontaneous or electrically elicited contraction and/or relaxation of the longitudinal and circular muscles of the colon and rectum and the anal canal in rat segment preparations were recorded simultaneously to display contractile potency and functional coordination of muscles in the large intestine. Spontaneous high-amplitude contractions, but not relaxations, appeared synchronously in the longitudinal and circular muscles of the colon and rectum. The anal canal showed contractions following the activity of rectal muscles. The colonic and rectal longitudinal muscle responded to electrical stimulation with frequency-dependent contraction. The response of the circular muscle of the colon initially consisted of frequency-independent relaxation followed by frequency-dependent contraction, which was more pronounced in the distal colon, while the rectal circular muscle responded with contraction. The responses appeared synchronously, demonstrating the coactivation of the nervous pathways that supply both muscles. The contractions of longitudinal muscles were more pronounced suggesting a dominant role of this layer in the coordinated motor activity. The local response of the internal anal sphincter was biphasic, comprising short contractions followed by relaxation, while the response of the anal canal was contraction. The contractile local responses increased from the colon to the rectum, while differences in the relaxations were not observed, indicating rather higher contractile potency than the relaxation ability of muscles in the distal part of the gut.