THE INNER SUBLAYER OF THE CIRCULAR MUSCLE COAT IN THE CANINE PROXIMALCOLON - ORIGINS OF SPONTANEOUS ELECTRICAL AND MECHANICAL-ACTIVITY

In the canine proximal colon, tissue near the submucosal surface of th e circular muscle layer produces spontaneous mechanical contractions, synchronized with electrical slow waves, Comparative physiological exa mination of tissue strips from various regions of the submucosa and ci rcular muscle coat revealed that the characteristic smooth muscle tiss ue of the innermost sublayer of the circular muscle is required for th is rhythmical phenomenon, Histological examination showed that tissues containing special smooth muscle cells form an inner sublayer of the circular muscle coat, These innermost muscle cells were distinguishabl e from the bulk circular muscle cells by the following features: 1) fl attened and shorter shapes of the cell and nucleus, 2) numerous caveol ae on the cell surface, 3) abundant mitochondria, and 4) frequent gap- junction formations, Neither slow waves nor spontaneous mechanical rhy thmicities were recorded from the submucosal connective tissue or from the bulk circular muscle tissue without the inner sublayer, The thick er smooth muscle cells found in the submucosal border specimens were i dentical in histological features to the bulk circular muscles which p roduced no slow waves and no spontaneous contractions, Cellular elemen ts in the interstitium, such as fibroblasts, mast cells and macrophage s, were found in all tissue strips that were physiologically examined, Nerve elements were found in all the specimens; however, there was a unique nerve network probably corresponding to the plexus entericus (s ubmucosus) extremus described by STACH (1972), It was concluded, there fore, that the inner sublayer characterized by special smooth muscle c ells with a delicate nerve plexus is essential for producing spontaneo us activities of the circular muscle coat in the canine proximal colon .