Oblique smooth muscle bundles between the circular and longitudinal muscle layers in the canine proximal colon

The region between the longitudinal and circular muscle layers of the canine proximal colon was investigated by light and electron microscopy. Unique smooth muscle bundles, referred to as "oblique" muscles, were arranged in branching/anastomosing bundles, forming a network intermeshed with the nerve plexus and vasculature. These smooth muscles were concentrated along the ganglia and primary connectives of the myenteric plexus, thus showing a periodic distribution roughly corresponding to that of the ganglia. The oblique smooth muscle bundles bridged the distance between the circular and longitudinal muscle layers and blended into the inner aspect of the longitudinal muscle layer and the outer aspect of the circular muscle layer. A few oblique muscle cells occurred within the sheath of the ganglia and primary strands, i.e., perineurium. Oblique muscles may serve to electrically couple the circular and longitudinal muscle layers, thus explaining the synchronous electrical activities of these muscles. It is possible that the pacemaker activity in the myenteric region is generated by these muscle cells. Oblique muscles may also serve to protect nerve elements and the vasculature from the mechanical stress of peristalsis.