Motility of the Small Intestine

Anatomic Considerations

Contractions of the small intestine are effected by the activities of two layers of smooth muscle cells: an outer layer with the long axis of the cells arranged longitudinally and an inner layer with the long axis of the cells arranged circularly. In general, the circular muscle layer is thicker, and both layers are more abundant in the proximal intestine; they decrease in thickness distally to the level of the ileocecal junction.

The small intestine is richly innervated by elements of the autonomic nervous system. Within the wall of the intestine itself lie neurons, nerve endings, and receptors of the enteric nervous system. These neural elements tend to be concentrated in several plexuses. The most prominent, the myenteric or Auerbach plexus , lies between the circular and longitudinal layers of smooth muscle cells. Plexal neurons receive input from other neurons within the plexus, from receptors located in the mucosa and muscle walls, and from the central nervous system by way of the parasympathetic and sympathetic nerve trunks. Plexal neurons provide integrated output to smooth muscle cells of both muscle layers, to epithelial cells, and perhaps to endocrine and immune cells. Many neurotransmitters are present in the enteric nervous system, including acetylcholine, norepinephrine, vasoactive intestinal peptide, enkephalin, and other peptides. Furthermore, many nerves express nitric oxide synthase activity.

Interstitial cells of Cajal (ICCs) also are prominent in and adjacent to the enteric nerves and muscular layers of the intestine. At least two major classes of ICCs have been identified: those responsible for generating slow waves and those involved in mediating neural input to the smooth muscle cells.

Extrinsic innervation is supplied by the vagus nerve and by nerve fibers from the celiac and superior mesenteric ganglia (see Fig. 2.1 ). Many of the fibers within the vagus are preganglionic, whereas many from the abdominal ganglia are postganglionic. Some of the fibers within the vagus are cholinergic, whereas some from the abdominal plexuses are adrenergic. In addition, nerves that contain somatostatin, substance P, cholecystokinin (CCK), enkephalin, neuropeptide Y, and other transmitters have been identified in afferent and efferent vagal and splanchnic nerves. The exact pathways and physiologic roles of these nerves are being elucidated.