ABSTRACT
The smooth muscle in the wall of the large intestine is active almost constantly, although this background level of activity can be modified markedly by both neural and hormonal influences. The large intestine receives input from the sympathetic branch of the autonomic nervous system (ANS) via thoracolumbar segments of the spinal cord. The parasympathetic input to the large intestine up to the splenic flexure comes from the vagus nerve (cranial nerve X) and from the splenic flexure onwards from sacral segments of the spinal cord. The transit time through the large intestine for material introduced into the cecum is 2-3 days. The part of the colon in which food remains longest is the transverse colon; this has been shown by instillation of radio-opaque dyes and imaging of the large intestine over a number of days. While the fecal material is in the colon, the last small amounts of water and electrolytes are absorbed from it (see below), and its passage through the colon, anal canal and the anus is eased by the secretion of mucus from numerous mucous cells in the wall of the large intestine. Major propulsive motions of the large intestine normally follow a meal, but the pathways involved in such reflexes are not well understood. The gastroileal reflex leads to opening of the ileo-cecal valve to allow more chyme to enter the lower parts of the ascending colon, whereas the gastrocolic reflex elicits powerful propulsive contractions of the colonic smooth muscle to propel feces towards the rectum. Distention of the rectum then leads to the urge to
