The colon reabsorbs salts and water. About 1500 mL of fluid enters the colon each day, but only 50 to 100 mL of fluid is excreted in feces. Anatomically, the colon is divided into the cecum, ascending colon, transverse colon, and descending colon. Segmental contraction of the circular smooth muscle divides the colon into numerous haustra (sacculations). There is some retrograde peristalsis, which delays aboral movement of chyme. Mass movement (three per day) is primarily responsible for the aboral movement of the chyme in the colon. During a mass movement, the haustra relax and a peristaltic wave initiates a prolonged contraction of the colon until an entire segment is contracted. After a few minutes, the segment relaxes and haustra reappear. The enteric nerves primarily inhibit colonic smooth muscle contraction. Hirschsprung’s disease is characterized by congenital absence of enteric nerves, and the colon is obstructed by a tonic contraction.
Chyme entering from the small intestine is mostly a liquid. There is progressive absorption of water along the length of the large intestine, resulting in the mostly solid feces being excreted. Enhanced motility decreases the time for water absorption and can cause diarrhea, the excretion of watery feces. Conversely, impaired motility allows time for too much water to be reabsorbed and can cause constipation.
Parasympathetic nerves modulate colonic motility by interacting with the enteric nervous system. Parasympathetic innervation is provided by the vagus for the cecum, ascending colon, and transverse colon, and by parasympathetic nerves originating in the sacral spinal cord for the descending colon. Sympathetic activity inhibits colonic contractions. This reflex control coordinates colonic motility with events in the remainder of the GI system. In the colocolonic reflex, distention causes relaxation in adjacent areas. This is partly mediated by sympathetics. In the gastrocolic reflex, an increase in colonic activity follows stretching of the stomach.
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