Propulsive movements, colon

The functions of the colon are (1) absorption of water and electrolytes from the chyme and (2) storage of fecal matter until it can be expelled. The proximal half of the colon, illustrated in Figure 12.2, is concerned principally with absorption, and the distal half with storage. Because intense movements are not required for these functions, the movements of the colon are normally sluggish. Although sluggish, the movements still have characteristics similar to those of the small intestine and can be divided into mixing movements and propulsive movements. 

Gastrointestinal smooth muscle motility is affected from the stomach to the colon. In general, the walls of the viscera are relaxed, and both tone and propulsive movements are diminished. 

Figure 106,2. Opioid-induced bowel dysfunction (OBD). Opioids increase intestinal fluid absorption, inhibit intestinal secretions and peristalsis, and block propulsive movements in the colon. Constipation and other symptoms associated with OBD occur in 15-90% of cancer patients and may be more distressful and debilitating than pain. Tolerance rarely develops to these symptoms and traditional therapy including stimulants, lubricants, arxf bulk laxatives may not be effective.

A second form of motility in the large intestine is mass movement. Three or four times per day, typically after a meal, a strong propulsive contraction occurs that moves a substantial bolus of chyme forward toward the distal portion of the colon. Mass movements may result in the sudden distension of the rectum that elicits the defecation reflex. 

The anatomy of the distal colon, with its thick muscular walls, suggests a predominantly propulsive activity. Studies with single administrations of pellets or Pulsincap devices suggested that the distal part of the transverse colon area is difficult to treat since this area and the descending colon function as a conduit. Steady-state measurements confirm this assertion (48) and Weitschies group have also reported data showing mass movements propel objects quickly through the distal transverse colon. 

Colonic motor function also has cyclic slow waves in the proximal colon. These contractions are primarily retrograde in the proximal colon, allowing segmentation and liquid reabsorption. In the distal colon a propulsive mass movement occurs intermittently. This may be stimulated by food ingestion and is termed the gastrocolonic reflex. 

Peristaltic waves of the type seen in the small intestine only rarely occur in other parts of the colon. Instead, most propulsion occurs by (1) the haustral contractions discussed previously and (2) mass movements. 

From the transverse colon to the sigmoid, mass movements take over the propulsive role. These movements usually occur only a few times each day, most abundantly for about 15 min during the first hour after eating breakfast. 

Neostigmine augments GI motor activity the colon is particularly stimulated. Propulsive waves are increased in amplitude and frequency, and movement of intestinal contents is thus promoted. The effect of anti-ChE agents on intestinal motility probably represents a combination of actions at the ganghon cells of Auerbach s plexus and at the smooth muscle fibers (see Chapter 37). 

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