Movements of the Small Intestine

1. Mixing Contractions (Segmentation Contractions)

• When a portion of the small intestine becomes distended with chyme, stretching of the intestinal wall elicits localized concentric contractions spaced at intervals along the intestine, lasting a fraction of a minute.
• These contractions cause "segmentation" of the small intestine, dividing it into spaced segments resembling a chain of sausages.
• As one set of segmentation contractions relaxes, a new set begins at new points between the previous contractions, "chopping" the chyme 2 to 3 times per minute and promoting progressive mixing with intestinal secretions.

• Maximum frequency in the duodenum and proximal jejunum: ~12 per minute (set by the basic electrical rhythm / slow waves of the intestinal wall)
• In the terminal ileum: 8 to 9 contractions per minute

• Segmentation contractions become exceedingly weak when excitatory activity of the enteric nervous system is blocked by atropine
• Although slow waves initiate contractions, effective contraction requires background excitation mainly from the myenteric (Auerbach's) nerve plexus

2. Propulsive Movements

A. Peristalsis

• Chyme is propelled by peristaltic waves that can occur anywhere in the small intestine and move toward the anus at a velocity of 0.5 to 2.0 cm/sec (faster proximally, slower terminally)
• Waves are normally weak and die out after traveling only 3 to 5 cm (rarely exceed 10 cm)
• Net forward movement of chyme averages only 1 cm/min
• Transit from pylorus to ileocecal valve takes 3 to 5 hours
• The function of peristalsis is both to propel chyme toward the ileocecal valve and to spread chyme along the intestinal mucosa for absorption

B. Control of Peristalsis - Nervous and Hormonal Signals

• Chyme entry into the duodenum causes stretching of the duodenal wall, increasing peristaltic activity
• Gastroenteric reflex - distention of the stomach, conducted through the myenteric plexus down along the small intestinal wall
• Hormones: Gastrin, CCK, insulin, motilin, and serotonin all enhance intestinal motility

• Hormones: Secretin, glucagon, and GLP-1 inhibit motility

C. Propulsive Effect of Segmentation Movements

• Segmentation movements, although lasting only seconds, often travel ~1 cm in the anal direction
• The distinction between segmentation and peristaltic movements is not always sharp - segmentation also contributes to propulsion

D. Peristaltic Rush

• Intense irritation of the intestinal mucosa (e.g., in severe infectious diarrhea) can cause powerful and rapid peristalsis - the "peristaltic rush"
• Initiated partly by autonomic nervous system/brainstem reflexes and partly by enhancement of myenteric plexus reflexes
• Powerful contractions travel long distances within minutes, sweeping intestinal contents into the colon
• Function: relieves the small intestine of irritative chyme and excessive distention

3. Movements Caused by the Muscularis Mucosae and Muscle Fibers of the Villi

• The muscularis mucosae can cause short mucosal folds that increase surface area exposed to chyme, thereby increasing absorption
• Individual muscle fibers extend into the intestinal villi, causing them to contract intermittently
• Villous contractions (shortening, elongating, shortening again) "milk" the villi, promoting lymph flow from the central lacteals into the lymphatic system
• These contractions are initiated mainly by local nervous reflexes in the submucosal (Meissner's) nerve plexus in response to chyme

4. The Ileocecal Valve - Preventing Backflow

• The ileocecal valve protrudes into the cecum lumen and is forcefully closed when cecal pressure rises - it resists reverse pressure of at least 50 to 60 cm of water
• The wall of the ileum for several centimeters upstream has a thickened circular muscle called the ileocecal sphincter, which remains mildly constricted, slowing emptying into the cecum
• After a meal, the gastroileal reflex intensifies ileal peristalsis and promotes emptying into the cecum
• Prolonged chyme retention in the ileum facilitates absorption; normally only 1500 to 2000 mL of chyme empty into the cecum each day

• When the cecum is distended: contraction of the ileocecal sphincter intensifies and ileal peristalsis is inhibited, delaying further emptying
• Cecal irritation (e.g., inflamed appendix) causes intense sphincter spasm and partial ileal paralysis, blocking emptying
• These reflexes are mediated via the myenteric plexus and extrinsic autonomic nerves (especially prevertebral sympathetic ganglia)