Small Intestinal Contractions
The small intestine exhibits several distinct types of contractions that aid in digestion, with peristalsis being the primary propulsive contraction and the Migrating Motor Complex (MMC) being the characteristic fasting pattern 1.
Primary Contraction Patterns
Peristalsis
- Peristalsis is the coordinated wave of muscle contraction that propels intestinal contents distally through the small bowel 2.
- These contractions involve both circular and longitudinal muscle layers working in coordinated fashion to move chyme forward 3.
- Peristaltic waves can be graded in amplitude according to stimulus intensity and depend on the spread of nervous activity along the bowel rather than just fluid displacement 2.
- Local muscle tone and contraction are essential for both initiation and maintenance of peristaltic propagation 2.
Migrating Motor Complex (MMC)
- The MMC is the stereotypic pattern of contractions occurring during fasting states, characterized by recurring bursts of intense electrical activity 1.
- During nocturnal sleep, normal MMC activity is clearly evident and serves to clear the small bowel of cellular debris and residual food 1, 4.
- The MMC cycle comprises three phases, with Phase 3 being the most prominent feature—consisting of intense contractions that start in the stomach or duodenum and migrate distally 4.
- In humans, Phase 3 recurs approximately every 110 minutes and is critical for preventing bacterial overgrowth by clearing debris 4.
- When the MMC is impaired, the small bowel cannot clear debris, predisposing to gut stasis and bacterial overgrowth 1.
Additional Contraction Types
Segmentation Contractions
- The small intestine produces mixing contractions that promote efficient digestion and absorption through localized folding and kneading of contents 3, 5.
- These contractions create mixing through repeated radial and longitudinal muscular contraction, causing displacement of chyme around and between adjacent villi 5.
Slow Wave Contractions
- Electrical oscillations (slow waves) generated by interstitial cells of Cajal orchestrate waves of muscle contraction throughout the small intestine 6.
- There is a frequency gradient from high at the duodenum to low at the terminal ileum, creating organized propulsion 6.
Clinical Relevance
Caveat: In patients with enteric neuropathies, gut coordination is disrupted and the presence of chyme can cause severe painful non-propulsive large contractions, which explains abdominal pain shortly after eating 1. These uncoordinated contractions (bursts) may be hyperactive but fail to achieve effective propulsion 1.