Contraction above the bolus and relaxation below it
Expansion
Distension by a bolus triggers the myenteric (peristaltic) reflex:
- Behind the bolus (orally): circular muscle contracts and longitudinal muscle relaxes, driven by acetylcholine and substance P
- Ahead of the bolus (aborally): circular muscle relaxes and longitudinal muscle contracts, driven by nitric oxide, vasoactive intestinal peptide and ATP
The net effect is propulsion in the aboral direction, the law of the gut.
Contrast this with segmentation, the other main motility pattern: alternating contractions that divide and mix the contents without net propulsion, which dominates in the small intestine during digestion and maximises contact with the mucosa.
Two clinical points follow. Loss of the inhibitory nitric oxide neurones prevents relaxation ahead of the bolus, which is the mechanism in achalasia and in the aganglionic segment of Hirschsprung disease. And opioids increase non-propulsive segmenting activity while inhibiting propulsion, which is why they cause constipation yet also cause cramps.