Peristalsis

Peristalsis is driven by the coordinated contraction of circular and longitudinal smooth muscle layers in the wall of the organ. A wave of contraction moves aborally (away from the mouth), preceded by a wave of relaxation that allows the lumen to open. This pattern is generated by the enteric nervous system, often called the 'second brain,' which integrates signals from local sensory neurons and interneurons. The myenteric plexus (Auerbach's plexus) plays a central role, releasing excitatory neurotransmitters like acetylcholine and inhibitory ones like nitric oxide to coordinate the wave. In the esophagus, peristalsis is triggered by swallowing and is reinforced by the vagus nerve, whereas in the intestine, peristalsis is largely autonomous, modulated by hormones and mechanical stretch. The interstitial cells of Cajal act as pacemakers, generating slow waves that set the rhythm of contractions.¹

In the esophagus, peristalsis propels a bolus from the pharynx to the stomach, with a primary wave initiated by swallowing and secondary waves clearing residual food. In the stomach, peristaltic waves mix and grind food, gradually pushing chyme through the pylorus. The small intestine exhibits segmentation (non-propulsive mixing) and peristalsis that moves chyme toward the colon; the migrating motor complex (MMC) sweeps the gut between meals. In the colon, peristalsis is slower and often segmental, facilitating water absorption and forming feces. Defecation involves a mass peristaltic rush that empties the rectum. Peristalsis also prevents reflux by maintaining a high-pressure zone at the lower esophageal sphincter.²

Peristalsis is not exclusive to the digestive tract. In the ureters, peristaltic waves transport urine from the kidneys to the bladder, driven by smooth muscle in the renal pelvis and ureteric wall. In the bile ducts, peristalsis helps move bile from the liver to the gallbladder and duodenum. The fallopian tubes exhibit peristaltic contractions that assist in transporting the ovum toward the uterus. Even the lymphatic vessels rely on peristaltic-like contractions to propel lymph. These systems share similar neural and hormonal regulation, though the pacemaker mechanisms differ. For example, ureteric peristalsis is myogenic, originating in the renal pelvis, and is modulated by the autonomic nervous system.³

Peristalsis has several surprising facets. The migrating motor complex (MMC) is a distinct pattern of peristaltic waves that occurs during fasting, clearing residual debris; it is often called the 'housekeeper' of the gut. Peristalsis can be influenced by emotions and stress via the brain-gut axis, which is why anxiety can alter bowel habits. In some animals, such as earthworms, peristalsis is the primary mode of locomotion, not just digestion. The phenomenon of 'reverse peristalsis' occurs in vomiting, where contractions move contents upward. Also, the interstitial cells of Cajal are not neurons but specialized mesenchymal cells; their loss is linked to diabetic gastroparesis. Finally, the first description of peristalsis is attributed to the ancient Greek physician Galen, who observed it in animal dissections.⁴

Enteric nervous system

The intrinsic nervous system of the gastrointestinal tract, comprising the myenteric and submucosal plexuses.

Myenteric plexus

A network of neurons located between the circular and longitudinal muscle layers, controlling peristalsis.

Interstitial cells of Cajal

Pacemaker cells that generate slow waves, coordinating smooth muscle contractions.

Migrating motor complex

A cyclic pattern of peristaltic activity during fasting that clears the gut.

Achalasia

A disorder of the esophagus characterized by failure of the lower esophageal sphincter to relax and loss of peristalsis.