Peristalsis in Gastrointestinal Tract

Peristalsis

• Peristalsis is a reflex response that occurs in the entire gastrointestinal tract, from the esophagus to the rectum.
• It is initiated by the stretching of the gut wall by the contents of the lumen.
• The stretch causes a circular contraction behind the stimulus and an area of relaxation in front of it.
• The wave of contraction then moves in an oral-to-caudal direction, propelling the contents of the lumen forward at rates of 2-25 cm/s.
• Peristaltic activity can be increased or decreased by autonomic input to the gut, but its occurrence is independent of extrinsic innervation.

Mechanism of Peristalsis

• Local stretch releases serotonin, which activates sensory neurons that activate the myenteric plexus.
• Cholinergic neurons in the myenteric plexus activate neurons that release substance P and acetylcholine, causing smooth muscle contraction behind the bolus.
• Cholinergic neurons in the myenteric plexus also activate neurons that secrete NO and vasoactive intestinal polypeptide (VIP), producing relaxation ahead of the stimulus.

Segmentation

• Segmentation is a motility pattern that occurs when a meal is present, and is designed to retard the movement of the intestinal contents along the length of the intestinal tract.
• It provides for the mixing of the intestinal contents (chyme) with the digestive juices.
• A segment of bowel contracts at both ends, and then a second contraction occurs in the center of the segment to force the chyme both backward and forward.
• Retrograde movement of the chyme occurs routinely in the setting of segmentation.
• This mixing pattern persists for as long as nutrients remain in the lumen to be absorbed.

Basic Electrical Rhythm (BER)

• The smooth muscle of the gastrointestinal tract has spontaneous rhythmic fluctuations in membrane potential between -65 and -45 mV.
• The BER is initiated by the interstitial cells of Cajal, stellate mesenchymal pacemaker cells with smooth muscle-like features.
• These cells are located in the outer circular muscle layer near the myenteric plexus in the stomach and small intestine, and at the submucosal border of the circular muscle layer in the colon.
• The BER itself rarely causes muscle contraction, but spike potentials superimposed on the most depolarizing portions of the BER waves do increase muscle tension.