Peristalsis is a reflex response that is initiated when the gut wall is stretched by the contents of the lumen, and it occurs in all parts of the gastrointestinal tract from the oesophagus to the rectum.
The stretch initiates 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 that vary from 2 to 25 cm/s.
Peristaltic activity can be increased or decreased by the autonomic input to the gut, but its occurrence is independent of extrinsic innervation.
It appears that local stretch releases serotonin, which activates sensory neurons that activate the myenteric plexus.
Cholinergic neurons passing in a retrograde direction in this plexus activate neurons that release substance P and acetylcholine, causing smooth muscle contraction behind the bolus
cholinergic neurons passing in an anterograde direction activate neurons that secrete NO and vasoactive intestinal polypeptide (VIP), producing the relaxation ahead of the stimulus.
- Segmentation and mixing:
- When the meal is present, the enteric nervous system promotes a motility pattern that is related to peristalsis but is designed to retard the movement of the intestinal contents along the length of the intestinal tract to provide time for digestion and absorption.
- This motility pattern is known as segmentation, and it provides for ample mixing of the intestinal contents (known as chyme) with the digestive juices.
- A segment of bowel contracts at both ends, and then a second contraction occurs in the centre of the segment to force the chyme both backward and forward.
- Unlike peristalsis, therefore, retrograde movement of the chyme occurs routinely in the setting of segmentation.
- It presumably reflects programmed activity of the bowel dictated by the enteric nervous system, and can occur independent of central input, although the latter can modulate it.
- Basic electrical activity and regulation of motility-
- Except in the oesophagus and the proximal portion of the stomach, the smooth muscle of the gastrointestinal tract has spontaneous rhythmic fluctuations in membrane potential between about −65 and −45 mV.
- This basic electrical rhythm (BER) is initiated by the interstitial cells of Cajal, which are stellate mesenchymal pacemaker cells with smooth muscle-like features that send long multiply branched processes into the intestinal smooth muscle.
- In the stomach and the small intestine, these cells are located in the outer circular muscle layer near the myenteric plexus.
- The BER itself rarely causes muscle contraction, but spike potentials superimposed on the most depolarizing portions of the BER waves do increase muscle tension.
- The depolarizing portion of each spike is due to Ca2+ influx, and the repolarizing portion is due to K+ efflux.
- acetylcholine increases the number of spikes and the tension of the smooth muscle, whereas epinephrine decreases the number of spikes and the tension.
- The rate of the BER is about 4/min in the stomach. It is about 12/min in the duodenum and falls to about 8/min in the distal ileum. In the colon, the BER rate rises from about 2/min at the cecum to about 6/min at the sigmoid.
- The function of the BER is to coordinate peristaltic and other motor activity, such as setting the rhythm of segmentation; contractions can occur only during the depolarizing part of the waves.
- Migrating motor complex–
- Oesophagus motility–
- Reflex response that is triggered by afferent impulses in the trigeminal, glossopharyngeal and Vagus nerve.
- Impulses are integrated in nucleus tractus solitarius and nucleus ambiguous.
- The efferent fibres pass to the pharyngeal musculature and tongue via trigeminal, facial and hypoglossal nerves.
- Swallowing is initiated by voluntary action of collecting the oral contents on the tongue and propelling them backward into pharynx.
- This starts a wave of involuntary contraction in the pharyngeal muscles that pushes the material into the esophagus.
- Inhibition of respiration and glottic closure are part of the reflex response.
- A peristaltic ring contraction of the esophageal muscle forms behind the material, which is then swept down the esophagus at a speed of approximately 4 cm/s.
- When humans are in an upright position, liquids and semisolid foods generally fall by gravity to the lower esophagus ahead of the peristaltic wave.
- However, if any food remains in the esophagus, it is cleared by a second wave of peristalsis. It is therefore possible to swallow food while standing on one’s head.
- Lower esophageal sphincter–
- The musculature of the gastroesophageal junction (lower esophageal sphincter; LES) is tonically active but relaxes on swallowing.
- The tonic activity of the LES between meals prevents reflux of gastric contents into the esophagus.
- The LES is made up of three components
- The esophageal smooth muscle is more prominent at the junction with the stomach (intrinsic sphincter)
- Fibers of the crural portion of the diaphragm, a skeletal muscle, surround the esophagus at this point (extrinsic sphincter)
- the oblique or sling fibers of the stomach wall create a flap valve that helps close off the esophagogastric junction and prevent regurgitation when intragastric pressure rises.
- The tone of the LES is under neural control.
- Release of acetylcholine from vagal endings causes the intrinsic sphincter to contract, and release of NO and VIP from interneurons innervated by other vagal fibers causes it to relax.
- Contraction of the crural portion of the diaphragm, which is innervated by the phrenic nerves, is coordinated with respiration and contractions of chest and abdominal muscles.
- Gastric motility and emptying-
- Receptive relaxation- When food enters the stomach, the fundus and upper portion of the body relax and accommodate the food with little if any increase in pressure.
- vagally mediated and triggered by movement of the pharynx and esophagus.
- Intrinsic reflexes also lead to relaxation as the stomach wall is stretched.
- Peristaltic waves controlled by the gastric BER begin soon thereafter and sweep toward the pylorus.
- The contraction of the distal stomach caused by each wave is sometimes called antral systole and can last up to 10s.
- Peristalsis then begins in the lower portion of the body, mixing and grinding the food and permitting small, semiliquid portions of it to pass through the pylorus and enter the duodenum.
- In the regulation of gastric emptying, the antrum, pylorus, and upper duodenum apparently function as a unit.
- Contraction of the antrum is followed by sequential contraction of the pyloric region and the duodenum.
- In the antrum, partial contraction ahead of the advancing gastric contents prevents solid masses from entering the duodenum, and they are mixed and crushed instead.
- The rate at which the stomach empties into the duodenum depends on the type of food ingested.
- Food rich in carbohydrate leaves the stomach in a few hours. Protein-rich food leaves more slowly, and emptying is slowest after a meal containing fat.
- The rate of emptying also depends on the osmotic pressure of the material entering the duodenum.
- Hyperosmolality of the duodenal contents is sensed by “duodenal osmoreceptors” that initiate a decrease in gastric emptying, which is probably neural in origin.
- Fats, carbohydrates, and acid in the duodenum inhibit gastric acid and pepsin secretion and gastric motility via neural and hormonal mechanisms. The messenger involved is probably peptide YY. CCK has also been implicated as an inhibitor of gastric emptying.