Introduction

Peristalsis is primarily found throughout the gastrointestinal tract and is the involuntary propulsion of food. This movement begins in the pharynx, once a food bolus is formed, and ends in the anus. Along with segmentation or mixing of food, peristalsis is an essential part of providing the body with nutrients. 

The gastrointestinal tract is innervated by the enteric nervous system (ENS), and the parasympathetic nervous system activates peristalsis through the myenteric plexus. The myenteric plexus and interstitial cells of Cajal (ICC) contribute to the contraction and relaxation of the circular and longitudinal muscles found within the gastrointestinal tract.[1][2][3]

Issues of Concern

Inhibition of the myenteric plexus via medications such as atropine may alter the functionality of peristalsis.[4] Peristalsis has implications in disorders such as GERD, small intestinal bacterial growth, gastroparesis, and achalasia, which will be a topic of further discussion.

Cellular

The layers of the GI tract include the mucosa, submucosa, muscularis externa, and serosa. Peristalsis occurs both in the skeletal and smooth muscles. For peristalsis in the skeletal muscle, the vagal nucleus triggers the neurons, and in smooth muscle, it is triggered by the dorsomotor nucleus via the vagus nerve.[5]

The muscularis externa is comprised sequentially of the inner circular layer, the myenteric plexus, and the outer longitudinal layer. When the circular muscle and longitudinal muscle contract, the circular muscle decreases the diameter of the muscle, and the longitudinal decreases the length. The circular and longitudinal muscles contract and relax, leading to the peristaltic movement. When there is a bolus of food, circular muscles behind the bolus contract and relax in the front, whereas longitudinal muscles behind the bolus relax and contract in the front. On a chemical level, when the bolus passes through the intestine, serotonin (5-hydroxytryptamine) is released by enterochromaffin cells, and sensory neurons are activated. The contraction of circular muscle behind the bolus is due to excitatory transmitters such as acetylcholine, substance P, and neuropeptide Y. The relaxation of circular muscle in front of the bolus is due to inhibitory motor neurons to both muscle layers. The same concept applies to longitudinal muscles where the excitatory transmitters behind the bolus instead inhibit longitudinal muscle.[6][7]

The myenteric plexus has a network from the esophagus to the internal anal sphincter. ICCs are found through intermittent spaces of smooth muscle and show a dense distribution in the myenteric plexus. ICCs are known as pacemaker cells because they create slow-wave patterns that contribute to the peristaltic movement. There is an influx of calcium for a slow wave pattern, which increases the chance for an action potential. Spike potentials occur when the membrane potential is depolarized above the threshold and results from muscle distension and acetylcholine. However, spike potentials do not cause contractions in the stomach.[8][9]

Development

In gestation, gastric peristalsis can begin anywhere from 14 to 23 weeks. At 24 weeks, gastric emptying and peristalsis start to develop faster than before.[10] Primary and secondary peristalsis decrease with aging.[11]

Organ Systems Involved

The organ system involved includes parts of the gastrointestinal tract: pharynx, esophagus, stomach, small intestine, large intestine, and rectum. Peristalsis is mainly found within the smooth muscle, and other areas of this type of movement are found in bile ducts, glandular ducts, and ureters.

Function

There are two types of peristaltic contractions: primary and secondary. The primary peristaltic wave helps to move food forward. The secondary peristaltic wave initiates if the primary peristaltic wave does not work and food is not able to pass. Due to the distention of the esophagus, stretch receptors become stimulated, and the secondary wave clears the bolus.

Mechanism

The stretching of the gut stimulates peristalsis during the development of the food bolus, which activates the ENS, parasympathetic activation, and chemical activation of the gut.

Swallowing partially occurs due to the ENS and has three phases: oral, pharyngeal, and esophageal. During the pharyngeal phase of swallowing, peristalsis first occurs due to the pharynx and then goes to the esophageal phase. Gravity assists with the downward movement of the bolus as well. Gastric peristalsis occurs at the site of the corpus and antrum, with peristaltic contractions increasing in the antrum. Intestinal peristalsis occurs in the small and large intestines. In the colon, there is a mass movement, and peristaltic contraction helps to move the chyme. Strong peristalsis is apparent when the stomach and small intestine are empty due to the migrating motor complex (MMC). When there is a lack of MMCs, there can be increased intestinal bacterial growth.[9]

Related Testing

Motility disorders involving peristalsis can be tested by esophageal, antroduodenal, colonic, and anorectal manometry.[12] High-resolution manometry can help clinicians test abnormal and normal peristalsis. A 24 hour pH catheter can also measure acid reflux, which can be due to disorders involving issues with peristalsis.[13] A barium esophagogram is useful for the detection of transit and swallowing.[14]

Pathophysiology

Disorders involving peristalsis:

Gastroesophageal Reflux Disease (GERD)

GERD occurs when stomach contents travel backward up into the esophagus, which can be due to an impaired lower esophageal sphincter (LES). Normally, the LES contracts after the passage of food; however, it can remain open in GERD, allowing stomach acid to travel in a retrograde fashion. Many factors contribute to GERD, one of which is dysfunctional peristalsis and bolus movement, which is a finding in patients with the condition. Usually, esophageal peristalsis assists in clearing refluxate, but this is not always the case in GERD.[15]

Hirschsprung Disease

In Hirschsprung disease, the ENS is nonfunctional, leading to intestinal blockage, among other issues. The ENS controls motility, and a lack of this system, in turn, affects peristalsis where it can no longer function properly and contents cannot pass. Both the myenteric plexus and submucosal plexus are affected. The risk of Hirschsprung’s disease increases with Down syndrome and normally requires surgery.[16]

Gastroparesis

Gastroparesis occurs when the stomach cannot move its contents forward even without an obstruction. The pathophysiology behind gastroparesis is complex; however, there is a lack of communication between smooth muscle and ICCs, thus affecting peristalsis. Diabetic gastroparesis can also occur. Individuals who have elevated blood glucose levels can have neuronal damage, which affects the myenteric plexus.[17]

Achalasia

Achalasia is a disorder in which the LES does not relax to allow food to travel from the esophagus to the stomach. In both children and adults, achalasia involves dysfunctional peristaltic movement of the esophagus, greater LES pressure, and the inability of the sphincter to relax.[12] A lack of peristalsis is most likely due to the dysfunction of the myenteric plexus in the esophagus. Specifically, there is a problem with inhibitory transmitters preventing peristaltic movement leading to greater excitation, adding to the LES pressure.[18]

Reverse Peristalsis (vomiting)

 A factor that leads to the vomiting reflex is reverse peristalsis, and as the name implies, contents are moved backward instead of forward. This action begins in the small intestine and moves towards the stomach.

Clinical Significance

Abnormal peristalsis can be detected and classified. High-resolution manometry helps to define weak and absent peristalsis. In the recent Chicago classification, weak peristalsis is when there are breaks in 20-mmHg isobaric contours and occurs in greater than twenty percent of swallows. Failed peristalsis is present in greater than thirty percent of swallows.[14]

Peristalsis plays a significant role in multiple common diagnoses that health care professionals see daily. From common disorders, including constipation or diarrhea where peristaltic contractions can be decreased or increased, to rarer forms of pathologic peristalsis having involvement in Hirschsprung disease, it is an essential aspect of the gastrointestinal system.

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