The predominant form of contraction in small intestine is segmentation contraction.
The most common movements seen in the small intestine when chyme is present are stationary contractions and relaxations of short segments of the small intestine. The presence of chyme in the small intestine distends the small intestinal wall; the stretch elicits/triggers local contractions and relaxations at intervals along the intestine.
This pattern of contraction called segmentation contractions differ from peristalsis because there is little apparent net forward movement of intestinal contents toward the large intestine.
Physiologically, segmentation contraction also called mixing contractions primarily mixes chyme with digestive enzymes; however, a degree of propulsion of chyme towards the colon occurs. Therefore, segmentation contractions serve to both mix chyme with digestive juices and at the same time propel the intestinal content towards the colon.
Perhaps, the rate of propulsion is extremely slow because segmentation contractions are essentially mixing waves. Thus, we could consider segmentation contractions as the progressive mixing of chyme with intestinal secretions.
Each contracting segment of the small intestine is only about 4cm long and the contraction lasts for only a fraction of a minute.
The nature of segmentation contraction is that they essentially produce continuous division of the small intestine into spaced segments (i.e. segmentation of the small intestine) particularly mixing chyme in the lumen and bringing it into close contact with the intestinal wall.
BER of small intestine determines the frequency of movements in the small intestine
Basic electrical rhythm (electrical slow waves) of the small intestine determines the frequency of segmentation movements. More specifically, the electrical activities of pacemaker cells (interstitial cells of Cajal) of the small intestine initiate these segmentation contractions.
Interstitial cells of Cajal (GI pacemaker cells) lie in close association with the two muscle layers of the muscularis externa. Slow waves of intestinal basic electrical rhythm produce undulating changes in membrane potential of pacemaker cells.
Slow waves on their own are incapable of triggering significant muscle contractions. However, once slow waves attain threshold voltage, pacemaker cells fire true action potentials, which radiate to adjacent intestinal smooth muscle fibers through interconnecting gap junctions.
Although pacemaker cells set the basic pace/frequency for the rate of the segmentation contractions, excitatory signals from parasympathetic nerves to the gut enteric neurons together with excitatory hormones are necessary if the segmentation contractions must be strong enough.
This is evident following the administration of an acetylcholine-blocking drug called atropine. Atropine blocks the effect of the neurotransmitter acetylcholine; as a result, the segmentation contractions become significantly weak and practically ineffective.
Thus, even though the intestinal basic electrical rhythm (intestinal slow waves) set the frequency of the segmentation contractions, segmentation contractions would be exceedingly ineffective and inefficient in the absence of steady background excitation primarily from the myenteric nerve plexus of the gut.
Concerning the frequency of intestinal basic electrical rhythm, each of the three regions/subdivisions of the small intestine normally has a different frequency of basic electrical rhythm.
Of the three segments, the duodenum has the highest/maximal frequency of about 12/min, however, it is important to note that segmentation contractions in the duodenum at 12 contractions per minute is only possible in conditions of high excitatory signals. In the distal/terminal ileum, the maximal frequency falls to about 8/min.
Generally, along the length of the small intestine, each successive segment has a maximum frequency of BER slightly lower than that of the segment above.
Waves of peristaltic contractions in the small intestine propels chyme at an exceedingly slow velocity
Peristalsis occurs along the length of the small intestine and propels chyme through the small intestine towards the large intestine. Peristaltic contractions in the small intestine are exceedingly weak and usually cease after propelling chyme for just 4-5cm.
The forward movement of chyme along the small intestine is therefore very slow; so slow that it takes approximately 3-5 hours for chyme to travel from the pyloric sphincter to the ileocecal valve.
Actually, it is extremely rare for a particularly peristaltic wave along the small intestine to travel farther than 8cm before it dies out. On the average, peristaltic wave along the small intestine moves chyme at a slow velocity of 1cm/minute.
Physiologically, the exceedingly slow velocity of peristalsis in the small intestine is extremely important since it allows enough time for, the digestion of certain food constituents as well as the absorption of the products of the digestion.
Neural factors and hormones can alter movements in the small intestine
The enteric nervous system, autonomic nerves, and hormones are capable of altering the intensity of segmentation contractions. Stimulation of parasympathetic nerves to the small intestine increases the force of contraction whereas stimulation of sympathetic nerves to the small intestine decreases the force of contractions.
Although these factors alter the force of intestinal smooth muscle contractions, they are unable to cause significant alterations in the frequency of BER of the small intestine.
Concerning peristalsis in the small intestine, the arrival of chyme in the duodenum after a meal stretches the duodenal wall. The stretch induces a rise in peristaltic activity along the small intestine. The myenteric plexus is principally responsible for this response.
Moreover, hormones such as gastrin, CCK, motilin, serotonin, and insulin all promote intestinal motility. For instance, motilin stimulates an inter-digestive gut contraction called migrating motor complex.
Ileocecal valve regulates emptying of the small intestine
As chyme reaches the terminal part of the small intestine just before it gets into the cecum, it encounters a valve called ileocecal valve. This valve regulates the passage of material between the ileum and cecum. The valve therefore, regulates both the passage of chyme from ileum to cecum as well as prevents the backflow of fecal matter into the ileum from the cecum.
Different reflexes initiated at different segments along the GI tract alter the contractility of various portions of the small intestine. An example is the gastro-ileal reflex.
Usually, the ileocecal valve closes and prevents the passage of chyme until when the individual eat another meal. By the time the stomach is emptying, a gastro-ileal reflex enhances peristalsis in the ileum ultimately creating enough pressure that forces the chyme through the ileocecal valve into the cecum.
Migrating motor complex replaces gut segmentation contractions in between meals
A unique pattern of peristaltic wave replaces the segmentation contractions of the gut when the gut is largely empty. When the gut is empty (i.e. when absorption of most of the meal is complete), entero-endocrine cells found in the intestine release an intestinal hormone called motilin. Motilin stimulates both autonomic nerves and enteric neurons of the gut to induce a pattern of gut contraction called migrating motor complexes.
Migrating motor complex also called migrating myoelectric complex (MMC) begin in the lower region of the stomach, travels a short distance of about 1.5 ft or so along the small intestine before dying out. As the wave of peristalsis ceases, another peristaltic wave starts some distance (farther down the small intestine) away from where the preceding wave died out.
Consequently, this repeated wave of peristaltic activity each traveling just a short distance slowly migrates down the entire length of the small intestine to the ileocecal juncture.
By the time, a MMC reaches the end of the ileum, a short period of quiescence lasting about 40 minutes elapse before new waves begin in the stomach and the cycle repeats. It takes about 90 minutes for a cycle of migrating motor complex to travel from stomach to ileocecal juncture.
Physiologically, migrating motor complexes serve to clear the small intestine of remnants of food and bacteria that may collect in the relatively sterile small intestine. An increase in biliary and pancreatic secretions accompanies each MMC. Secretions from the accessory glands assist in the cleansing function of the MMC, consequently, preventing a build-up of bacterial cells in the small intestine.
MMC can be quite noisy and troublesome, however, the good news is that one can put off a MMC by eating something since the ingestion of food abolishes MMCs and restores the normal quiet digestive pattern of motility. As the individual takes a meal, MMC ceases and the normal digestive pattern of motility resumes.