Mixing and Propulsion of Gastrointestinal Contents Page!

There are 2 major types of movement of the stomach and small intestine.   These movements are in addition to the constant tone maintained by the wall of the gut; constant tone generated by the smooth muscle of the gut helps prevent the gut from being over-distended... or, from remaining distended following "constant use."

1) Mixing is accomplished by peristalsis or by churning (local constrictive contractions)

2) Propulsion is accomplished by peristalsis; a ring of contraction moving in both directions.  Peristalsis begins on the orad side of distension, and the portion of the wave moving in the orad direction quickly dies out... such that the net direction of peristaltic waves is to push digesta analward.

The usual stimulus for peristalsis is distension. The myenteric plexus is strongly innervated by parasympathetics except in the small intestine.  Peristalsis will occur without the myenteric plexus (eg. in the atropinized gut) but the myenteric plexus is required for effective, coordinated peristalsis.

Receptive Relaxation and "The Law of the Gut"

Peristaltic waves on the orad side of the digesta, attempt to push digestive contents analward.  Where orad peristaltic waves occur, they die out quickly to generally prevent digesta from being moved in the orad direction.  Meanwhile, an area of receptive relaxation occurs analward from the area distended by digesta.  This is "the law of the gut."  According to the Law of the Gut, then, analward relaxation helps peristaltic waves push digesta in the analward direction.

There also exists a "gradient" of activity with 3 contractions/min in the stomach.  Beginning with the duodenum, contraction frequency is maximal at approximately 12 contractions/min.  The number of contractions gradually declines as we move analward through the intestine, with 7-9 contractions per minute in the ileum and just a few contractions per day occurring in the colon.

Storage: The smooth muscle of the stomach exhibits plasticity.  It can change fiber length dramatically while maintaining constant tone.  Tone in the stomach is relatively low until it is filled to about 1 liter.  The stretching force on the stomach wall increases proportionally to the increase in radius of the curvature of the body of the stomach.  Thus, large distension causes only a slight increase in pressure on the walls (Laplace's Law). Stretch receptors in the stomach reflexively inhibit motor activity of the stomach (vagal reflexes).

Mixing: Digestive juices are secreted by the gastric glands which cover almost the entire outer wall of the body of the stomach.  The basal electrical rhythm of the stomach is about 3 waves/minute moving antrally along the longitudinal smooth muscle and spreading spontaneously to the circular smooth muscle.  In the full stomach, the waves begin near the midpoint of the stomach for mixing.  As the stomach empties, the waves begin further up to move the last vestiges of stored food into the antrum.  As the wave moves down, they become constrictive rings at the incisura angularis (the point at which we enter the pyloric antrum) which forces the antral contents towards the pylorus.  This is very important to mixing as each constrictor ring only empties a few ml's through the small pyloric sphincter.  The intensity of antral peristalsis is regulated in response to signals from both the stomach and the duodenum.  The "pyloric pump" is a strong antral peristaltic wave that forces a few ml's of chyme through the pyloric sphincter.

Signals that increase peristalsis in the stomach include: 1) distension (vagally) and locally (myenterically) and 2) gastrin from the antral mucosa (both inhibit pyloric tone as well, to assist in passage of digesta out of the stomach).  Distension of the duodenum signals for decreased peristaltic force in the stomach and increased pyloric tone (to slow stomach emptying to allow time for digesta to be cleared from the duodenum).  Gastrin causes increased peristaltic activity, increased HCl secretion and relaxation of the pyloris in the stomach.  This also causes constriction of the gastroesophageal sphincter, preventing gastro-esophageal reflux.  Thus, gastrin is very important to stomach emptying.

enterogastric reflex

Reflex nervous signals are sent from the duodenum to the stomach during stomach emptying.  These signals are intended to increase secretion and mixing in the stomach but the greatest effect is to slow stomach emptying.  This reflex is entirely vagally mediated (vagus --> brainstem --> vagus) and is elicited by:

1) duodenal distension

2) irritation of the duodenal mucosa (mechanical)

3) increased acidity of the duodenum (from the chyme) 3.5-4.0

4) the osmolality of the chyme entering the duodenum (primarily hypertonic)

5) a.a.'s and f.a.'s (breakdown products of proteins and fats) in the chyme entering the duodenum

Movements of the Small Intestine

The movements of the small intestine are also divisible into mixing contractions and propulsive contractions.  But, all contractions of the small intestine cause some mixing and some propulsion.  Mixing movements depend primarily on reflex signals generated in the myenteric plexus and elicited by distension.  Local myenteric reflexes elicited by stretch receptors start longitudinal muscle contractions.  Intense irritation of the intestine causes a peristaltic rush.  Chyme is blocked when it reaches the ileocecal valve.  When you eat the next meal, the gastroileal reflex intensifies ileal peristalsis and forces the remaining chyme through the ileocecal valve and into the cecum.  Gastrin, which helps relax the hepatopancreatic sphincter, also relaxes the ileocecal valve.  So, in the small intestine, we see both segmental and peristaltic contractions, but both ultimately have mixing and propulsive effects.

Mixing movements are characterized by repeated contractions of the circular muscle within a segment of the gut and peristaltic waves in both directions.  Propulsive movements are characterized by a circular contraction (to prevent orad flow) superimposed by an analward (distal) peristaltic contraction wave.

liver, bile, gall bladder contraction and relaxation of the hepatopancreatic sphincter

The liver continually secretes bile which is stored and concentrated in the gallbladder.  Recall that bile and some portion of the pancreatic exocrine secretion must pass through the hepatopancreatic sphincter into the duodenum.  The hepatopancreatic sphincter must relax to allow bile and exocrine pancreatic secretions to enter the duodenum.  Fat in the small intestine elicits gall bladder contractions via CCK.  CCK causes rhythmic contraction of the gallbladder and relaxation of the hepatopancreatic sphincter.  Duodenal peristalsis also causes relaxation of the hepatopancreatic sphincter in front of the wave.  Gastrin also causes the hepatopancreatic sphincter to relax.

feedback control of the ileocecal valve

Recall that the ileocecal valve separates the ileum from the colon.  The ileocecal valve enters the colon at the juncture between the cecum and the ascending colon.  The ileocecal valve is critical in preventing backflow of fecal material into the small intestine.  The valve relaxes in the presence of gastrin.  The degree of contraction of the ileocecal valve (sphincter) is also strongly reflexly controlled by the cecum.  Distension of the cecum causes reflex contraction of the ileocecal sphincter; again, this reflex is locally mediated by the myenteric plexus.  Thus, distension of the cecum strongly closes the ileocecal valve!