The following types of movements are found in the human small intestine: 1. Rhythmic segmentation or Ludwig’s pendulum 2. Peristalsis.
These are rings of contraction occurring at regular space of intervals in which a portion of the intestine is divided into segments. The contraction is followed by relaxation. The contraction takes place at the site of maximum distention. It can be studied under X-ray after barium meal. The opaque column of barium meal becomes broken into several small segments.
At the next moment each of these segments is subdivided by a fresh batch of contractions, the previous group having disappeared in the meantime. The halves of the adjacent segments so divided run together and form new segments. These are again subdivided and thus the process goes on (Fig. 9.50).
According to Friedman, there are two types of segmenting contractions in the duodenum:
(1) One type consisted of a contraction localised in a segment less than 2 cm and was eccentric in appearance.
(2) The other was concentric and consisted of a local contraction involving a segment usually longer than 2 cm and uniform circumferentially.
In animals the groups of contractions succeed at the rate of 20 – 30 per minute. In man the rate is slower. The frequency is inversely proportional to the distance from the stomach. A cyclic changes in the electrical potential occurred in the duodenum known as basic electrical rhythm (B.E.R.) originates near the entrance of the bile duct and moves down the duodenum.
In the duodenum it is about 17 per minute, in the ileum it is about 12 per minute. In addition, irregular bursts of spike potential superimposed on B.E.R. appear in an electrical record. So the contraction is segmental and not peristaltic since spike potential and contractions do not precede more than a few centimeters. The duration of electrical cycle is about 3.5 sec. and hence the rhythm is 17 -18 per minute.
The frequency variation at different region is due to gradient in the physiological properties, viz., rhythmicity, irritability, variation in latent period and drug susceptibility. Muscular contraction occurs at intervals of some multiple of 3.4 seconds and do not travels vary far along the intestine. The vagi (Fig. 9.51) and the splanchnic nerves (Fig. 9.52) regulate the activity of the intestine, and adrenal medulla in psychic conditions, but action of these nerves is reversed in the control of ileocaecal sphincter.
These are the most fundamental movements of the intestine and are due to outstanding property of smooth muscle that is rhythmicity. Circular muscle is responsible for the most visible movement. They are myogenic in nature and are intendant of all nerves.
Segmentation movement does not cause forward passage of food materials. It helps- (1) in digestion due to proper mixing of food with enzymes of digestive juices, (2) in absorption due to- (a) constantly changing the layer of fluid in contact with mucosa, (b) change in pressure, (3) in the improvement of intestinal circulation.
Type # 2. Peristalsis:
Peristalsis is described to be a composite wave, consisting of a wave of relaxation followed by a wave of constriction. It is a translatory movement and travels down the gut in an aboral direction (away from the mouth). Bayliss and Starling have demonstrated that a stimulus applied to a given point on the intestinal wall causes contraction above and relaxation below the stimulated point (Fig. 9.53 & 9.54). This is a local reflex of smooth muscles and their intrinsic plexuses. This is called The Law of Intestine or Myenteric Reflex. It is suggested that peristalsis depends on this reflex.
Usually two types of contraction, viz., peristaltic and rhythmic segmenting are present simultaneously, the former is superimposed on the latter and responsible for rise into the tone level of intestinal muscle without any interruption in the rhythm of the segmenting contraction. Peristaltic wave travels for varying distances—some few cms and other a few metres depending on the intensity of stimulus.
Segmental contractions sometimes recur frequently maintaining its character and travel aborally as peristaltic movements. A peristaltic wave induced by strong stimulus may sweep over the entire length of small intestine what is called rush wave or peristaltic rush.
The peristaltic waves move aborally and not orally and are due to the gradient of rhythmicity, conductivity and irritability. The impulse arises in the most irritable point and travels in the less irritable, i.e., the aboral side and not in the oral side due to the long refractoriness.
Type of Peristalsis:
Three types of peristaltic movements are present in the small gut:
i. First Type:
It is a slow gentle wave moving at the rate of 1 – 2 cm per second which dies out easily after travelling a short distance.
ii. Second Type:
It is a very swift wave travelling the entire length of small gut at the rate of 2 – 25 cm (average about 10 cm) per second. According to Alvarez the latter is true peristalsis. Due to its rapid speed it is also known as rush peristalsis.
iii. Third Type (Antiperistalsis):
In every respect it is same as peristalsis excepting that its direction is opposite. It moves in the oral direction. It is present in the second and third parts of duodenum only in man. Weak antiperistalsis also occurs in the terminal part of the ileum, thus preventing a rapid passage of the ileal contents into caecum.
In the duodenum it helps through admixture, as well as causes duodenal regurgitation into the stomach. These antiperistaltic movements occur due to presence of sensitive receptor area in this region which responds to the qualities of chyme and concerned with delaying the passage of chyme into lower portion of the gut facilitating more scope of digestion and absorption.
Peristalsis depends on both nervous and chemical factors. The vagi and sympathetic have got influence on peristaltic movements. Stimulation of vagus increases and that of sympathetic inhibits peristalsis. Vagotomy on the other hand decreases the peristaltic activity only to a minor extent. The local nerve plexus (Auerbach’s plexus) helps in the co-ordination of peristaltic movements.
Distention of the intestine, normally caused by presence of food, causes peristaltic movements due to a stretch reflex called myenteric reflex. Reflex inhibition of whole of the small intestine may take place due to stretching of lower part of the small intestine (such as intestine and intestinal reflex) or stretching of gall-bladder and urinary bladder, etc.
These inhibitions may be removed by stimulation of splanchnic nerves (sympathetic). Presences of local nerve plexus (myenteric plexus) are required for this and the afferent receptors of which are present in the mucous membrane of the intestine. Liberation of 5-hydroxytryptamine (serotonin) from the enterochromaffin cells is a possible mediator in this reflex action. Role of a basic polypeptide, substance P as a mediator has also been suggested.
Role of Endocrines:
Hormones also exert great influence. Pituitrin excites the movements, as also thyroxine. Adrenaline inhibits the movements.
This is a special manifestation of peristaltic movements in the ileum. Peristalsis is generally very sluggish in the last part of the ileum. But after a meal, brisk peristalsis is set up reflexly in this region. This is called gastro-ileal reflex. The purpose is to drive out the ileal contents into caceum and thus making room for fresh supply.
i. Chief function is the propagation of the food onwards.
ii. Other functions are same as of segmentation movement.
The metabolic gradient theory of Alvarez is important in this connection. He observes that excitability frequency of movement, strength of contraction, tone of the intestine gradually diminish from above downwards along the intestinal canal. Even the normal direction of peristalsis is from above downwards.
The latent period of the intestinal muscles gradually becomes longer in the lower parts of the small intestine. This peculiarity, according to Alvarez, is due to the difference in the degree of metabolic activity between the upper and lower parts of intestine. The metabolic rate is much higher in the upper part than in the lower part and it is this gradient upon which this difference depends.
In certain pathological conditions of intestine, such as, inflammation, obstruction, etc., the metabolic rate of the diseased part may be higher than those above it. So that under such conditions, antiperistalsis will start at the site of lesion and proceed towards the stomach. This theory explains the phenomenon of faecal vomiting during intestinal obstruction.
Movements of Duodenum:
It is seen that in the intestine, duodenum is the place, where many peculiarities of movements are seen. It possesses peristalsis and segmentation movements all along. The second and third parts possess the additional movement of antiperistalsis. The first part, i.e., from the end pyloric sphincter to the opening of bile duct, also known as duodenal cap, possess special receptive area which responds to the qualities of chyme. It has got its own rhythmicity different from its adjacent parts.