Movements of Small Intestine | Physiology

Movements of the Small Intestine 

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Keywords: Law of the intestine, Myentric plexus, Ileocecal valve, Peristalsis, Retroperistalsis, BER.

Introduction:

The innermost layer of the wall in the gastrointestinal tract is the mucosal membrane. It is worth remembering that the gastrointestinal tract comprises four layers from outside to inside: the serous layer, muscular layer, submucosa, and mucous layer.

The mucosal membrane has three layers:

1. Epithelium,

2. Lamina propria, and

 3. Muscularis mucosa from inside to outside.

 The muscularis mucosa is present outside the lamina propria and separates it from the submucosa. It is present from the esophagus to the upper rectum. The muscularis mucosa is also known as lamina muscularis mucosa. Lamin stands for a thin layer. Muscularis mucosa comprises several thin layers of smooth muscle fibers arranged in different ways, keeping the mucosal surface and underlying glands in a constant state of gentle movements to expel glandular contents and enhance contact between epithelium and the contents of the lumen. It folds in the lumen, so the surface area increases for digestion and absorption. It goes into the villi, which protrudes into the gastric lumen. When it contracts, it pushes the absorbed fat into the lacteals and, when it contracts alternately, causes side-to-side movements to absorb the nutrients. It has no role in major movements of the small intestine.

Types of movements of the small intestine :

1. Segmental

2. Peristalsis

3. Pendular

4. Tonic

5. Antiperistasis

6. Peristaltic rush

Aims of movements

1. Digestion

2. Absorption

3. Propulsion

Segmental movement of the small intestine is due to the contraction of the circular muscle layer. This movement causes the mixing of food. When a small segment of the small intestine contracts, a small part of the small intestine distal to it dilates. Now the dilating segment contracts, and the segment distal to it dilates. In this way, the mixing of food occurs. This segmental movement is due to the small intestine's intrinsic nervous system, which produces  BER=Basic electrical rhythm.

BER  or slow waves are generated by the 'interstitial cells of Cajal' between the longitudinal and circular muscle layers. The frequency of BER is highest in the duodenum at 15/minutes and decreases slowly towards the ileum, where it is 10 /minutes. The slow wave's rhythmic fluctuations are in the membrane -potential ranging between -65 to 40 mv. The slow waves do not cause action potentials in this range, but when it becomes more than -70mv, they will produce action potential and cause contractions. The slow waves are localized to a short segment of the intestine, causing contraction to that segment and segmental movement of the intestine.

 

Peristalsis: When BER increases, it causes 2-3 minutes of peristalsis. Peristalsis is the progressive contraction of successive portions of the small intestine's circular smooth muscles. The myenteric plexus is responsible for this. According to the "law of the intestine, this wave is from upward to downward."

The law of the intestine is also known as the ‘Starling law of the intestine.’

 The ‘law of the intestine’ states that a  stimulus to the small intestine initiates contraction in the segment on the oral side of the stimulation. As a result, the segment on the aboral side dilates.

When a part of  the small intestine is stimulated, two events take place:

1. Segment of the small intestine on the oral side contracts. The ascending part of the myenteric plexus releases acetylcholine and substance P, causing contraction.

 2. At the same time, the aboral segment dilates. The descending myenteric plexus releases VIP and NO, which cause dilatation. Receptors are chemoreceptors and stretch receptors that send impulses to the myenteric plexus.

Extrinsic nervous system: Parasympathetic will stimulate, and sympathetic will inhibit the motility of the small intestine.

Hormonal effects: Gastrin, acetylcholine, substance P, Motilin, CCK-PZ, and serotonin are stimulatory.

Secretin and glucagon are inhibitory.

The gastroroileal reflex, or gastroenteric reflex, is caused by a stretch of the stomach's mucosa; impulses increase the motility of the small intestine.

The impulses generated by stretch receptors act by :

1. Myentric plexus

2. In the vasovagal reflex, the efferent  goes to the deglutition center present in the medulla oblongata, from where motor fiber returns to the small intestine, making a reflex arc

3. Impulses go to the sympathetic ganglion, and efferent fibers go to the small intestine.

Anti-peristalsis or retroperistalsis reverses the peristaltic wave in the small intestine that carries contents of the small intestine upwards in the opposite direction.

Physiologically, retroperistalsis occurs at two sites

1. In the first part of the duodenum, protect the duodenum from high acidity of chyme and

2. At the terminal ileum to assist water and electrolyte absorption.

Pathologically, it occurs before vomiting. When the emetic center in the brain is stimulated, it initiates a vomiting reflex, and retroperistalsis appears to get rid of the food. It's better to say chyme.    

Peristalsis rush or peristaltic reflex is a wave of intense contraction that affects a segment of the small intestine. This appears as a protective measure to remove the local mechanical obstruction of the small intestine. The powerful contraction causes:

1. Intestinal colic - severe cramping abdominal pain.

2. Local ischemia –due to compression of blood vessels.

3. Accumulation of toxins due to ischemia

And pathogens-hypotension and toxemia occur –a severe condition that may be fatal if it persists for a long time.

The Ileocecal valve and ileocecal sphincter are present at the junction of the ileum and caecum. The Ileocecal valve is one-way and prevents backward flow from the caecum.

Pendular movements: in this movement, intestinal contents move to and fro in a segment of about 15-20cm and contribute to mixing the food and digestive juices.

Tonic movements: A prolonged contraction of one segment of the small intestine isolates it from another part of the small intestine. This movement helps in absorption.

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