In much of the gastrointestinal tract, smooth muscles contract in sequence to produce a peristaltic wave which forces a ball of food called a bolus while in the esophagus and gastrointestinal tract and chyme in the stomach along the gastrointestinal tract. Peristaltic movement is initiated by circular smooth muscles contracting behind the chewed material to prevent it from moving back into the mouth, followed by a contraction of longitudinal smooth muscles which pushes the digested food forward. Catastalsis is a related intestinal muscle process.(Google)
Movement of small intestine
The small intestine is responsible for removing nutrients from food and bringing water and electrolytes in the body. It also moves food through a specific part of the digestive tract, into the large intestine
Once processed and digested by the stomach, the milky chyme is squeezed through the pyloric sphincter into the small intestine. Once past the stomach a typical peristaltic wave will only last for a few seconds, travelling at only a few centimeters per second. Its primary purpose is to mix the chyme in the intestine rather than to move it forward in the intestine. Through this process of mixing and continued digestion and absorption of nutrients, the chyme gradually works its way through the small intestine to the large intestine.
During vomiting the propulsion of food up the esophagus and out the mouth comes from contraction of the abdominal muscles; peristalsis does not reverse in the esophagus.
Types of contraction.(Guyton pg# 786)
There are two main types of contractions that occur in the small intestine as well as other parts of the gastrointestinal tract. These movements are known as mixing contractions or segmentation contractions and propulsive contractions. While these contractions are categorized separately, both perform similar functions. Mixing movements mainly churns food but also propels it. Propulsive movements primarily push food towards the anus but also spreads it out for allowing the digestive enzymes to mix with it.
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Mixing or Segmentations contraction
When a portion of small intestine becomes distended with chyme,stretching of the intestinal wall elicits localized cocentric contractions spaced at intervals along the intestine and lasting a fraction of minute.The contraction cause segmentation of the small intestine.They divide the intestine in to spaced segments that have the appearance of a chain.As one set of contraction relaxes a new set often begins,but the contraction this time occur mainly at new points between the previous contractions.Therefore the segmentation contraction chop the chyme two to three times per minute.
The segmentation contractions become exceedingly weak when the excitatory activity of the enteric nervous system is blocked by the drug atropine. (Guyton pg # 786)
While mixing contractions also propels food through the small intestine, its main function is to churn the chyme with intestinal enzymes and mechanically break down some particles.(Google)
Propulsive Movements or contraction
Chyme is propelled through the small intestine by peristaltic waves.These can occur in any part of small intestine ,and they move towards the anus at a velocity of 0.5 to 2.0 cm/sec,faster in the proximal intestine and slower in the terminal intestine.They normally are very weak and die out after traveling only 3 to 5 cm.This means that 3 to 5 hours are required for passage of chyme from the pylorus to the ileolic valve.(Guyton pg #787)
These contractions are concentric meaning that they run around the intestine as illustrated in the diagram below. These spaced segments relax and constrict a number of times thereby “chopping†the chyme and mixing it back and forth with enzymes, mucus, water and ions in the intestine. These contractions occur about 10 to 12 times per minute in the duodenum (first part) and jejunum (mid) of the small intestine and diminish to about 8 or 9 contractions per minute in the terminal ileum (end portion).(Google)
Control of Movement through the Small Intestine
A number of hormonal and nervous factors initiate and maintain peristalsis and mixing. Nerve impulses which travel throughout the small intestine are triggered by the gastroenteric reflex (refer to Defecation Reflex) and localized distention of the duodenum. These impulses are propagated via the enteric nervous system, mainly the myenteric plexus, which is the gut’s own internal network of nerves. It is further regulated by impulses from the central nervous system via the vagus nerve and sympathetic trunk..
The digestive hormones, cholecystokinin (CCK), gastrin and motilin increase intestinal motility while secretin decreases the activity. In addition, serotonin and insulin can increase motility while glucagon can decrease motility.(Google)
Ileocecal Valve
The ileocecal valve controls the entry of chyme into the colon and prevents backflow of fecal matter into the ileum of the small intestine.A principal function of the ileolic valve is to prevent backflow of fecal contents from the colon in to the small intestine. It has valve lips that closes when the pressure in the cecum builds up. In addition, the end portion of the ileum, just proximal to the ileocecal valve, has a muscular sphincter called the ileolic sphincter. This is constantly constricted and restricts the flow of chyme from ileum into the cecum unless appropriately stimulated to relax and thereby open. The ileocecal sphincter ensures that intestinal contents stay within the small intestine long enough for sufficient absorption of nutrients.(Guyton pg # 788)
Motility disorders in small intestine
Problems in the small intestine occur when food is blocked, unable to move, or contains bacteria that cannot be broken down by small intestine cells. Intestinal Dysmotility is a dysfunction that hampers peristalsis, or the contractions that move food forward. When food cannot move forward, cramps and headaches occur in the patient because the digestive process has been stopped unnaturally. Bacterial Overgrowth is a condition that leads to immediate expulsion in the small intestine. If there is an overload of bacteria, the small intestine cannot handle it, and stomach pains and vomiting occur. Cancer can strike the small intestine, although it usually is detected in other parts of the body first. Worms and parasites can occasionally affect small intestine health, although these can most often be removed by careful surgeons or specialized medicines.(Google)
Movement in large intestine
Movement of food through the large intestine is caused by smooth muscle contraction called “peristalsis” the best way to describe this is to imagine how a snake moves its abdominal muscles to move.
As opposed to the more continuous peristalsis of the small intestines, faecal contents are propelled into the large intestine by periodic mass movements. These mass movements occur one to three times per day in the large intestines and colon, and help propel the contents from the large intestine through the colon to the rectum.(Google)
Principal functions of colon are
It absorbs water from indigestible food matter and then passes useless waste material from the body. It absorbs all the calcium. Water is added to undigested food to digest it easily.It absorbs water from indigestible food matter and then passes useless waste material from the body. Actually no digestion takes place in the large intestine this organ is almost entirely for water absorption. In the colon, the major process that takes place is the reabsorption of water.(Guyton pg # 788)
Types of movements or contractions
The movements still have characteristics similar to those of small intestine can be divided into mixing movements or Haustrations and propulsive movements or Mass movements.
Mixing movement
In the same manner the segmentation movements occur in the small intestine large circular contractions occur in the large intestine,about 2.5 cm of circular muscle contracts.At the same time longitudinal muscles contracts.These combined contractions cause the unstimulated portion of the large intestine to bulge outward in to bag like sacs called haustrations.
Mass movements
Much of the propulsions in the cecum and ascending colon results from the slow but persistent haustral contractions requiring as many as 8 to 15 hours to move the chyme from the ileolic valve through the colon while the chyme itself becomes fecal in quality a semisolid slush instead of semifluid.A seriesof mass movements usually persists for 10 to 30 minutes .then they cease but reurn perhaps a half day later.When they have forced mass of feces into the rectum the desire for defecation is felt.(Guyton pg #789)
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Another very important type of motility that occurs in the large intestine is the high amplitude propagating contraction (HAPC). These contractions only occur 6-8 times per day in healthy people, but they are extremely strong contractions, which begin in the first part of the large intestine and sweep around all the way to the rectum; they stop just above the rectum.(Google)
Motility Disorders of the Large Intestine
Constipation
Constipation is usually described as infrequent bowel movements (less than 3 per week), passage of hard stools, and sometimes difficulty in passing stools. The sensations associated with constipation can include a constant feeling of needing to go, or a sensation of bloating or fullness. In children, constipation often leads to fecal incontinence.
Diarrhea
The symptoms of diarrhea are frequent, loose or watery stools, and a subjective sense of urgency. Patients with diarrhea also may worry about loss of control over bowel movements. An excessive number of high amplitude propagating contractions can be a cause of diarrhea
Hirschsprung’s disease
Hirschspurng’s disease is a rare congenital (a person is born with it) disorder that is caused by absence of nerve cells (ganglion) in the rectum and/or colon. Usually the problem involves only the bottom portion of the colon, but in some it involves the entire colon or even part of the small intestine. The part of the bowel that lacks nerve cells (aganglionic) cannot propel stool toward the anus, and therefore results in obstruction, severe constipation, or inflammation (enterocolitis). Although symptoms usually begin within a few days after birth, some people don’t develop them until childhood or even adulthood.
Irritable bowel syndrome (IBS)
The term “irritable bowel syndrome,” or IBS, is used to describe a group of symptoms that occur together. These symptoms include abdominal discomfort or pain and altered bowel habit, either constipation or diarrhea. Some people have both constipation and diarrhea, just at different times. Bloating or distention of the abdomen is also common. IBS symptoms are believed to be caused partly by abnormal motility.(Google)
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