The stomach and the small intestine both play an important role in the digestive system of humans. The stomach, which is a j-shaped muscular organ, plays an important role in food storage and in mixing food and gastric juices together. While the small intestine is the receiver of the food digested by the stomach and where food is further broken down through chemical processes and food nutrients are absorbed by the intestinal walls and transported through the blood vessels in a process called diffusion.
The gastrointestinal system has a control method to regulate conditions at the lumen of the tract. Below are some factors which initiate the reflexes:
Distension of wall by volume of luminal contents
Chyme concentration of specific products
All throughout the process of digestion, the movement of the muscles in the stomach and small intestines plays an important part in propelling the food being processed by the digestive tracts and in mixing the ingested food with gastrointestinal chemicals that will further aid in the digestion process. This movement in the muscles is what is known as gastro-intestinal motility.
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In general, motility means the ability of organisms to move and digest food through the digestive tract or what is known as peristaltics which describes both the movement of the stomach or gut motility, or the movement in the intestinal region or intestinal motility.
What is the similarity and differences between stomach motility and intestinal motility? Stomach motility is similar to
the motility in the small intestines because both serve the purpose of propelling digested food towards its next step of digestion. The difference is in the stomach the food propelled through contractions in the stomach walls are moving towards the small intestine to be further digested, while in the small intestines the food is fully digested and it is the waste product of the digestive process - the parts of the meal that cannot be fully broken down by the digestive enzymes - which is propelled through intestinal motility towards the large intestine and is then transformed into waste material or fecal matter.
Both stomach motility and intestinal motility serves the purpose of mixing digested food with intestinal enzymes and digestive juices to aid in liquefying solid food into liquids that can be absorbed by the intestinal walls and transported via blood vessels. The difference is in the enzymes that are mixed in with the food and the end result of the process.
In the stomach, motility will aid in liquifying ingested food by crushing, grounding, and mixing it with gastric juices till the stomach comes up with a liquified form called chyme. In the small intestine, motility helps in mixing digestive enzymes produced from the pancreas and bile with the chyme produced by the stomach and which was moved to the small intestine through motility. Motility in the small intestines also helps not just in the breakdown of foodstuff but more importantly in transforming food into a liquid that could be
dispersed and absorbed by the epithelium of the intestinal walls and thus facilitate enzymatic digestion and absorption of the nutrient molecules found in food.
The motility of the stomach and the intestine also has two distinct movements corresponding to the differences in their function in the digestive process. In the stomach muscular movement can be seen in two aspects; first in the upper stomach the muscular movement are low frequency sustained contractions aimed at relaxing the muscular walls of the stomach in order to function as the storage are of food prior to liquifying it into chyme. The motility in the lower part of the stomach is manifested in strong peristaltic waves of contraction that increases in amplitude when nearing the pylorus or the end of the stomach that leads to the small intestine. This rapid movement corresponds to the role of the lower part of the stomach which grinds food and liquifies it as chyme before transporting it towards the small intestine. Upon the arrival of food, the stomach releases peristaltic waves. Small amounts of chyme are released through the pyloric sphincter intro the duodenum. The pacemakers cells in the longitudinal smooth muscle layer generates these waves.
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The gastrin hormone is released by G-cells in the stomach and results to the succeeding secretion of pepsinogen, HCl and intrinsic factor from the stomach's parietal cells. It also leads to an increase in the stomach's motility. The Gastric inhibitory peptide (GIP) meanwhile, lessens the gastric acid and motility. The hormone enteroglucagon
also functions to decrease the stomach movements while the small intestine is still digesting food. Secretin meanwhile is produced in the small intestine and results to effects in the pancreas but also decreases the stomach's acid secretion.
If in the stomach muscle movement or motility serves to relax the muscles of the upper stomach so as to play its part as a reservoir of food while at the same time lower part is muscles move in an agitated manner so as to function as a grinder of food; in the small intestine motility also performs a similar function. This function or characteristic of the movement associated with motility is called as patterns of motility.
Both the stomach and intestine move in such a way as to induce propulsion of food. This is achieved when a ring of muscles in the digestive tract contracts at the end near the oral cavity and then moves toward the anal tract. This propels the contents of the digestive tract towards the direction of the anus, but as the ring of muscle moves, the muscle on the other side relaxes and facilitates a smooth passage of the digested material.
Both the stomach and intestine also moves to mix food by moving muscles in what is called as segmentation. In segmentation rings of muscle form segments which contracts and relaxes which in turn chop and mix ingested material. This is clear shown by the movement in the muscles of the small intestine, while in the stomach
this kind of movement can be seen in the alternating contraction and relaxation of the longitudinal muscles of the stomach which was already discussed above.
As with the other parts of the digestive system both stomach and intestinal motility is controlled by the excitatory and inhibitory signals that emanate from the enteric nervous system. The enteric nervous system is made up of two networks of neurons which are embedded in the walls of the digestive tract and extends from the oesophagus to the anus. However the part that plays an important part in digestive tract motility is the myenteric plexus.
Differences in the secretion are also evident in the stomach and small intestine. Secretions in the stomach include mucus which covers the entire luminal surface down to the glands referred to as "mucous neck cells". A bicarbonate-rich mucus is secreted which coats and lubricates the gastric surface. Hydrochloric acid is also secreted by the stomach's parietal cells into the lumen. This acid plays a major role in neutralizing microorganisms like bacteria. Pepsinogen is also released by the stomach into gastric juice by mucous and chief cells. The last of the four major products of the gastric epithelium is the hormones. Gastrin, as previously discussed, is important in controlling acid secretion and gastric motility.
In the case of the small intestine, quantities of water are secreted to the lumen of the small intestine during digestion. Two different processes creates an osmotic gradient that pulls water into the lumen of the intestine,
the increase in luminal osmotic pressure as a result from influx and digestion, and the crypt cells actively secreting electrolytes which causes water secretion.
In spite of the aforementioned differences, motility and secretion in the stomach and small intestine works in sync with the rest of the system to digest food and nutrients needed by the human body.