Regulation of the Gastrointestinal System

 

The regulation of gastrointestinal system is needed to achieve main function of the gastrointestinal system. To ensure efficient assimilation of food the main functions has to be regulated. The main physiological functions of the gastrointestinal system are motility, secretion, absorption and digestion. There are two types of regulation. They are,

1.    Endocrine regulation

2.    Neural regulation

 

Endocrine regulation is mediated by the release of hormones.

Neural regulation – central nervous system and gut connections (intrinsic innervation), enteric nervous system that comprises both secretory and secreto-motor neurons.

In endocrine regulation, all the hormones are polypeptides. These are secreted by mucosa of nerve cells and gland cells in gastrointestinal tract. Some hormones are secreted by nerves.

Enteroendocrine cells are present in the mucosa of stomach, small intestine, colon. Some of the substances which act as hormones are secreted by ECL cells. Example, histamine. Gut hormones are, Gastrin, Secretin, Cholecystokinin, Somatostatin.

 

·       Gastrin

Gastrin is secreted by G cells in gastric mucosa -antrum part of stomach. Gastrin is secreted in respond to Vagal stimulation, Distension in the lumen of stomach, Amino acids, peptides in the lumen other blood born factors. When Gastrin secretion is increased, increases gastric acid, pepsin secretion, Trophic action (growth of mucosa of stomach, small intestine and large intestine), Stimulation of gastric motility, Increases insulin secretion after a protein meal.

•        CCK- (Cholecystokinin)

Cells mucosa of upper Small intestine

nerves in the distal ileum and colon. It is also found in neurons in the brain,

Secretion stimulated by

-        contact of intestinal mucosa with products of digestion

(Peptides, Amino acids, Fatty acids)

Functions of CCK

1. Contraction of gall bladder

2. Relaxes the sphincter of Oddi

3. Increases secretion of pancreatic juice rich in enzymes

4.Secretion of bile into the intestinal lumen

5.contraction of the pyloric sphincter-inhibits gastric emptying, along with secretin, prevents the reflux of duodenal contents into the stomach

6.  enhance the motility of the small intestine and colon

 

Secretin

S cells located in glands of upper S.I.

Secretion is stimulated by

-        Digested protein products

-        Acid in contact with mucosa of upper Small intestine.

 

•        Functions of secretin

1.    Increases secretion of HCO3 – by duct cells of pancreas of biliary tract,

(Increases secretion of alkaline, watery pancreatic juice)

3. Decreases gastric acid, secretion

4. Inhibits gastric motility, contracts the pylorus & slows gastric emptying

5. Relaxes the LOS

 

          Somatostatin

•        is secreted as a paracrine by D cells in the pancreatic islets

•        & by D cells in the gastrointestinal mucosa.

Functions of somatostatin

1.    Inhibits gastric acid secretion

2.    Inhibits gastric motility

3.    inhibits exocrine pancreatic secretion

4.       Inhibits gallbladder contraction

5.        Inhibits the absorption of glucose, amino acids, and triglycerides

 

Gastrin, CCK, secretin, and glucagon also stimulates insulin secretion.

 

Hence gastrin, CCK, secretin, and GIP not only facilitates digestion it also makes sure the absorbed nutrients are utilized.

 

Neural regulation

Extrinsic innervation   -      Autonomic N.S.

1.Parasympathetic cholinergic N.

-Vagus N

-2. Sympathetic noradrenergic- sphincters contract decreases smooth M activity

 

Enteric Nervous system

•        This is a network of nerve fibers present in the wall of the gastrointestinal tract

Reflexes enteric nervous system

consists  of 2 networks of nerve fibres

 - Intrinsic to GIT

 

 1.Myenteric plexus

  (Auerbach’s plexus)-Between longitudinal and circular muscles of GIT

-        Motor control

 

Submucous plexus

   Meissner’s plexus in the submucosa

  -  Innervates the glandular epithelium, intestinal endocrine cells, sub mucosal blood vessels.

   Control of intestinal secretions

 

•        Connected to CNS by parasympathetic and sympathetic nerve fibres

•        Can function independently.

 

vagal efferent generally end on cholinergic nerve cells of the

myenteric and submucous plexuses.