Gastrointestinal Physiology

By Jon Trister, MD

Gastrointestinal Physiology.
It is important to understand GI function.

Neurological consideration.
Glossopharyngeal nerve (Cranial nerve IX): mixed, sensory and motor nerve.
Sensory function:Oropharynx,Eustachian tube,Middle ear,Posterior third of the tongue,Carotid sinus, Carotid body, Taste to posterior third of the tongue, Supplies parasympathetic fibers to the parotid gland via otic ganglion,contribute to the pharyngeal plexus.
Motor function: Stylopharyngeus muscle.

Vagus nerve (Cranial nerve X):
a)Parasympathetic control of the heart, lungs, gastrointestinal tract.
b)Sympathetic control of the peripheral chemoreceptors.
Nuclei:
Dorsal nucleus of the vagus nerve:Parasympathetic output to the intestines;
Nucleus ambiguus:Parasympathetic output to the heart;
Solitary nucleus:Receives afferent taste information and afferent info from visceral organs.
Spinal trigeminal nucleus:afferent impulses arising from the outer ear, the dura of the posterior cranial fossa and mucosa of the larynx.

Accessory nerve ( Cranial nerve XI):Originates in the C1-C2 segments): Innervates sternocleidomastoid and trapezius muscles.
It enters the scull via foramen magnum, travels along the inner wall toward jugular foramen and leave the scull again fia Jugular foramen together with IX and X nerves.

Cranial nerves IX-X-XI provide neurological regulation of the gastrointestinal function.
They pass through jugular foramen which has very complex anatomy.JUgular foramen is formed by the anterolateral part of the temporal bone and posteromedial part of the occipital bone.
Its subdivided into larger posterolateral compartment ( pars venosa) containing the jugular bulb and tenth and eleventh cranial nerves and a smaller anteromedial compartment ( pars nervosa) containing the ninth cranial nerve.These two parts are usually separated by a fibrous bridge connecting the jugular spine of the petrous temporal bone to the jugular process of the occipital bone.This bridge could be osseos or fibrous.
Size of the right jugular foramen is usually larger than the left.
Dura over the jugular foramen had a two perforations forming a glossopharyngeal meatus through ninth nerve passed to enter the pars nervosa and vagal meatus through which vagus and accessory nerves entered the anteromedial part of the pars venosa and jugular bulb.The glossopharyngeal and vagal meati consistently separated by a dural septum.(Albert Rhoton etc, J.Neurosurgery/Volume 42/May 1975).

This complex anatomical entity is a subject to somatic dysfunction as the result of trauma, congenital abnormalities, tumors,dural strains, inter- and intraosseous strains of the temporal and occipital bones.
Those pathologies may alter anatomical relationship of the structures occupying the jugular foramen and subsequently to their physiological dysfunction.

Digestion begins with imaginary stimulation: thinking , seeing and smelling food send signals to the brain via afferent nerves. In the brain processing of information is occurs and efferent pathway-parasympathetic- glossopharyngeal nerve is activated to stimulate salivary glands to produce saliva rich in digestive enzymes-amylase ( initial breakdown of carbohydrates) and lipase (initial breakdown of triglycerides).
Glossopharyngeal nerve ( IX)
[originates in upper medulla, passes through the Jugular foramina]: Stimulate salivary glands.
Vagus (X) [Originate in medulla oblongata and passes through the Jugular foramina] Control digestive tract from the esophagus to the mid-colon.
Accessory nerve (XI) [Originate at the junction of medulla oblongata and spinal cord and passes via jugular foramen with IX and X nerves]
1.Mastication stimulates production of the saliva by oral salivary glands (IX).Mastication stimulates gastric secretion of Pepsin and water (X). 
 
Stomach:Distention of the esophagus stimulates the oral salivary glands.Distention of the gastric fundus stimulates secretion of the Pepsin,HCL, Intrinsic Factor and water.
Distention of the antrum stimulates Gastrin secretion. In addition, the cells lining the stomach secrete a thick mucus coating that protects the stomach from acid and pepsin.
Distention of the duodenum stimulates secretion of the Pepsin, HCL, Intrinsic Factor and water. Pepsin is active in acidic environment. Pepsin is very specific in digestive capability: it break down only [protein linkages containing certain amino acids : tryptophan , tyrosine, Methionine and leucine.
The presence of food stimulates the vigorous contraction of the muscles of the stomach wall, mixing of the food particles with digestive juices and push semi digested food into duodenum.
Antral drainage inhibit secretion of the  Pepsin, HCL, Intrinsic Factor and water

Gastrin amplifies antral production of the HCL, Intrinsic Factor, Pepsinogen and water
Gastrin amplifies duodenal secretion of Cholecystokinin
Gastrin is produced by G cells of antrum, in the duodenum, small intestine and pancreas.
HCL inhibit production of Gastrin.HCL stimulates secretion of Secretin by duodenum
 
Duodenum very complex process and regulated by nervous system and hormones:
Gastrin stimulate the formation of the Cholecystokinin.Lipids in the jejunum stimulate secretion of the Cholecystokinin.Carbohydrates and HCL in duodenal bulb stimulate secretion of the Cholecystokinin.Cholecystokinin induce gallbladder contraction and opening of the hepato-pancreatic ampulla.
Cholecystokinin stimulates production of the Insulin, HCO3, Glucagon in the pancreas
Cholecystokinin inhibit antral secretions and bulbar acidity.Secretin reduce HCL production and stimulate HCO3 production by pancreas which protect duodenal bulb from acid.Secretin stimulate Insulin release and inhibit Glucagon release.Duodenum also secretes mucus for protection from destructive effect of HCL.
Hypertonic Glucose solutions, Acidity in the bulb, Lipids in the jejunum limit the formation of the HCL and diminish gastric motility.If Lipids, proteins and carbohydrates evacuated at the same time disturbing of digestive function might occurs.
Enterokinase in duodenum convert into Trypsin and inhibit Action of the biliary salts.Biliary salts (weak concentration) stimulate Lipase secretion and inhibit (strong concentration) Lipase secretion. 
Gastrin Stimulates: HCL and Water secretion by stomach; secretion of Pepsin, Secretion of Intrinsic Factor, Secretion of Cholecystokinin in the duodenum.
HCL: Inhibits secretion of Gastrin; Stimulates secretion of Secretin; Stimulates production of pepsin from pepsinogen.Anti-bacterial function; Converts Ferric iron to Ferrous iron(absorbable)
Cholecystokinin: Stimulates action of biliary salts, Contraction of Gallbladder, dilatation of choledocho-pancreatic ampula,inhibit gastrin and HCL production; stimulates HCO3, insulin and glucagon release
Secretin: Inhibit HCL, Glucagon, stimulate Insulin release ,stimulates HCO3 production by Pancreas

Pancreas:
Endocrine function:
Alpha cells produce and secrete Glucagon, which pancreas release in response to low blood sugar in the serum.Glucagon stimulates breakdown of glycogen ( liver and muscles) to glucose , which is released into bloodstream to serve as a source of cellular energy.
Beta cells secrete insulin in response to high glucose level in the serum.Insulin ( among other functions) drive glucose into muscles and liver to store in the form of glycogen.
Delta cells produce and secrete somatostatin , a complex hormone that is also produced by intestinal lining cells as well as certain neurons in the brain.In the pancreas somatostatin can inhibit release of insulin and glucagon (!?!) to help maintain blood sugar level in a very narrow range.

Exocrine function:
The cells of exocrine pancreas arranged in acini- special clusters along the network of channels forming pancreatic duct-to secrete pancreatic enzymes.
Proteolytic enzymes:Trypsin, Chymotrypsin, Carboxypeptidase A, Carboxypeptidase B,Elastase, Amynopetodases, Dipeptidases, Tripeptidases,Pepsin.

Each of these enzymes has precise and specific function.
Trypsin will break down a protein only at amino acid linkages containing either arginine or lysine.
Chymotripsyn will break down a protein at amino acid linkages containing tryptophan,phenylalanine, tyrosine.
DIpeptidase will break down a protein consisting two amino acids
Tripeptidase will break down a protein consisting three amino acids
All of these enzymes are produced in in-active forms to prevent self-destruction:
Trypsinogen, Chymotripsynogen,Procarboxypeptidase,Prodipeptidases,Protripeptidases
The acing cells also secrete multiple lipases and amylases.
The acinar cells store the inactive precursors in little sacs, or vacuoles, in the cytoplasm, until they are needed for digestion.As ling as these pre-enzymes remain in their inactive form, they pose no threat to the pancreas themselfs.
The body has a remarkable mechanism for signaling the acinar cells of the pancreas to begin manufacturing and secreting stored precursor enzymes.
First, the presence of the food in the mouth and in the stomach stimulate vagal nerve to release its neurotransmitter-Acetylcholine which stimulate release of pre-enzymes into the common space of cul-de-sac.
The presence of food in duodenum stimulates lining cells of duodenum to secrete the hormone Cholecystokinin into bloodstream. This hormone like vagus nerve stimulates the acinar cells to produce and release stored enzymes into duct of Wirsung and than empty pre-enzymes into the duodenum via Vater papilla.
The cells lining the small intestine produce proteolytic enzyme calle enteropeptidase, whose function is specifically to cleave off the six-amino-acid tail of trypsinogen, leaving the active and very powerful trypsin.
Trypsin activates other proteolytic enzymes making them active, to digest protein-cascade reactions.
This semiliquid food boluses that make their way from the stomach to intestine are extremely acidic, from stomach HCL.But Trypsin and Chemotrypsin can perform their digestive function only in an alkaline environment.To solve this problem, lining cells of small intestine sense the load of incoming food, they release another hormone, Secretin into the bloodstream.Secretin stimulate pancreas to produce NaHCO3, powerful antacid quickly neutralizes acid produced earlier by stomach.Now, the activated Enzymes have an ideal environment in which to begin digestive work.