Secreted by: G-cells

Secretion stimulated by: amino acids in the stomach lumen, distension of the stomach by food and by vagus nerve activity

Secretion mediated by: (negative feedback control) acid in the gastric antrum, the paracrine action of somataostatin and via several hormones released by the small intestine (enterogastrones)

Function: Increases secretory activity of the stomach;

Trophic (growth promoting effect) on stomach mucosa

Secreted by: I-cells

Secretion stimulated by: the products of fat and protein digestion being present in the small intestinal lumen

Function: Promotes digestive activity in the small intestine and regulates the delivery of chyme from the stomach;

Increased pancreatec secretion;

Contraction of the gall bladder;

Poromtion of intestinal motility;

Inhibition of gastric emptying;

Trophic effect on the pancreas

Secreted by: S-cells

Secretion stimulated by: the presence of acid and amino acids in the duodenum

Function: Increases bicarbonate secretion by the pancreas and biliary system, therby neturalizaing acid delivery to the duodenum from the stomach (this will allow digestive enzymes in the small intestine to operate at their pH optima);

Inhibits acid production by the stomach

Secreted by: X-cells in the body of the stomach

Secretion stimulated by: hypoglycemia;

low body weight;

secretion is highest during the interdigestive period

Function: Potent stimulator of food intake;

Stimulates growth hormone release

Secreted by: M-cells

Secretion stimulated by: an ill-defined clock, which exists within the enteric nervous system that determines the interval between meals and prompts motilin secretion when appropriate

Function: Stimulates a pattern of contraction seen in the distal stomach and small intestine between meals (migrating motor complex (MMC));

Acts on other neurons within the ENS to begin the MMC contractions

Secreted by: K-cells

Secretion stimulated by: Glucose and fat being present in the small intestine

Function: In large (pharmacologic) doses: Causes inhibition of motor and secretory activity in the stomach;

In small (physiological) doses: Promotes the secretion of insulin

Secreted by: L-cells; made by the cleavage of pro-glucagon

Secretion stimulated by:  the presence of glucose in the small intestinal lumen

Function: Promotes insulin secretion; Inhibits glucagon secretion by pancreatic islet cells

Chemical modification of some GI peptides in the amino acid side chains (e.g. amidation, sulphation, etc.0

EXAMPLE: DESULPHATION OF A SINGLE AMINO ACID IN CCK-33 PRODUCES A PEPTIDE WITH THE GASTRIN PATTERN OF HORMONAL ACTIVITY

Secreted by: Enterochromaffin cells (present in large quantities in the intestinal mucosa within the EC cells)

Secretion stimulated by:  the response to mechanical stimulation (e.g. distension) of the gut wall

Function: Increased intestinal motility and secretion; Exerts  its effects largely through interactions with ENS

Secreted by: D-cells

Secretion stimulated by:  the presence of glucose in the small intestinal lumen

Function: May be released both into the blood to act as a hormone, but is also a paracrine mediator; Potent inhibitor: inhibits pancreatic secretion, gastric secretion and motility, gallbladder contraction and nutrient absorption; Inhibits gastrin, secretin, GIP and motilin; Vasoconstrictor

Secreted by: mast cells and enterochromaffin-like (ECL) cells in the stomach mucosa

Secretion stimulated by:  the presence of glucose in the small intestinal lumen

Function: Has important stimulatory effects on gastric acid secretion in the stomach; Local edema and vasodilation in the event of local infection or injury from mast m mast cell degranulation

-Requires the confomational alteration of myosin head (unlike striated muscle, which has movement of the troponin-tropomysin complex on the actin filament)

-SR is arranged via caveoli (analogous to T tubules)

-Can contract for long periods at lowlevels of energy consumption and low myosin cross bridge recycling rates

-Controlled by nerves, circulating hormones, by stretch of the muscle and other tissue factors

CONTRACTION:

-Ca2+ is the key to contraction

1) Ca2+ binds calmodulin forming the C-C complex

2) The C-C complex activates the myosin light chain kinase (MLCK)

3) MLCK phosphorylates myosin

4) Myosin then interacts with actin causing contraction

RELAXATION:

1) Muscle relaxation occurs as Ca2+ falls due to pumping of Ca2+ out of the cytoplasm via ATPases in the cell membrane and SR

2) C-C complex dissociates and MLCK begins to inactivate

3) MLC phsophatase will predominate

4) Myosin will dephosphorylate causing the muscle to relax

-Block/lack of ENaC channel

-Causes excess reabsorption of Na+ and loss of K+

-Loss of an apical Cl- membrane

-Characterized by failure of many secretory epithelia, but most notably the airway and pancreatic duct

-Patients suffer from blockage of small airways and pancreatic ducts with thick mucus that is not being lubricated by fluid and electrolyte secretions

-Repeated airway infection leading to destruction of the lungs is the eventual cause of dealth in most cases

-Primary developmental disorder of the Enteric Nervous System, in which there are no ganglia in the submucosal or myenteric plexuses in part of the large bowel

-The extent of aganlionosis is variable, but rectosigmoid aganglionosis accounts for 75% of all cases

-Increased tone and no propulsion within the aganglionosis segment resulting in massive distension of the colon proximal to the segement and frequent attacks of intestinal obstruction

1) Acetylcholine

2) Substance P

3) Vasoactive Intestinal Polypeptide (only in secretory enterocytes)

1) Nitric Oxide

2) ATP

3) Vasoactive Intestinal Peptide (only in smooth muscle)

-Involve changes in the sensory and motor physiology of the GI tract

-Example: Irritable bowel syndrome, characterized by a hypersensitive gut wall where mild/moderate distension is sensed as pain

-Targets the salivary glands, especially the parotid glands

-Peak incidence is between 5-9 years of age

-In post-adolescent males, it can involve the testes, causing sterility

-In severe cases, the pancreas can be attacked producing diabetes, and the CNS (only rarely is it involved)

-Mass vaccination has reduced the incidence of mumps in recent years

Secreted by: Salivary acinar cells

Function: Breaks down starch (up to 75% in a meal before being denatured by gastric acid); can remain active int he stomach because a high proportion of a meal remains unmixed for many minutes in the stomach

Secreted by: Small salivary glands present on the surface of the tongue

Function: Helps with the digestion of fats and is not denatured in the stomach by gastric acid

-May be a congenital lack of salivary glands

-Causes significant difficulty with swallowing and also tooth disease due to bacterial overgrowth

1) Upper Zone: Closely related to the pharyngeal musculature and conssits of striated muscle

2) Middle Zone: (Main body) consists of smooth muscle

3) Lower Zone: Consists of lower esophageal sphincter

-LES function is diminished and reflux occurs chronically, a patient has Gastro-Esophageal Reflux Disease

-Often causes the symptom of pyrosis (heartburn) and is associated with mucosal damage

-"Absence of relaxation"

-Disorder of ENS , affecting esophageal smooth muscle

-Loss of peristalsis and a failure of the LES to relax properly

-Patinets have great difficulty swallowing, frequently aspriate their food and suffer malnourishment

Secreted by: oxyntic (parietal cells) in the Gastric mucosa of the stomach

Function: Aids in the digestion of proteins via denaturation; Cleaves pepsinogens

Secreted by: peptic (chief cells) in the gastric mucosa of the stomach.

Function: Cleaved to produce active proteolytic pepsins

Secreted by: Exocrine glands in the gastric mucosa of the stomach

Function: Lubricates ingested solids, aiding gastric motility functions; Forms part of the gastric mucosal barrier against acid and pepsin attack

Secreted by: Exocrine glands in the gastric mucosa of the stomach

Function: Secreted into the surface mucus and forms part of the gsatric mucosal barrier

Secreted by: Exocrine glands in the gastric mucosa of the stomach

Function: Forms a complex with Vitamin B12 in the small intestine.  It is necessary for the absorption of Vitamin B12 in the ileum

Secreted by: Exocrine glands in the fundus that has an acidic pH optimum

Function: Contributes a small amount to triglyceride hydrolysis

-Spends most of the time in a quiescent state, which is interrupted at ~90 minute intervals by a powerful peristaltic wave, the migrating motor complex (MMC)

-Large indigestible components of food that remain in the stomach after a meal are flushed out into the small intestine

1) Ingestion of a meal requires that the proximal stomach relaxes

2) RECEPTIVE RELAXATION and ACCOMMODATION occur to allow food to enter the stomach and to store it without causing a rise in intragastric pressure - both phenomena are mediated largely by vago-vagal reflexes

3) Once food is ingested, the proximal stomach exhibits slow sustained contractions that gradually press food into the distal stomach

4) Tonic contraction of the proximal stomach determines intra-gastric pressure (the main determinant of the gastric emptying of liquids)

-Contractions of the stomach serve to grind the food (trituration) and to mix it with gastric juice.

-These powerful contractile waves seen at this time give rise to this term of "antral systole."

-When the stem cell divides, one of the daughter cells remains to become the next stem cell, the other goes on to divide many times, with cells migrating both upwards and downwards to differentiate into the different cell types present

-Allows for gastric epithelium to rapidly regenerate following injuries that are restricted to the epithelial cell layer

-The composition changes with secretion rate

-At all rates, it is approximately isotonic with plasma

-At low flow rates, it resembles interstitial fluid

-At high flow rates, it becomes a solution primarily of HCl

1) Active transporter H+/K+ ATPase is used to pump H+ out of the cell up an immense concentration gradient

2) K+ secretion via a luminal membrane K+ channel provides the K+ needed by the H+/K+ ATPase to pump H+ out of the cell

3) OH inside the cell produced along with H+ is neutralized by combining it with CO2 to form bicarbonate

4) Most of the CO2 comes from oxyntic cell metabolism

5) HCO3 produced leaves the cell in such quantity that the gastric venous plasma becomes alkaline

OVERALL RESULT: normally encountered after eating a meal, when stomach acid is released into the stomach causing a temporary increase in pH of the blood.

-Oyntic (parietal) cell has receptors for gastrin, acetylcholine and histamine

-Gastrin and Ach stimulate secretion via an increase in intracellular Ca2+

-Histamine causes an increase in cAMP

-Prostaglandin E2 reduces cAMP production

-Mediated by the vagus nerve

-Even the thought of a meal is sufficient to increase acid production

-2 ways for acid production stimulation:

1) Direct release of Ach by nerve terminals on oyntic cells

2) Release of gastrin

-Acid secretion accounts for at least 50% of the response to a meal

-Entry of food into stomach neutralizes acidity and can cause pH to rise as high as 6, which allows gastrin secretion to occur

-Distension of the stomach is a key stimulus during the phase

-Long loop vago-vagal reflexes operate

-Partially digested peptides and amino acids in the proximal part of the small intestine activate duodenal G cells to produce gastrin.

-5-10% of the total gastric secretion comes from the intestinal phase

-In the presence of acid, fatty acids and hyperosmotic fluids in the small intestine all stimulate the release of these hormones from small intestinal endocrine cells that inhibit gastric function

Examples:

-Secretin: Released in response to acidity in the duodenum and is the primary enterogastrone

-GIP: Released in response to glucose and to fatty acids and at high levels can inhibit the oxyntic cells

-CCK: Released in response to protein and fat digestion products

-Created from the cleavage of pepsiongens to pepsin

-Occurs below a pH of 5 and is virtually instananeous below pH of 2

-Autocatalyses conversion of pepsinogen to pepsin