NBME REVIEW:GI PHYSIOLOGY.

By Dr Abiodun Mark Akanmode.

GASTROINTESTINAL TRACT.

NERVOUS CONTROL OF THE GI.

The neural control of the GI is under the control of a vast network called the enteric nervous system(myenteric and meissener’s plexus).

The enteric nervous system is innervated by the autonomic nervous systems.

An increase in sympathetic activity slows processes.

An increase in parasympathetic activity promotes digestive and absorptive processes.

ENDOCRINE CONTROL OF THE GI.

SWALLOWING.

Swallowing is a reflex controlled from the brain stem.

Events during swallowing: Relaxation of upper esophageal skeletal

muscle sphincter (UES) peristaltic wave Relaxation of lower esophageal smooth

muscle sphincter (LES) via VIP acting as an inhibitory transmitter

Relaxation of proximal stomach (receptive relaxation)

GASTRIC MOTILITY

Stimulation: Increased parasympathetic activity via

acetylcholine and gastrin release Local distension.

Inhibition: Low pH of stomach contents inhibits the release

of gastrin. Feedback from duodenal overload (neural and

hormonal)

STOMACH EMPTYING

Liquids > CHO > protein > fat (> = faster than)

The pylorus of the stomach acts as a sphincter to control the rate of stomach emptying.

A wave of contraction closes the sphincter so that only a small volume is moved forward into the duodenum.

CCK, GIP, and SECRETIN increase the degree of pyloric constriction and slow stomach emptying

SMALL INTESTINAL MOTILITY

Rhythmic contractions in adjacent sections create segmentation contractions, which are mixing movements.

Waves of contractions preceded by a relaxation of the muscle (peristaltic) are propulsive.

The ileocecal sphincter, or valve between the small and large intestine, is normally closed.

Distension of the ileum creates a muscular wave that relaxes the sphincter

MIGRATING MYOELECTRIC COMPLEX (MMC)

Are waves of electrical activity that sweep through the intestines in a regular cycle during fasting.

Moves undigested material from the stomach and small intestine into the colon.

Repeats every 90-120 minutes during fasting.

When one movement reaches the distal ileum, a new one starts in the stomach.

Correlated with high circulating levels of motilin, a hormone of the small intestine

DEFECATION Defecation is a reflex involving the central nervous

system.

A mass movement in the terminal colon fills the rectum and causes a reflex relaxation of the internal anal sphincter and a reflex contraction of the external anal sphincter.

Voluntary relaxation of the external sphincter accompanied with propulsive contraction of the distal colon complete defecation.

Lack of a functional innervation of the external sphincter causes involuntary defecation when the rectum fills.

SECRETIONSSALIVA. The basic unit of salivary glands are clusters of cells

called acini, which secrete a fluid containing: Water, electrolytes, mucus, enzymes.

The fluid flows out of the acini into collecting ducts.

Within the ducts, the composition of the secretion is altered, such as: much of the sodium being actively reabsorbed potassium is secreted, and large quantities of bicarbonate ion are secreted.

The fluid goes through small collecting ducts within salivary glands which lead into larger ducts, eventually forming a single large duct that empties into the oral cavity as saliva.

SALIVA. Parotid gland secretions are entirely serous (lack mucin).

Submandibular and sublingual gland secretions are mixed mucus and serous.

The ANS controls both the volume and type of saliva secreted.

Other composition of saliva includes:-ptyalin.-mucus-glycoprotein.-immunoglobulins.-lysozymes.

SECRETIONS:GASTRIC SECRETION.

The epithelial cells that cover the gastric mucosa secrete a highly viscous alkaline fluid (mucin plus bicarbonate) that protects the stomach lining from the caustic action of HCL.

Fluid needs both mucin and bicarbonate to be protective.

Nonsteroidal anti-inflammatory drugs such as aspirin decrease the secretion of the mucin and bicarbonate.

Surface of the mucosa studded with the openings of the gastric glands

SECRETIONSGLANDS:

Parietal cells: -HCl -Intrinsic factor combines with vitamin B12 and is reabsorbed in the distal ileum.

Chief Cells: -Pepsinogen is converted to pepsin by H+.

-Pepsin is active only in the acid pH medium of the stomach.

-Pepsin begins the digestion of protein but is not essential for life.

Mucous Neck Cells: - Secrete the protective mucus, HC03

combination

CONTROL OF ACID SECRETION:

There are 3 natural substances that stimulate parietal cells:

Acetylcholine (ACh), acting as a transmitter; its release is stimulated by sight/smell of food and reflexly in response to stomach distension( vagovagal reflex).

Locally released histamine; stimulated by Ach and gastrin

The hormone gastrin; stimulated by release of GRP

CONTROL OF PANCREATIC SECRETIONS Most of the regulation is via two hormones: secretin

and cholecystokinin Secretin:

Released from the duodenum in response to acid entering from the stomach.

Action on the pancreas is the release of fluid high in HC03.

This released HC03--rich fluid is the main mechanism that neutralizes stomach acid entering the duodenum.

Cholecystokinin ( CCK): Released from the duodenum in response to partially

digested materials (e.g., fat, petides, and amino acids) Action on the pancreas is the release of enzymes

(amylases, lipases, proteases).

SUMMARY OF DIGESTION.

ABSORPTION:CARBOHYDRATE:

Only monosaccharides (glucose, galactose, fructose) are absorbed by enterocytes.

Glucose and galactose are taken up by SGLTl (Na+ dependent).

Fructose is taken up by facilitated diffusion by GLUT-5.

All are transported to blood by GLUT-2.

D-xylose absorption test: distinguishes GI mucosal damage from other causes of malabsorption.

ABSORPTION:PROTEIN.

Luminal membrane: amino acids are transported by secondary active transport linked to sodium. Small peptides uptake powered by a Na-H anti porter.

Basal membrane: simple diffusion of amino acids, although it is now known some protein-mediated transport also occurs

ABSORPTIONFAT:

The end products of triglyceride digestion, 2-monoglycerides and fatty acids, remain as lipid-soluble substances that are then taken up by the micelles.

Digestive products of fats found in the micelles and absorbed from the intestinal lumen may include:

• Fatty acids (long chain)

• 2-Monoglyceride

• Cholesterol • Vitamins A, D, E, K • Bile salts, which stabilize the micelles.

The digested lipids then diffuse across the brush border in the lipid matrix. In the mucosal cell, triglyceride is resynthesized and forms lipid droplets (chylomicrons).

These leave the intestine via the lymphatic circulation (lacteals).

VITAMIN/MINERAL ABSORPTION

Iron :as Fe2+ in duodenum.

Folate: Absorbed in jejunum.

B12: Absorbed in terminal ileum along with bile acids, requires intrinsic factor

BILE:

Composed of bile salts (bile acids conjugated to glycine or taurine, making them water soluble), phospholipids, cholesterol, bilirubin, water, and ions.

Functions: Digestion and absorption of lipids and fat-

soluble vitamins. Cholesterol excretion (body's only means of

eliminating cholesterol) Antimicrobial activity (via membrane

disruption)

ENTEROHEPATIC CIRCULATION.

BILIRUBIN:

Product of heme metabolism.

Bilirubin is removed from blood by liver, conjugated with glucuronate, and excreted in bile.

Direct bilirubin-conjugated with glucuronic acid; water soluble.

Indirect bilirubin-unconjugated; water insoluble