Physiology I - Gastrointestinal Physiology

Front 1

Innervation of the GI tract

Back 1

The autonomic nervous system of the GI tract comprises both extrinsic and intrinsic nervous systems

Front 2

Extrinsic innervation

Back 2

Parasympathetic; vagus and pelvic nerves, excitatory on the functions of the GI tract

Sympathetic; inhibitory on the functions of the GI tract,

Front 3

Intrinsic innervation

Back 3

Local reflexes to relay information within the GI tract

Control most functions of the GI tract,

- Myenteric plexus (motility)

- Submucosal plexus (secretion and blood flow)

Front 4

What are the official GI hormones?

Back 4

- Gastrin

- Cholecystokinin (CCK)

- Secretin

- Glucose-dependent insulinotropic peptide (GIP)

Front 5

Gastrin

Back 5

- G cells of stomach

- Stimulated by; small peptides and amino acids, distention of stomach, vagus (GIP),

- Inhibited by; H+ in stomach, somatostatin

- Actions; increase gastric H+ secretion (parietal cells), stimulates growth of gastric mucosa

Front 6

CCK

Back 6

- I cells of duodenum and jejunum

- Stimulated by; small peptides and amino acids, fatty acids

- Actions; contraction of gallbladder and relaxation of sphincter of Oddi, increased; pancreatic enzyme and bicarbonate secretion and growth of exocrine pancreas/gallbladder, inhibit gastric emptying

Front 7

Secretin

Back 7

- S cells of duodenum

- Stimulated by; H+ in duodenum, fatty acids in duodenum

- Actions; increase pancreatic bicarbonate secretion and biliary bicarbonate secretion, decreased gastric H+ secretion

Front 8

GIP

Back 8

- Duodenum and jejunum

- Stimulated by; fatty acids, amino acids and oral glucose

- Actions; increased insulin secretion, decreased gastric H+ secretion

Front 9

Motilin

Back 9

Increases GI motility and is involved in interdigestive myoelectric complexes

Front 10

Pancreatic polypeptide

Back 10

Inhibits pancreatic secretion

Front 11

Glucagon-like peptide-1 (GLP-1)

Back 11

Binds to pancreatic beta cells and stimulates insulin secretion. Analogue of GLP-1 may be helpful in the treatment of type II diabetes mellitus

Front 12

Paracrines

Back 12

- Diffuse short distances to act on target cells located in the GI tract

- The GI paracrine are somatostatin and histamine

Front 13

Somatostatin

Back 13

- Secreted in response to H+ in the lumen

- Inhibited by vagal stimulation

- Inhibits the release of all GI hormones and gastric H+ secretion

Front 14

Histamine

Back 14

- Secreted by mast cells of the gastric mucosa

- Increases gastric H+ secretion directly and by potentiating the effects of gastrin and vagal stimulation

Front 15

Neurocrines

Back 15

- Synthesized in neurons of the GI tract, moved by axonal transport down the axons and released by action potentials in the nerves

- GI neurons are; VIP, GRP and enkephalins

Front 16

VIP

Back 16

- Released from neurons in the mucosa and smooth muscle of the GI tract

- Produces relocation of GI smooth muscle, inkl. lower esophageal sphincter

- Stimulates pancreatic HCO3- secretion

- Inhibits gastric H+ secretion

Front 17

GRP

Back 17

- Released from vagus nerves that innervate the G cells

- Stimulates gastrin release from G cells

Front 18

Enkephalins

Back 18

- Secreted from nerves in the mucosa and smooth muscle of GI tract

- Stimulate contraction of GI smooth muscle

- Inhibit intestinal secretion of fluid and electrolytes

Front 19

Leptin

Back 19

- Secreted by fat cells

- Decreases appetite

Front 20

Ghrelin

Back 20

- Secreted by gastric cells

- Increases appetite

Front 21

Slow waves

Back 21

Are oscillating membrane potentials inherent to the smooth muscle cells of some parts of the GI tract (Cajal cells)

Front 22

Gastric motility

Back 22

Gastric contractions are increased by vagal stimulation and decreased by sympathetic stimulation

Front 23

Gastric emptying

Back 23

- Fastest when the stomach contents are isotonic

- Fat inhibits gastric emptying (CCK)

- H+ in the duodenum inhibits gastric emptying

Front 24

Small intestinal motility

Back 24

Parasympathetic stimulation increases intestinal smooth muscle contraction and sympathetic stimulation decreases it

Front 25

Segmentation contractions

(small intestine)

Back 25

Mix the intestinal contents

Front 26

Peristaltic contractions

(small intestine)

Back 26

Propel the chyme through the small intestine toward the large intestine

Coordinated by the enteric nervous system

Front 27

Gastroileal reflex

(small intestine)

Back 27

Mediated by the extrinsic ANS and possibly by gastrin.

The presence of food in the stomach triggers increased peristalsis in the ileum and relaxation of the ileocecal sphincter.

Front 28

Segmentation contractions

(large intestine)

Back 28

In the proximal colon mic the contents and are responsible for the appearance of haustra

Front 29

Gastrocolic reflex

(large intestine)

Back 29

The presence of food in the stomach increases the motility of the colon and increases the frequency of mass movements.The reflex has a rapid parasympathetic component that is initiated when the stomach is stretched by food. A slower, hormonal component is mediated by CCK and gastrin.

Front 30

Formation of saliva

Back 30

By the parotid, submandibular and sublingual glands.

Front 31

Regulation of saliva production

Back 31

- Controlled by parasympathetic and sympathetic nervous systems (increased by both)

Front 32

Parasympathetic regulation of saliva production

Back 32

- Cranial nerves VII and IX

- By increasing transport processes in the acinar and ductal cells and by causing vasodilation

Front 33

Sympathetic regulation of saliva production

Back 33

Increases the production of saliva and the growth of salivary glands, although the effects are smaller than those of parasympathetic stimulation

Front 34

Which factors affect saliva production?

Back 34

Increased by;

food, smells, conditioned reflexes and nausea

Decreased by; sleep, dehydration, fear, anticholinergic drugs

Front 35

What stimulates gastric H+ secretion?

Back 35

- Vagal stimulation

- Gastrin

- Histamine

Front 36

What inhibits gastric H+ secretion?

Back 36

- Negative feedback of gastrin

- Somatostatin; inhibit release of histamine and gastrin

- Prostaglanding

Front 37

Which drugs block gastric H+ secretion?

Back 37

- Antropine

- Cimetidine

- Omeprazole

Front 38

Antropine

Back 38

A cholinergic muscarinic antagonist, inhibits H+ secretion by blocking the direct pathway, which uses ACh as a neurotransmitter. However, atropine does not block H+ secretion to completely because it does not inhibit the indirect pathway, which uses GRP as a neurotransmitter.

Front 39

Pancreatic secretion

Back 39

Much higher bicarbonate concentration than plasma - to neutralize the acidic chyme. Contains enzymes essential for the digestion of protein, carbohydrate and fat.

Isotonic

Front 40

Stimulation of pancreatic secretion

Back 40

- Secretin

- CCK

- ACh

Front 41

Composition of bile

Back 41

Bile contains;

bile salts, phospholipids, cholesterol and bile pigments (bilirubin)

Front 42

Production of bile

Back 42

- Produced continuously by hepatocytes

- Stored in the gallbladder

- The bile acids are conjugated with glycine or taurine to form bile salts

Front 43

Carbohydrates

Back 43

Absorbed in the small intestine by Na+-dependent cotransport (glucose, galactose) and facilitated diffusion (fructose).

Front 44

Proteins

Back 44

Absorbed in the small intestine by Na+-dependent cotransport (amino acids) and H+-dependent cotransport (di- and tripeptides)

Front 45

Digestion of proteins

Back 45

- Endopeptidases; degrade proteins by hydrolyzing interior peptide bonds

- Exopeptidases; hydrolyze one amino acid at a time from the C terminus of proteins and peptides

- Pepsin; pepsinogen activated to pepsin when low pH

- Pancreatic proteases; trypsinogen activated to trypsin by enterokinase

Front 46

Lipids

Back 46

- Absorbed in the small intestine

- Micelles form with bile salts in intestinal lumen

- Diffusion of fatty acids, monoglycerides and cholesterol into cell

- re-esterification in cell to triglycerides and phospholipids

- Chylomicrons form in cell to and are transferred to lymph

Front 47

Bilirubin production and excretion

Back 47

Hemoglobin is degraded to bilirubin by the reticuloendothelial system. Bilirubin binds to albumin in the circulation system. In the liver bilirubin is conjugated with glucuronic acid. A portion of conjugated bilirubin is excreted in the urine, and a portion into bile. In the intestine, it is converted to urobilinogen, which is returned to the liver and excreted as stercobilin and urobilin in feces.