Regulation of GI function

Question 1

What is the innervation of the GI tract

Answer 1

You have contributions from enteric nervous system, PNS, and SNS.

1. ENS (Meisners and Auerbachs)
2. PNS (primary innervation)
   Preganglionics are long. Vagal nerves come from the brain and innervate every part of the gut tube except the descending and sigmoid colon. Preganglionics from the sacral spinal cord innervate the lower parts of the GI tract
   -released acetylcholine contracts smooth muscle, relaxes internal sphincters, increases salivary, gastric, and pancreatic secretions (gets that shit going)
3. SNS - they all originate from the thoracolumbar region (T5-L2) and travel in the sympathetic chain. The sympathetics innervating the esophagus synapse at the superior cervical ganglion.
   Others can synapse on the 3 prevertebral ganglia. Celiac, superior mesenteric and inferior mesenteric.
   -
   (Norepinephrine released from postganglionics of SNS STOPS the GI tract - relaxes smooth muscle, contracts internal sphincters, reduces WATERY glandular secretions

Question 2

What are the neurotransmitters in the ENS (neurocrines)

5 of them

Answer 2

Vasoactive Intestinal Polypeptide (VIP) - relaxes smooth muscle, increases salivary, gastric and pancreatic secretions

• profound watery diarrhea with hypokalemia.
• less contraction, more secretion

Neuropeptide Y: relaxes smooth muscle and reduces glandular secretions (acts like sympathetic)
-less contraction, less secretion

Substance P - is co-secreted with Ach and contracts smooth muscle and increases salivary gland secretions.
-more contraction, more secretion

Enkephalins (opiates)- contracts smooth muscle and sphincters, and reduces glandular secretions. Overall opiates cause constipation because they disrupt normal contraction and relaxation)
-more contraction, less secretion

Gastrin releasing Peptide - increases gastrin secretion

Question 3

What are the 4 GI hormones?

Answer 3

Gastrin, Cholecystokinin, Secretin, Incretins (GIP, GLP-1)

Gastrin: produced by G cells in the antrum. Gastrin goes to the ECL cells by circulation which secrete histamine (paracrine)
-histamine activate parietal cells to secrete HCL. Gastrin can also directly affect the parietal cells.

CCK: produced by I cells in the duodenum and jejunum is response to fat and protein. CCK contracts gallbladder, relaxes sphincter of Oddi, stimulates secretion of primarily enzymes from exocrine pancreas, inhibits gastric emptying
-CCKalpha receptors are selective for CCK, while CCKb receptors binds CCK and gastrin.

Secretin: the antigastrin- it is secreted by S cells in the duodenum and jejunum in response to acid and fatty acids. It will promote secretion of bicarbonate from pancreas and bile to neutralize stomach acid. This is necessary to protect lumen and not denature enzymes.

Incretins - GIP is secreted by K cells in duodenum, GLP-1 is secreted by L cells in the ileum and colon, released from intestine after glucose loads, they increase insulin release from the pancreas.
-this is why oral glucose load leads to much higher insulin levels relative to IV glucose.

Question 4

How is gastrin secretion regulated?

Answer 4

Dietary peptides and vagal nerves stimulate gastrin secretion.

The neurotransmitter used by these vagal nerves are gastrin releasing peptide.

Acid in the antrum will stimulate D cells which release somatostatin in a paracrine fashion to inhibit G cells

Question 5

what are the external reflexes of the GI tract

3 stimulatory

3 inhibitory

Answer 5

——STIMULATORY—–

Gastrocolic - presence of food in the stomach causes evacuation of the colon.

Gastroileal reflex - entry of food in stomach results in relaxation of ileo-cecal sphincter leading to emptying of ileum.

Colonocolonic reflex - distension in one part causes relaxation in another part promoting aboral movement.

—-INHIBITORY——————

Enterogastric (duodenogastric)- signals from small intestine inhibit stomach secretion. CCK tells the stomach to stop.

Ileogastric reflex - ileal distension leads to decreased gastric emptying

Colonoileal - signals from the colon inhibit emptying of the ileal contents into the colon.

Question 6

What are pathophysiological long distance reflexes (3)

Answer 6

All 3 cause total paralysis of the gut.

Intestino-intestinal reflex - over distension of a segment due to intestinal obstruction leads to overall inhibition of intestinal muscle action.

Colonic-intestinal reflex - over-distention in colon causes general inhibition of intestinal muscle activity.

Peritoneo-intestinal reflex: handling of intestine during surgery or peritoneal irrigation results in generalized inhibition of intestines.

Question 7

what are the anti-reflux mechanisms (infants vs pregnant women)

4 things

Answer 7

1. High tone of LES (infants and pregant women DONt have it)
2. Secondary esophageal peristalsis (infants dont have it, pregnant women rely on it)
3. Pinching of LES by diaphragm (infants have it, this is why baby throw up so much) Women don’t have because during pregnancy there is elevation of the diaphragm.
4. LES will contract when gastric or abdominal pressure increases (infants don’t have it, and women don’t have it)