Lecture 16

Question 1

What are the layers of the gut track from outer to inner?

Answer 1

Serosa, longitudinal smooth muscle layer, myenteric plexus (of Auerbach), circular smooth muscle layer, meissner’s plexus (submucosal plexus), submucosa, mucosa

Question 2

Describe the serosa

Answer 2

Outer layer of connective tissue and simple squamous epithelium that is continuous with the mesentery; missing in some parts where it is replaced by adventitia connecting to abdominal wall

Question 3

What is the function of the mesentery?

Answer 3

Pathway for blood vessels to gut tube

Question 4

Describe the submucosa

Answer 4

Incorporates blood vessels and submucosal plexus

Question 5

Describe the mucosa

Answer 5

Forms a continuous sheet lining entire GI tract; made of loose CT with sensory nerves, blood vessels, and some galnds; includes muscularis mucosa (thin layer of smooth muscle that creates ridges and folds)

Question 6

Describe propulsive movements

Answer 6

Stimulation at any point in the gut can cause contractile ring to occur, with the usual stimulus being distension; requires presence of functional myenteric plexus; can occur in any direction but usually dies out when traveling in the oral direction

Question 7

Describe mixing movements

Answer 7

May be caused by perstaltic contractions themselves; at other times, local intermittent constrictive contractions occur every few centimeters in the gut wall

Question 8

Describe slow waves

Answer 8

Complex interactions among the smooth muscle cells and the interstitial cells of Cajal (smooth muscle cell electrical pacemakers); set baseline for intermittent spike potentials; occur spontaneously and intensity varies between 5-15mV with a frequency range of 3-12 per minute; set the maximum frequency of contraction for each part of GI tract

Question 9

Describe spike potentials

Answer 9

Action potentials that excite muscle contraction; occur automatically when resting membrane potential of GI smooth muscle becomes more positive than -40mV; last 10-40 times as long as typical action potential in a large nerve fiber; responsible for opening calcium-sodium channels in GI smooth muscle cells

Question 10

What is the realtionship between slow wave potentials and spike potentials?

Answer 10

Higher the slow wave potential, greater the frequency of the spike potentials

Question 11

What are factors that cause depolarization of the GI tract?

Answer 11

Stretching of muscle, stimulation by acetycholine (parasympathetic) or lack of sympathetic stimulation, and stimulation by specific GI hormones

Question 12

What are factors that cause hyperpolarization of the GI tract?

Answer 12

Norepinephrine and epinephrine (sympathetic)

Question 13

What causes tonic contractions?

Answer 13

Continuous repetitive spike potentials, hormones, or continuous entry of calcium ions

Question 14

Describe the pattern of motility

Answer 14

Local stimulus like stretch from bolus that opens sodium channels; slow waves pass locally through gap junctions and spikes occur causing action potentials

Question 15

What are the 3 types of GI reflexes?

Answer 15

Reflexes entirely within the gut wall enteric nervous system

Reflexes from the gut to the prevertebral sympathetic ganglia and then back to the GI tract

Reflexes from the gut to the spinal cord/brain stem and then back to the GI tract

Question 16

Describe reflexes that are entirely within the gut wall enteric nervous system

Answer 16

Account for much of the GI secretion, peristalsis, mixing contractions, and local inhibitory effects

Question 17

Describe reflexes from the gut to the prevertebral ganglia and back to the GI tract

Answer 17

Transmit signals long distance to other areas of the tract; cause evacuation of the colon (gastrocolic reflex); inhibit stomach motility and secretion (enterogastric reflex); empty ileal contents into the colon (colonoileal reflex)

Question 18

What are the 4 “official” GI hormones?

Answer 18

Gastrin, CCK (cholecystokinin), secretin, and gastric inhibitory peptide (GIP)

Question 19

What do we call other hormones, including motilin, that aren’t “official” GI hormones?

Answer 19

“Candidate” hormones

Question 20

In terms of gastrin, what is the stimulus for secretion, how is the secretion inhibited, where is it secreted from, and what are its actions?

Answer 20

Secretion: small peptides and AA in stomach lumen, especially F and Y, distension of stomach, vagal stimulation mediated by GRP (gastrin-releasing peptide)

Secretion Inhibited: acid from stomach and somatostatin

Source: secreted from G cells of small intestines in response to stimuli associated with ingestion of a meal

Actions: stimulates gastric acid secretion by parietal cells and stimulates mucosal growth by stimulating synthesis of RNA and new protein

Question 21

In terms of CCK, what is the stimulus for secretion, where is it secreted from, and what are its actions?

Answer 21

Secretion: small peptides and AA, fatty acids and monoglycerides

Source: I cells of the small intestines

Actions: stimulates pancreatic enzyme secretion, pancreatic bicarbonate secretion, growth of exocrine pancreas, and contraction of gallbladder and relaxation of sphincter of Oddi for secretion of bile; inhibits gastric emptying and appetite

Question 22

In terms of secretin, what is the stimulus for secretion, where is it secreted from, and what are its actions?

Answer 22

Secretion: acid and fat in lumen of duodenum

Source: S cells of the small intestines

Actions: stimulates pepsin secretion, pancreatic bicarbonate secretion, biliary bicarbonate secretion, and growth of exocrine pancreas; inhibits gastric acid secretion by parietal cells

Question 23

In terms of GIP, what is the stimulus for secretion, where is it secreted from, and what are its actions?

Answer 23

Secretion: ONLY GI hormone released in response to protein, fat, and carbohydrate; released in response to orally administered glucose

Source: K cells of duodenum and jejunum

Actions: stimulates insulin release to bring blood glucose down; inhibits gastric acid secretion by parietal cells

Question 24

In terms of motilin, what is the stimulus for secretion, where is it secreted from, and what are its actions?

Answer 24

Secretion: fat, acid, and nerve action

Source: M cells of duodenum and jejunum

Actions: stimulates gastric and intestinal motility; secreted during fasting

Question 25

What is receptive relaxation?

Answer 25

Initiated by stomach distension; proximal region of the stomach relaxes to accommodate ingested meal; CCK increases distensibility of proximal end of stomach

Question 26

What is the gastroileal reflex?

Answer 26

Presence of food in stomach triggers peristalsis in ileum

Question 27

What are segmentation contractions in the small intestine?

Answer 27

Mix intestinal contents by sending chyme in both directions without a net forward movement

Question 28

What is the gastrocolic reflex?

Answer 28

Food in stomach increases frequency of mass movements; when stomach is stretched with food, there is a rapid parasympathetic component; slower CCK and gastrin component is also involved

Question 29

What are the principal functions of the colon?

Answer 29

Absorption of water and electrolytes from fecal matter; storage of fecal matter until it can be expelled

Question 30

What are haustrations?

Answer 30

Produced by large circular constrictions plus contractions of the teniae coli

Question 31

What is the function of saliva?

Answer 31

Initial starch digestion and triglyceride digestion; lubrication of food and protection of mouth and esophagus

Question 32

What are some factors that increase/decrease saliva production?

Answer 32

Increase: food in mouth, smells, conditioned reflexes, nausea

Decrease: sleep, dehydration, fear, anticholinergic drugs

Question 33

What types of secretory cells are found in gastric glands?

Answer 33

Mucous neck cells, chief (peptic) cells, parietal (oxyntic) cells

Question 34

Describe chief cells

Answer 34

Secrete pepsinogen (inactive form of pepsin) which is a proteolytic enzyme with a pH range of activity between 1.8-3.5; release is stimulated by Ach from vagus nerve or gastric nervous plexus in response to acid in stomach

Question 35

What do parietal cells secrete?

Answer 35

HCl

Question 36

What inhibits gastric secretions?

Answer 36

Somatostatin and prostaglandins