GUT Introduction to the Functions and Control of the Alimentary Tract

Question 1

What are the digestive functions of stomach?

Answer 1

1. Accommodation & storage
2. Mechanical and enzymatic breakdown
3. Slow delivery of chyme to duodenum

Question 2

How does the stomach allow large volume of food storage?

Answer 2

Fundus and body of stomach (thinner muscle tone) relaxes, allowing
large volume (~1.5L) of food storage

Question 3

What inhibits smooth muscle tone, to accomodate large volume of food and what transmitters are involved?

Answer 3

Vagal reflex inhibits smooth muscle tone – mechanoreceptors → fundic relaxation
– Vasoactive intestinal peptide (VIP) and nitric oxide (NO)

Question 4

What does the antrum region of the stomach do?

Answer 4

Antral region mixes/grinds food with gastric secretions → Digestion

Question 5

What is the purpose of the colon/rectum?

Answer 5

storage of indigestive residues and faecal matter

Question 6

What system enables the storage of food in the stomach?

Answer 6

ANS enables the storage of food in the stomach

Question 7

What is mucus secreted by and what is the function of mucus?

Answer 7

Mucus (secreted by goblet cells and mucus neck cells) – acts as a
lubricant by acting as a barrier that protects the stomach and colon
especially from gastric acid (prevents trauma)

Question 8

What is the function of lipase?

Answer 8

converts triglycerides to fatty acids and glycerol

Question 9

What is pepsin secreted by and what is the function of pepsin?

Answer 9

-secreted by chief cells or peptic cells as pepsinogen
-protein digestion

Question 10

What is HCl secreted by and what is the function of HCl?

Answer 10

-Secreted by parietal cells
-important in defence

Question 11

What are intrinsic secretions secreted by and what is the function??

Answer 11

(secreted by parietal cells) – for vitamin B12 absorption

Question 12

What are paractine secretions often called?

Answer 12

Often called “local hormones”

Question 13

What are paracrine secretions secreted from and what do they act on and via what?

Answer 13

Secreted from cells in the mucosa, but unlike hormones, the chemical
acts locally on adjacent cells via the interstitial fluid

Question 14

Where is Gastrin secreted from?

Answer 14

stomach (G-cells in antrum)

Question 15

What is secretin secreted from?

Answer 15

duodenal mucosa

Question 16

What is Pancreozymin-cholecystokinin secreted from?

Answer 16

duodenal mucosa

Question 17

What is insulin secreted from?

Answer 17

pancreas (beta-cells)

Question 18

Where does absorption mainly occur?

Answer 18

Mainly in small intestine

Question 19

Where does absorption of fluid occur?

Answer 19

small intestine and colon

Question 20

What is Gut motility and what does it allow?

Answer 20

The movements of the muscular wall (mostly smooth muscle except
extreme ends of the upper oesophagus/rectum) allows:
-Movement from one region to another (law of gut); mass evacuation
-Mechanical degradation, e.g. gastric antrum
-Mixing lumen contents, e.g. small intestine
-Transport of nutrients, water and of urea and electrolytes
-Digestion and absorption

Question 21

How may some drugs and some products of normal metabolism leave the body?

Answer 21

i. Saliva
ii. Bile
iii. Faeces
iv. (Vomit)

Question 22

What is the largest mucosal surface in the body?

Answer 22

The intestine is the largest mucosal surface in the body and is probably exposed to the heaviest burden of environmental antigens

Question 23

What is the largest lymphoepithelial organ?

Answer 23

intestine

Question 24

What are the defence mechanism of the guts?

Answer 24

1. Sight, smell and taste alerts us to harmful food substances
2. Vomit reflex
3. Acid in stomach (HCl) kills most harmful bacteria
4. Mucus secretions
5. Natural bacterial flora prevents colonisation of harmful bacteria
6. Aggregation of lymphoid tissue (e.g. Peyer’s patches) able to mount a
   response to food-borne antigens - analyse and respond to pathogenic
   microbes
7. Peyer’s patches: located in the lamina propria layer of the mucosa and
   extending into the submucosa of the ileum

Question 25

What is the liver involved in?

Answer 25

It is involved in carbohydrate, nitrogen and lipoprotein metabolism as well as the production of bile and excretion of bilirubin

Question 26

What is the function/mechanisms of the digestive tract?

Answer 26

1. Storage 2. Paracrine secretions 3. Exocrine secretions 4. Endocrine secretion 5. Absorption 6. Motility/transport 7. Excretion/transport 8. Defence 9. Metabolism

Question 27

What is the parasympathetic nervous system involved in the gut?

Answer 27

1.Stimulates digestive activity 2.Stimulates gallbladder 3.Relaxes rectum

Question 28

What is the sympathetic nervous system involved in the gut?

Answer 28

1.Inhibits digestive activity 2.Stimulates glucose release by liver

Question 29

What are the steps involved in relaxation of the gastric reservoir?

Answer 29

1. Mechanical stimuli in the pharynx are stimulated and send signals to the vagus center 2. Vagus center stimulates the inhibitory vagal fibres, releasing ACh which activate inhibitory enteric pathways that release NO, PACAP, VIP and/or ATP in order to relax muscle.

Question 30

What does a vagotomy lead to?

Answer 30

↓ accommodation and gastric compliance

Question 31

What are the steps in involved in movement of food into duodenum enabled by the ANS?

Answer 31

* Ripples of contraction move the food towards the antrum (thicker muscle layer) * Pyloric sphincter is often relaxed but closes upon arrival of peristaltic wave * Repulsion of chyme causes the opening of pyloric sphincter * Small partially digested material is squirted through the pyloric sphincter into duodenum * Repulsion of antral contents backwards towards the body allows mixing/grinding

Question 32

What is the sieving effect?

Answer 32

viscous and solid matter are retained in the stomach

Question 33

What type of hormones are produced by the gut?

Answer 33

All hormones produced by the gut are peptides (sequence of amino acids)

Question 34

What zone is present in the fundus?

Answer 34

Pacemaker zone

Question 35

What occurs spontaneously in the fundus?

Answer 35

Spontaneous depoloarisation and repolarisation

Question 36

Where does gastric action occur in the stomach?

Answer 36

In the body of the stomach

Question 37

Where does gastrin production occur?

Answer 37

In the pyloric antrum

Question 38

What is the pylorus made up of?

Answer 38

-Pyloric canal -Pyloric sphincter

Question 39

What are the steps involved in gastrin mediated effects?

Answer 39

1. Neurons secrete Gastrin-releasing peptide(GRP) which stimulate gastrin 2. Gastrin is secreted into circulation which acts on ECL cells that result in the secretion of histamine that act on parietal cells. This results in the secretion of HCl. 3. ECL cells are also activated by acetylcholine secreted from neurons which result in the secretion of histamine that act on parietal cells leading to the secretion of HCl. 4. Gastrin from G cells as well as acetylcholine from neurons act directly on parietal cells allowing for the secretion of HCl.

Question 40

What are the steps involved in paracrine control?

Answer 40

1. H+ stimulates via D cell, at the antral lumen, via positive feedback 2. Somatostatin acts on G cells via a negative feedback, causing gastrin to re-enter circulation and leave G cells.

Question 41

What does a vago-vagal reflex describe?

Answer 41

It describes a type of reflex in which both the afferent (“sensory”) and efferent (“motor”) axons are in the vagus nerve trunk

Question 42

What is the vago-vagal reflex within?

Answer 42

It is reflex circuit within the GIT

Question 43

What is the vago-vagal reflex pathway via?

Answer 43

Pathway is via the brain stem (medulla)

Question 44

When is the vago-vagal reflex active and what are the steps involved in it?

Answer 44

Thus the vago-vagal reflex is active during the receptive relaxation of the stomach in response to swallowing – reflex goes from stomach to brain and then back to stomach → active relaxation of smooth muscles of the stomach

Question 45

What does the vago-vagal reflex promote?

Answer 45

It also promotes motility and acid secretion

Question 46

What does the vago-vagal reflex pathway control?

Answer 46

Control of accommodation coordination of contraction/relaxation of GIT

Question 47

What is the vago-vagal pathway?

Answer 47

1. Vagal afferent stimulates NTS( Nucleus tractus solitarius) 2. NTS then stimulates the DMVN( Dorsal motor vagal nucleus) which transmits via the Vagal efferent to the stomach

Question 48

What is the NTS the main site of?

Answer 48

main site of termination of vagal afferents

Question 49

What is the DMVN the main of?

Answer 49

main site of origin of vagal efferents (motor) supplying the gut

Question 50

What are the 2 nerve fibers intrinsic to the gut in the enteric nervous system?

Answer 50

1. Myenteric plexus (Auerbach’s plexus) 2. Submucosal plexus (Meissner’s plexus)

Question 51

What does the myenteric plexus(auerbach's plexus) mediate?

Answer 51

mediate motor function

Question 52

What does the submucosal plexus(meissner's plexus) mediate?

Answer 52

mediate intestinal secretions

Question 53

What does the enteric nervous system reflex regulate?

Answer 53

Reflexly regulate GI functions entirely within the wall of the gut

Question 54

What is the enteric nervous system connected to the CNS by?

Answer 54

Connected to CNS by parasympathetic and sympathetic fibres

Question 55

What are the neurotransmitters involved in the enteric nervous system?

Answer 55

Neurotransmitters: ACh, NO, NA, 5-HT, GABA, ATP

Question 56

What is the function of the myenteric plexus?

Answer 56

Motor control

Question 57

What is the function of submucosal plexus?

Answer 57

Intestinal secretions

Question 58

What is the most prominent plexus?

Answer 58

Myenteric plexus(auerbach's plexus)

Question 59

What does cholinergic innervation do the myenteric plexus(auerbach's plexus) do?

Answer 59

↑ gastric motility and secretion

Question 60

What does adrenergic stiimulation do the myenteric plexus(auerbach's plexus) do?

Answer 60

↓ gastric motility and secretion

Question 61

What intestine is peristalsis slower in?

Answer 61

Slower in large intestine compared to the small intestine

Question 62

What is peristalsis?

Answer 62

Waves of propulsive contractions that move the contents of the gut towards the anus

Question 63

What initiates the propulsive contractions in peristalsis?

Answer 63

Distension initiates the propulsive contractions

Question 64

What fibres control movement in peristalsis?

Answer 64

Vagal inhibitory and excitatory fibres control movement

Question 65

What are the steps involved in peristalsis?

Answer 65

1. Distension causes signals to be sent along sensory neurons, via cholinergic interneurons, where they: a)Stimulate excitatory motor neurons, which results in the release of substance P and Ach transmitter that act on circular muscles -This causes Contraction of propulsive segment b)Stimulate inhibitory motor neurons, which secrete VIP and NO transmitter that act on circular muscles -This causes relaxation in receiving segment