GUT Introduction to the Functions and Control of the Alimentary Tract Flashcards
What are the digestive functions of stomach?
- Accommodation & storage
- Mechanical and enzymatic breakdown
- Slow delivery of chyme to duodenum
How does the stomach allow large volume of food storage?
Fundus and body of stomach (thinner muscle tone) relaxes, allowing
large volume (~1.5L) of food storage
What inhibits smooth muscle tone, to accomodate large volume of food and what transmitters are involved?
Vagal reflex inhibits smooth muscle tone – mechanoreceptors → fundic relaxation
– Vasoactive intestinal peptide (VIP) and nitric oxide (NO)
What does the antrum region of the stomach do?
Antral region mixes/grinds food with gastric secretions → Digestion
What is the purpose of the colon/rectum?
storage of indigestive residues and faecal matter
What system enables the storage of food in the stomach?
ANS enables the storage of food in the stomach
What is mucus secreted by and what is the function of mucus?
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)
What is the function of lipase?
converts triglycerides to fatty acids and glycerol
What is pepsin secreted by and what is the function of pepsin?
-secreted by chief cells or peptic cells as pepsinogen
-protein digestion
What is HCl secreted by and what is the function of HCl?
-Secreted by parietal cells
-important in defence
What are intrinsic secretions secreted by and what is the function??
(secreted by parietal cells) – for vitamin B12 absorption
What are paractine secretions often called?
Often called “local hormones”
What are paracrine secretions secreted from and what do they act on and via what?
Secreted from cells in the mucosa, but unlike hormones, the chemical
acts locally on adjacent cells via the interstitial fluid
Where is Gastrin secreted from?
stomach (G-cells in antrum)
What is secretin secreted from?
duodenal mucosa
What is Pancreozymin-cholecystokinin secreted from?
duodenal mucosa
What is insulin secreted from?
pancreas (beta-cells)
Where does absorption mainly occur?
Mainly in small intestine
Where does absorption of fluid occur?
small intestine and colon
What is Gut motility and what does it allow?
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
How may some drugs and some products of normal metabolism leave the body?
i. Saliva
ii. Bile
iii. Faeces
iv. (Vomit)
What is the largest mucosal surface in the body?
The intestine is the largest mucosal surface in the body and is probably exposed to the heaviest burden of environmental antigens
What is the largest lymphoepithelial organ?
intestine
What are the defence mechanism of the guts?
- Sight, smell and taste alerts us to harmful food substances
- Vomit reflex
- Acid in stomach (HCl) kills most harmful bacteria
- Mucus secretions
- Natural bacterial flora prevents colonisation of harmful bacteria
- Aggregation of lymphoid tissue (e.g. Peyer’s patches) able to mount a
response to food-borne antigens - analyse and respond to pathogenic
microbes - Peyer’s patches: located in the lamina propria layer of the mucosa and
extending into the submucosa of the ileum
What is the liver involved in?
It is involved in carbohydrate, nitrogen and lipoprotein metabolism as
well as the production of bile and excretion of bilirubin
What is the function/mechanisms of the digestive tract?
- Storage
- Paracrine secretions
- Exocrine secretions
- Endocrine secretion
- Absorption
- Motility/transport
- Excretion/transport
- Defence
- Metabolism
What is the parasympathetic nervous system involved in the gut?
1.Stimulates digestive activity
2.Stimulates gallbladder
3.Relaxes rectum
What is the sympathetic nervous system involved in the gut?
1.Inhibits digestive activity
2.Stimulates glucose release by liver
What are the steps involved in relaxation of the gastric reservoir?
- Mechanical stimuli in the pharynx are stimulated and send signals to the vagus center
- 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.
What does a vagotomy lead to?
↓ accommodation and gastric compliance
What are the steps in involved in movement of food into duodenum enabled by the ANS?
- 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
What is the sieving effect?
viscous and solid matter are retained in the stomach
What type of hormones are produced by the gut?
All hormones produced by the gut are peptides (sequence of amino
acids)
What zone is present in the fundus?
Pacemaker zone
What occurs spontaneously in the fundus?
Spontaneous depoloarisation and repolarisation
Where does gastric action occur in the stomach?
In the body of the stomach
Where does gastrin production occur?
In the pyloric antrum
What is the pylorus made up of?
-Pyloric canal
-Pyloric sphincter
What are the steps involved in gastrin mediated effects?
- Neurons secrete Gastrin-releasing peptide(GRP) which stimulate gastrin
- 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.
- 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.
- Gastrin from G cells as well as acetylcholine from neurons act directly on parietal cells allowing for the secretion of HCl.
What are the steps involved in paracrine control?
- H+ stimulates via D cell, at the antral lumen, via positive feedback
- Somatostatin acts on G cells via a negative feedback, causing gastrin to re-enter circulation and leave G cells.
What does a vago-vagal reflex describe?
It describes a type of reflex in which both the afferent (“sensory”) and
efferent (“motor”) axons are in the vagus nerve trunk
What is the vago-vagal reflex within?
It is reflex circuit within the GIT
What is the vago-vagal reflex pathway via?
Pathway is via the brain stem (medulla)
When is the vago-vagal reflex active and what are the steps involved in it?
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
What does the vago-vagal reflex promote?
It also promotes motility and acid secretion
What does the vago-vagal reflex pathway control?
Control of accommodation
coordination of contraction/relaxation of GIT
What is the vago-vagal pathway?
- Vagal afferent stimulates NTS( Nucleus tractus solitarius)
- NTS then stimulates the DMVN( Dorsal motor vagal nucleus) which transmits via the Vagal efferent to the stomach
What is the NTS the main site of?
main site of
termination of vagal afferents
What is the DMVN the main of?
main site of
origin of vagal efferents (motor) supplying the gut
What are the 2 nerve fibers intrinsic to the gut in the enteric nervous system?
- Myenteric plexus (Auerbach’s plexus)
- Submucosal plexus (Meissner’s plexus)
What does the myenteric plexus(auerbach’s plexus) mediate?
mediate motor function
What does the submucosal plexus(meissner’s plexus) mediate?
mediate intestinal secretions
What does the enteric nervous system reflex regulate?
Reflexly regulate GI functions entirely within the wall of the gut
What is the enteric nervous system connected to the CNS by?
Connected to CNS by parasympathetic and sympathetic fibres
What are the neurotransmitters involved in the enteric nervous system?
Neurotransmitters: ACh, NO, NA, 5-HT, GABA, ATP
What is the function of the myenteric plexus?
Motor control
What is the function of submucosal plexus?
Intestinal secretions
What is the most prominent plexus?
Myenteric plexus(auerbach’s plexus)
What does cholinergic innervation do the myenteric plexus(auerbach’s plexus) do?
↑ gastric motility and secretion
What does adrenergic stiimulation do the myenteric plexus(auerbach’s plexus) do?
↓ gastric motility and secretion
What intestine is peristalsis slower in?
Slower in large intestine compared to the small intestine
What is peristalsis?
Waves of propulsive contractions that move the contents of the gut towards
the anus
What initiates the propulsive contractions in peristalsis?
Distension initiates the propulsive contractions
What fibres control movement in peristalsis?
Vagal inhibitory and excitatory fibres control movement
What are the steps involved in peristalsis?
- 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
