The alimentary tract Flashcards
What are the 5 parts/regions of the stomach?
The cardia, the fundus, the body, the antrum and the pylorus
Which parts of the stomach can relax to accommodate for food that is yet to be digested?
The fundus and the body
What’s the function of the antral region of the stomach?
The antrum is responsible for the mixing and grinding of food with gastric secretions
What’s the function of the colon and rectum?
Storage of digestive residues and faeces
What do gastric juices consist of?
Mucus, pepsinogen, intrinsic factor and lipase
What’s the function of the mucus in gastric juices?
The mucus lubricates the stomach and colon
What’s the function of the lipase in gastric juices?
Lipase digests triglycerides into fatty acids and glycerol
What’s the function of the intrinsic factor in gastric juices?
The intrinsic factor helps with vitamin B12 absorption
What’s the function of the pepsin derived from pepsinogen in gastric juices?
Pepsin is important for protein digestion
Give an example of a paracrine secretion and its function
Somatostatin inhibits gastrin release in the stomach
Give 4 examples of exocrine secretions
Gastrin
Secretin
Pancreozymin-cholecystokinin
Insulin
What’s the function of gastrin and what releases it?
Gastrin is released from G cells in the pyloric antrum of the stomach, the duodenum and the pancreas. It acts to stimulate gastric acid secretion by parietal cells
What’s the function of secretin and where’s it produced?
Secretin is a peptide hormone secreted from S cells of the duodenum and jejunum. Secretin is a hormone that regulates water homeostasis throughout the body, as well as regulating secretions in the stomach, pancreas and liver. Secretin inhibits gastric acid secretion and gastric contractions. It stimulates bicarbonate-rich solution secretion from the gallbladder and duct cells of the pancreas
What’s the function of cholecystokinin and where does it act?
CCK, also known as pancreozymin, is a peptide hormone released from I cells of the duodenum and jejunum in response to partially digested lipids and proteins in the duodenum. It contributes, together with gastrin and secretin, to the gut hormone triad. It inhibits gastric emptying and stimulates acinar cells of the pancreas to release a juice rich in pancreatic digestive enzymes that catalyse the breakdown of fats, proteins and carbohydrates. It also plays a role in satiety by slowing gastric emptying. CCK directly and indirectly (via the DMVN) causes contraction of the gallbladder and relaxation of the Sphincter of Oddi, therefore causing bile secretion into the duodenum.
Where’s insulin produced and what does it do?
Insulin is secreted by beta cells in the islets of Langerhans of the pancreas. It stimulates glycogenesis, increases the number of GLUT4 transporters in adipose and muscle tissue, and inhibits glycogenolysis and gluconeogenesis
Where does nutrient absorption mainly happen?
In the small intestine
Where does fluid absorption mainly happen?
The colon and small intestine
Name 4 mechanisms for defence in the gut
- Sight, smell and taste
- Stomach acid that can kill most bacteria
- Vomit reflex
- Aggregations of lymphoid tissue called Peyer’s patches, which can mount an response to food-borne antigens
What are the 4 types of control of the alimentary tract?
Endocrine
Paracrine
Neurocrine
Metabolic
Name the 2 neurotransmitters responsible for stimulating contraction of gut muscle
Ach and SubP/SubK
Name the 2 neurotransmitters responsible for stimulating relaxation of gut muscle
VIP and NO
What controls gastric smooth muscle activity?
Contractie cholinergic neurons and relaxant NANC neurons in the myenteric plexus between the circular and longitudinal smooth muscle layers.
Gastrin and CCK and serotonin and motilin also affect smooth muscle
What provides parasympathetic innervation to the gut muscle and mucosa?
The vagus nerve
What provides sympathetic innervation to the gut muscle and mucosa?
The splanchnic nerves
What are the 3 types of relaxation of the fundus?
Receptive relaxation
Adaptive relaxation
Feedback relaxation
Upon arrival of a peristaltic wave, what happens to the pyloric sphincter?
It closes, but opens when chyme is repulsed through it
What receptors does gastrin bind to, and what happens when it does?
Gastrin is released from G cells and acts on CCKB receptors on target cells, triggering the release of histamine, as well as inducing insertion of K+/H+ ATPase pumps into the apical membrane of HCl secreting cells, causing gastric acid release into the stomach
What activates the pepsinogen released from chief cells to become pepsin?
High acidity alters the tertiary structure of pepsinogen, so it becomes pepsin
What is the NTS?
The nucleus tracts solitarius is the main site of termination of vagal afferent fibres. It’s a series of purely sensory nuclei in the medulla of the brainstem
What is the DMVN?
The dorsal motor vagal nucleus is the main site of origin of vagal efferents supplying the gut
What does the vagovagal reflex control?
The vagovagal reflex controls contraction of the gastrointestinal smooth muscle layers in response to distention of the tract by food
When is the vagovagal reflex activated?
During receptive relaxation of the stomach in response to swallowing of food
What are the 2 nerve fibres intrinsic to the gut and their functions?
The myenteric plexus (Auerbach’s plexus) is responsible for motor function.
The submucosal plexus (Meissner’s plexus) is responsible for intestinal secretions.
What type(s) of autonomic input does Auerbach’s plexus have?
Auerbach’s plexus has both sympathetic and parasympathetic input
What type(s) of autonomic input does Meissner’s plexus have?
Meissner’s plexus only has parasympathetic fibres
What’s the effect of cholinergic action in Auerbach’s plexus?
Increase in gastric motility and secretion
What’s the effect of adrenergic action in Auerbach’s plexus?
Decrease in gastric motility and secretion
Where does Meissner’s plexus provide secretomotor innervation?
To the mucosa nearest the lumen of the gut
What reflex causes the urge to go to the bathroom after a large meal?
The colo-colonic reflex of gut motility
What’s the rate of emptying of the stomach dependent on?
The material’s ability to be absorbed
What cells secrete pancreozymin?
Enteroendocrine cells in the duodenum
What are the 3 digestive functions of the stomach?
Accommodation and storage
Mechanical and enzymatic breakdown
Low delivery of chyme to the duodenum
What 2 mechanisms transport digesta from the gastric reservoir into the antral pump?
Tonic contractions that originate from the fundus
Peristaltic waves in the region of the gastric corpus
Only chyme of what size can pass through the pyloric sphincter out of the stomach?
2mm
What does the descending inhibitory reflex elicited by middle antrum contraction do?
The descending inhibitory reflex causes pyloric relaxation (VIP/NO)
What does the ascending excitatory reflex cause?
Pyloric contractions and increase in tone, preventing duodeno-gastric reflux
What is gastric emptying regulated by?
Negative feedback systems
What is antral over-distension regulated by?
The vago-vagal reflex
Describe the emptying of liquids from the stomach
Liquids rapidly disperse, emptying without lag time. The rate of emptying is influenced by nutrient control
Describe the emptying of solids from the stomach
There are 2 phases- lag time and the linear phase. The duration of lag time is related to the size of the particle. Large particles are triturated to smaller ones
Describe the emptying of fatty foods from the stomach
Fatty foods liquefy at body temperature and float on top of the liquid layer, therefore emptying slowly. Fats are potent inhibitors of gastric motor events and gastric emptying
Describe the emptying of indigestible solids from the stomach
Indigestible solids are not emptied in the immediate post-prandial period. MMC activity facilitates peristalsis for movement of indigestible substances through the GI tract
What is the migrating motor complex?
A distinct pattern of electromechanical activity during fasting, triggering peristaltic waves that facilitate transport of indigestible entities from the stomach to the colon
What’s the basal electric rhythm (BER)?
The spontaneous depolarisation and repolarisation of pacemaker cells in the smooth muscle of the stomach, small intestine and large intestine, which is spread through gap junctions in the smooth muscle of the GI tract
What’s the name of the pacemaker cells in the GI tract?
Interstitial cells of Cajal