GI System Flashcards
What is digestion?
The process by which foodstuffs are are degraded to produce smaller molecules which can be absorbed
What is absorption?
The processes by which nutrient molecules are absorbed by cells that line the GI tract and enter the bloodstream
Where can you find a myenteric plexus?
Through the whole length of the GI tract
Where can you find a submucosal plexus?
Only in the small and large intestine
Which layers make up the mucosa?
Epithelium, lamina propria, muscularis mucosae
What composes the muscularis externa?
An internal layer of circular muscle and an external layer of longitudinal muscle
What is the glycocalyx?
The network of proteins on the microvilli
What are the 2 routes of cellular transport in the small intestine?
Paracellular and transcellular
Which hormones are produced in the stomach/by which cells?
Gastrin (G cells) and somatostatin (D cells)
Which hormones are produced in the small intestine/by which cells?
Cholecystokinin (I cells), GIP (K cells), secretin (S cells), somatostatin (D cells)
The GI system is contained under which part of the nervous system?
Enteric
What happens to the membrane potential during GI smooth muscle contraction?
It oscillates between -40 mV and -60 mV in slow waves - frequency of slow waves determines frequency of muscle contraction
What type of contraction do sphincters have?
Long-lasting tonic contractions
What sphincters are in the GI tract?
Upper and lower oesophageal, pyloric, ileocaecal, internal and external anal
Where is the sphincter of Oddi?
Wraps around the ducts coming from the liver and gallbladder to the duodenum
Where do contractions begin in the stomach?
Corpus (body of stomach)
Where does mixing/grinding begin in the stomach?
Antrum (just before pylorus)
What is the most common type of movement in the small intestine?
Non-propulsive movements (caused by rhythmic contraction and relaxation of muscularis externa)
What is the name given to the process of moving GI contents towards the rectum/anus?
Segmentation
Which nerves control defection/the rectosphincteric reflex?
Somatic = pudendal nerve, parasympathetic = pelvic nerves
What important components are in saliva?
Mucin glycoproteins, lysozyme, a-amylase
What is the function of lysozyme in saliva?
Clean and protect cavity of the mouth
What is the function of a-amylase in saliva?
Reduce starch into oligosaccharides
What type of saliva does the parotid gland secrete?
Serous secretion, rich in a-amylase
What type of saliva do the submandibular and sublingual gland secrete?
Seromucous secretion
What type of saliva do the minor salivary glands secrete?
Mucous secretion, rich in mucin glycoproteins
What effect does acetylcholine have on the acinar and duct cells?
Stimulates fluid/protein secretion in the acinar cells, decreases absorption in the duct cells
What effect does noradrenaline have on the acinar and duct cells?
Stimulates protein secretion in the acinar cells, stimulates bicarbonate secretion in the duct cells
Is there any hormonal control of salivary gland secretion?
No!
Which branch of the autonomic nervous system stimulates saliva secretion?
Both sympathetic and parasympathetic branches
Where is B12 absorbed in the GI tract?
Stomach
What is the pH of gastric secretions?
0.9-1.5
What are the 5 main components of gastric secretions?
HCl, pepsins, intrinsic factor, mucus and bicarbonate ions
Which cell type secretes bicarbonate ions into the stomach?
Surface epithelial cells
What do the mucous neck cells of the stomach secrete?
A protective mucus
What do the parietal cells of the stomach secrete?
HCl and intrinsic factor
Which cell type secretes pepsinogens into the stomach?
Chief cells
Where are the G cells (gastrin-producing) in the stomach?
Antrum region
What is the secondary role of pepsin in the stomach?
Stimulate production of HCl
How is bicarbonate and mucus release regulated in the stomach?
Via acetylcholine and prostaglandins
What provides anatomical protection for the apical cells of the stomach?
Impermeability to H+ and the presence of tight junctions
What provides physiological protection for the apical cells of the stomach?
Thick layer of mucus impregnated with bicarbonate ions (mucus gel neutralisation zone)
Which agonist is involved in neuronal control of HCl secretion?
Acetylcholine and stimulation of ECL cells (indirect)
Which agonist is involved in hormonal control of HCl secretion?
Gastrin (secretion by G cells) stimulation of ECL cells (indirect)
Which agonist is involved in paracrine control of HCl secretion?
Histamine from ECL cell to activate cAMP within cell
Which nerve controls the cephalic phase of secretion?
Vagal nerve (entirely dependent)
What pH are pancreatic secretions?
7.8-8.4
Which type of digestive enzymes are released in an inactive form?
Proteases (trypsin, chymotrypsin, carboxypeptidases)
Where is cholecystokinin released from?
I cells in the small intestine
What is the effect of parasympathetic/enteric Ach release on pancreatic acinar cells?
Stimulation of enzyme release
How is NaCl secretion by the pancreatic acinar cells regulated?
Ach (neuronal) and CCK both increase intracellular calcium concentration
What is the most powerful stimulus for bicarbonate secretion from the pancreas?
Secretin
Which enzyme converts trypsinogen into trypsin?
Enteropeptidase
What is produced in the hepatocytes and stored in the gallbladder?
Bile salts, cholesterol, phospholipids - all together form a mixed micelle
What happens to bile acids once they have been utilised?
Absorbed by ileum then returned to the liver where they are taken up by hepatocytes (sodium-driven process)
What are the main 3 differences between the small and large intestines?
Large intestine does not have villi and is not involved in nutrient reabsorption; large intestine actively secrets K+ but small does not
What is the maximal absorptive capacity of the small intestine?
15-20L per day!!!
Which type of small intestine cell secretes salts?
Crypt-lining epithelial cells
Which type of small intestine cell absorbs salt/glucose?
Villus-lining epithelial cells
How is starch broken down in the small intestine?
Broken down by a-amylase into maltose, maltotriose and a-limit dextrins
How is maltose broken down in the small intestine?
Hydrolysed by maltase into 2 isolated glucose units
How is maltotriose broken down in the small intestine?
Hydrolysed by maltase into 1 glucose and 1 maltose
How is lactose broken down in the small intestine?
Hydrolysed into 1 unit glucose and 1 galactose
How is sucrose broken down in the small intestine?
Hydrolysed by sucrase into 1 fructose and 1 glucose
What is the SGLT1?
Sodium-coupled glucose transporter, transports glucose and galactose across the apical membrane of the SI
What is the GLUT5?
Glucose-transporter 5, transports fructose across apical membrane using facilitated diffusion (small intestine)
What is GLUT2?
Glucose-transporter 2, transports glucose, galactose and fructose across the basolateral membrane (small intestine) via facilitated diffusion
How many types of amino acid transport are there across the apical membrane?
7
