Week 7 - Digestion and absorption Flashcards
What are the two types of contractile response that are superimposed upon the basal tone of the smooth muscle of the gut?
segmentation and peristaltic contractions
What is segmentation?
small regions of smooth muscle first contract and then relax to promote the mixing of intestinal contents
circular smooth muscle contracts and relaxes
What is the purpose of peristaltic contractions?
propulsion of food along the GI tract
From what segments of the spinal cord do the preganglionic fibres of the gastrointestinal sympathetic innervation arise from?
T8 to L2
preganglionic - fibres from the CNS to the ganglion, use acytylcholine as their neurotransmitter
Where do the cell bodies of the postganglionic fibres of the GI sympathetic innervation lie?
within the coeliac, superior and inferior mesenteric and hypogastric plexuses
postganglionic - fibres from the ganglion to the effector organ, in symathetic division these neurons are adrenergic (adrenaline and noradrenaline are the primary neurotransmitters)
What is the effect of increased sympathetic discharge on gastrointestinal activity?
reduces the activity
sympathetic - ‘fight or flight’
chronic stress can cause digestive problems because increased stimulation of the sympathetic nervous sytem can produce almost complete inhibition of gut activity
What do the sympathetic fibres of the GI tract innervate?
smooth muscle of arterioles - fibres cause vasocontriction and redirection of blood away from the splanchnic bed
innervate some of the circular muscle of the large and small intestine
glandular tissue and secretory cells
What is the effect of parasympathetic input to the gut?
stimulates both motility and secretory activity
What nerve relays parasympathetic innervation to the oesophagus, stomach, small intestine, liver, pancreas, caecum, appendix, ascending colon and transverse colon?
vagus nerve
What area of the GI tract receives parasympathetic innervation from pelvic nerves via the hypogastric plexus?
the remainder of the colon not innervated by the vagus nerve
descending colon, sigmoid colon, rectum
What is the enteric nervous system (ENS)?
one of the main divisions of the nervous system that governs the function of the gastrointestinal system
What are the two main nerve plexuses that constitute the enteric nervous sytem?
myenteric plexus and submucosal plexus (both are intramural plexuses)
How many endocrine hormones are known to exist in the gut?
8 including gastrin, secretin and CCK
endocrine - circulating hormones
What syndrome is associated with extremely elevated levels of ghrelin in the blood?
Prader - Willi syndrome
ghrelin has a number of efects within the gut and elsewhere including stimulation of growth hormone secretion by the anterior pituitary and the stimulation of appetite
ghrelin secretion is enhanced during fasting and is at its lowest immediately after a meal
What are the three phases of gastric secretion?
cephalic, gastric, intestinal
What is the combined circulation to the stomach, liver, pancreas, intestine and spleen called?
splanchnic circulation
What immunoglobulin is found in saliva?
IgA
What are the three large pairs of salivary glands?
parotid, submandibular and sublingual
How is Vitamin C absorbed?
by sodium-dependant active transport in the jejunum
What binds to B12 in the gastric juice?
haptocorrin
B12 is released from food by the low pH of gastric juice and becomes bound to haptocorrin, a glycoprotein secreted by the salivary glands
What happens to B12 in the duodenum?
binds to intrinsic factor
haptocorrin is digested so B12 pairs up with intrinsic factor which is secreted by the parietal cells of the gastric mucosa
Where is the B12:IF complex absorbed?
lower ileum
What does B12 bind to in the capillary endothelial cells?
transcobalamin II ready to be transported into the blood
What is the function of the oesophagus?
lubricates the food
peristaltic waves move the food to the stomach
What is the function of the gall bladder?
storage and concentration of bile
What are the four layers of the gastrointestinal wall?
mucosa, submucosa, muscularis externa and serosa
What layers make up the mucosa?
epithelium, lamina propria, muscularis mucosa
What does saliva consist of?
water, mucoprotein, IgA, carbohydrates, inorganic ions, enzymes
What are the two types of salivary glands?
large paired - parotoid, submandibular, sublingual
small glands
Where are the small glands in the mouth found?
mucosa and submucosa
named after location
labial - lips
buccal - cheeks
lingual - tongue
What is the difference between the small glands and large paired salivary secreting glands?
small glands secrete continuosly
large paired glands secrete only on stimulation
Where are the large paired glands located?
outside the mouth and are connected by long ducts
What is the basic architecture of the salivary glands?
a number of lobules surrounded by a fibrous capsule
each lobule (acinus) is made up of balls of cells
drained by ductules that join to form larger ducts leading into the mouth
Where is the digestive enzyme salivary amylase stored?
zymogen granules in the serous acinar cells
What is the effect of parasympathetic stimulation on the salivary glands?
promotes abundant watery saliva rich in amylase and mucins
increase in blood flow - kallikrein promotes production of the vasodilator bradykinin
When does the cephalic phase of digestion occur?
before food has arrived in the mouth
anticipation of food - sight, smell, taste
Where do the neurogenic signals that initiate the cephalic phase of gastric secretion originate from?
cerebral cortex
appetite centers of the amygdala and hypothalamus
What triggers the secretion of acetylcholine by nerve endings the cephalic phase of gastric secretion?
parasympathetic vagal stimulation
What cell releases cholecystokinin?
I-cells of the duodenum
Which hormone causes increased pancreatic enzyme secretion and gall bladder contraction?
a) CCK
b) Gastrin
c) Lipase
d) Secretin
a) CCK
What is the role of pancreatic lipase?
cleaves triacylglycerides (TAG) in micelles
micelle - small aggregates of mixed lipids and bile acids
What are the two types of dietry lipid?
neutral fat and triglyceride
What two processes must occur for triglycerides to be digested?
emulsification and enzymatic digestion
Describe the action of pancreatic lipase on triglycerides
splits bonds linking fatty acids to 1st and 3rd carbon atoms of glycerol
very limited action without colipase
What cells are responsible for histamine secretion in the stomach?
ECL (enterochromaffin like cells)
On what cells of the stomach does gastrin act?
parietal cells
released by G cells of the pyloric antrum, duodenum and pancreas
What are the two mechanisms of action of gastrin?
binds to CCKB receptors on ECL cells to stimulate release of histamine
induces insertion of K+/H+ ATPase pumps into apical membrane of parietal cells - leads to secretion of H+ into stomach cavity
What stimuli causes G cells to release gastrin?
stomach distention
vagal stimulation
presence of partially digested proteins
hypercalcemia
