Topic 5: Gastrointestinal System
Which of the following will decrease/inhibit smooth muscle contractions? (A) Parasympathetic nerve activation (B) Adrenaline (C) Acetylcholine agonist (D) Phentolamine (E) Atropine
(B) Adrenaline
When does HCl secretion into the stomach occur? (A) Exercise (B) Under stress (C) After/during a meal (D) Sleeping
(C) After/during a meal
Which secretions contain protein digesting enzymes? (More than one answer) A) Bile (B) Saliva (C) Gastric secretions (D) Duodenal fluid (E) Pancreatic secretions
(C) Gastric secretions - Pepsinogen activated to pepsin due to acidic environment and (E) Pancreatic secretions - pancreatic proteases activated in the duodenal lumen
You take tablets containing acetylcholine and histamine. What would prevent all of the resulting gastric HCl secretion? (A) Histamine (H) receptor inhibitor (B) Muscarinic (M) receptor inhibitor (C) H+/K+ ATPase pump inhibitor (D) Gastrin (G) receptor inhibitor
(C) H+/K+ ATPase pump inhibitor
Cholycystokinin (CCK): (A) Stimulates gastric emptying (B) Is released from vagal (efferent parasympathetic) nerve terminals (C) Is released in the presence of fats in the duodenum (D) Inhibits gallbladder contraction (E) Stimulates acid secretion from parietal cells
(C) Is released in the presence of fats in the duodenum
Where would you find active protein digesting enzymes? (More than one answer) A) Gallbladder (B) Mouth cavity (C) Stomach (D) Duodenum (E) Pancreas
(C) Stomach and (D) Duodenum
Which of the following will increase ileum smooth muscle contraction? (A) Adrenaline (B) Noradrenaline (C) Atropine (D) Acetylcholine (E) Phentolamine
(D) Acetylcholine
Sympathetic nerve activation will cause the following effect on gut function: (A) Increased oesophageal peristalsis (B) Increased gastric acid secretion (C) Relaxation of the pyloric sphincter (D) Decreased intestinal contractions
(D) Decreased intestinal contractions
Which of the following will increase smooth muscle contractions? (A) Sympathetic nerve activation (B) Adrenaline (C) Noradrenaline agonist (D) Phentolamine (E) Atropine
(D) Phentolamine - Phentolamine blocks adrenergic (adrenaline/noradrenaline) receptors
Which of the following does NOT control the sphincter muscles? (A) Parasympathetic nerves (B) Sympathetic nerves (C) Myenteric plexus (D) Submucosal plexus
(D) Submucosal plexus - Only controls glandular secretions
Which of the following inhibits the muscurinic receptors of the ileum smooth muscle? (A) Parasympathetic nerve activation (B) Adrenaline (C) Acetylcholine agonist (D) Phentolamine (E) Atropine
(E) Atropine - Atropine is a muscarinic antagonist (blocking acetylcholine to attach to the receptor)
Which of the following organs produce both lipase and amylase? (A) Gallbladder (B) Mouth cavity (C) Stomach (D) Duodenum (E) Pancreas
(E) Pancreas
Which of the following is known as nature's antacid? (A) Micelles (B) Histamine (C) Cholecystokinin (D) Pepsin (E) Secretin
(E) Secretin
Where in the body is amylase present? (A) Saliva only (B) Stomach acid only (C) Pancreatic only (D) (A) and (B) (E) (B) and (C) (F) (A) and (C)
(F) (A) and (C)
What are the 5 main responses to the release of CCK?
1. Contraction of the gall bladder 2. Relaxation of the sphincter of oddi (hepatopancreatic sphincter) 3. Acinar cells of the pancreas release pancreatic enzymes (including pancreatic lipase) 4. Inhibits stomach contractions 5. Constricts the pyloric sphincter
What are the two gut hormone secreting cells and what hormones do they release?
1. Enterochromaffin-like (ECL) cells secrete histamine 2. G cells secrete gastrin
What protects the stomach mucosa from the corrosive acidic gastric juices?
1. HCO3- rich mucous 2. Mucosal epithelial cells joined by tight junctions 3. Damaged epithelial cells shed and rapidly replaced by division of undifferentiated stem cells
What are the four gastric glands located in the gastric pits of the stomach and what are their functions?
1. Mucous - secretes muscous 2. Parietal - secretes intrinsic factors + HCl 3. Chief - secretes pepsinogen 4. Enteroendocrine - gut hormone secreting cells
What are the two intrinsic nerve plexuses of the gut?
1. Myenteric plexus 2. Submucosal plexus
What are the three phases of gastric HCl secretion?
1. The cephalic phase 2. The gastric phase 3. Intestinal phase
What are the two main neurotransmitters of the enteric system and what are their resultant effects?
Acetylcholine and nitric oxide Acetylcholine is excitatory Nitric oxide is inhibitory
What are the exocrine cells of the pancreas called and what do they secrete?
Acinar cells Secrete enzymes
What causes the colon in Hirschsprung's disease, to become enlarged?
As there are no nerves innervating that section of the colon, there is not mass movements, therefore they are unable to stimulate motility, rectal contractions and relax our internal sphincter to allow defecation.
Which part of the composition of bile is responsible for fat emulsification? and how do they do this?
Bile salt and lecithin (phospholipid) - These emulsifying agents are amphipathic molecules. The hydrophobic side attaches onto the fat whilst the hydrophilic side faces outwards towards the watery chyme. This results with a net negative charge. Both factors help segregate the large fat globule into small emulsion droplets.
How is the hirschsprung's disease treated?
By surgically removing the aganglionic section
Based on your lectures, explain how the acidic chyme released from the stomach is neutralized in the duodenum. What happens when this is not neutralized?
By the HCO3- secretions from the pancreatic ductal cells together with the HCO3- secretions from the bile ductal cells. This is triggered by the release of secretin from the enteric endocrine cells stimulated the presence of acidity in the duodenum. Without these secretions, there will be no activation of our digestive enzymes and significant damage would occur on our duodenal mucosa.
How are the inactive proteases activated?
By the membrane bound enterokinase found in the duodenal lumen. It converts trypsinogen to trypsin which in turn activates other enzymes to become active.
What causes the release of cholcystokinin and secretin?
CCK: Presence of fatty chyme entering the duodenum Secretin: Presence of acidic chyme entering the duodenum
Name the main 5 composition of bile and it's main function in the body.
COMPOSITIONS: 1. Bile salts 2. Cholesterol 3. Lecithin 4. Electrolytes and H2O (main HCO3-) 5. Bile pigment (biliverdin) FUNCTION: - To emulsify fats in the small intestine, assisting in lipase breakdown of the fats.
Where do the parietal cell get their H+ and Cl- from?
Cellular metabolism -> CO2 combines with H2O in parietal cell-> H2CO3- 1. H+ gained from H2CO3 and pumped out P-cell via proton pump (H+/K+ ATPase) in exchange for K+ -> K+ then diffuses out 2. Left over HCO3- exchanged with Cl- -> Cl- goes into parietal cell -> diffuses into lumen of stomach The H+ and Cl- makes HCl which makes the acidic environment in the stomach
SHORT ANSWER: Q. Explain the role(s) of the gut hormones cholecystokinin (CCK) and secretin in the emulsification and chemical digestion of fats.
Cholecystokinin stimulates the contraction of the gall bladder allowing bile to enter the duodenal lumen. Secretin acts on the bile and pancreatic ductal cells to secrete fluids containing HCO3-
SHORT ANSWER: Q. Fatty foods are retained in the stomach for longer periods (than starches). Describe how CCK slows the release of fatty chyme from stomach (to the duodenum)
Due to the process of emulsification, fats get digested slowly in the duodenum. Therefore to prevent more fatty chyme entering the duodenum, there is an inhibitory feedback that invovles CCK. CCK inhibits stomach contractions and constricts the pyloric sphincter. This results in the slow release of fatty chyme into the duodenal lumen.
How is excitation spread through the muscle layers of the intestine?
Electrical stimulation is spread acorrs the gut via gap junctions
How does co-lipase assist in the breakdown of triglycerides into fatty acids and monoglycerides?
Emulsion of large fat globules (composed of triglycerides) requires the emulsifying agents bile salts and lecithin. This makes it it difficult for lipase to breakdown central regions of the fat. Co-lipase assists by attaching itself onto the bile salts and moving them and allowing space for lipase to enter.
What is the difference between exocrine and endocrine secretions?
Exocrine are ductal secretions whilst endocrine are ductless
A mass movement in the colon is characterized by a wave of sustained contraction that travels along a considerable length of bowel in the orthograde direction (toward the anus). Mass movements occur in the colon after each period of intense contractions of the migrating myoelectric complex occurs in the ileum. True or False?
False The statement is false because there is no relation between the timing of the MMC in the small bowel and mass movements in the colon.
How are HCl secretions enhanced?
INCREASE By the activation of parietal receptors. - G-type receptors activated by Gastrin - H2 receptors activated by histamine - Muscarinic receptors activated by ACh Activation of the parietal receptors embeds preformed proton pumps onto the membrane of the parietal cell, enhancing gastric HCl secretions.
Explain why proton pump inhibitors (PPIs) are now considered more effective than antihistamines for the treatment of digestive disorders involving inappropriate acid secretion (e.g. gastric ulcers)?
Inappropriate acid secretions can degrade the mucosal lining of the stomach and can cause gasrtic ulcers. Acid secretions derived from the parietal cells of the stomach are enhanced by increasing hormones that attach to its receptors to increase the number of proton pump inhibotrs attached onto its membrane. Antihistamine cna reduce this enhancement by inhibitng one of its pathways (the histamine receptor), however other pathways, such as gastrin can still enhance HCl secretions. PPIs on the other hand are more effective as it not only blocks this histamine pathway but also the rest of the pathways, including gastrin, blocking HCl secretions from the parietal cells altogether.
How do we treat constipation?
Increasing fibre content in our the diet. As fibre passes through the intestine, it becomes hydrated and swells cuasing distension in the large intestine. This activates stretch receptors which stimulates secretion and motility.
What is pancreatitis?
Inflammation of the pancreas
What is Hirschsprung's disease (congenital aganglionic megacolon) and what causes this?
It is the enlargement of the colon resulting with a 'megacolon'. This is due lack of innervation of nerves in a section of the colon, commonly found at the bottom section.
What is Steatorrhoea and what causes this?
It is the excretion of abnormal quantities of fat found within the faeces. This is due to reduced absorption of fat by the intestine.
What is retrograde peristalsis?
It is when fecal matter in the rectum moves back up into the sigmoid colon. It waits for the next mass movement. It becomes more frequent as we continue to put it off however it results with hard faeces as we are increasing it's absorption of water.
What is the defecation reflex?
It is when there is both involuntary and voluntary processes occurs simultaneously. The involuntary process involves the autonomic innervation of the smooth muscles of the internal anal sphincter. The voluntary process involves the somatic nerve innervation of skeletal muscles of the external anal sphincter.
What influences the enterohepatic circulation?
Levels of bile salts. If there is a decrease return in bile salt levels, there is an resultant increase in bile salt synthesis. If there is an increase return in bile salt levels, there is a resultant decrease in bile salt synthesis.
What is the pathway for bile secretion (into the gut lumen)?
Liver -> Small bile ducts (attached to each lobe) -> Join at the common hepatic duct -> Cystic duct -> Gall bladder -> Common bile duct -> Hepatopancretic duct -> Sphincter of oddi -> Duodenal lumen
What is MMC, what is its purpose and when does it start/initiated and stop?
MMC stands for migrating myoelectric complex. It is the peristaltic like movement from the stomach to the large intestine. It's purpose is to move undigested remains to the large intestine. It begins when most of the meal has been absorbed and is initiated by increase in chym pH or increase in plasma motilin Stops when a meal enters the stomach.
What are micelles and are their structures always maintained? If not, why and how is it maintained?
Micelles are aggregates of the larger fat globules. Their structure as micelles are not always maintained as they are constantly breaking and reforming as it is dependent on the changing concentration gradient of the monoglyceride and fatty acids present. Their structure is maintained under alkalimic pH environment.
When the monoglycerides and fatty acids are absorbed, in what form are they released in?
Monoglycerides/fatty acids go into the absorptive cells and are made into triglycerides. They are packaged in the ER and released as chylomicrons.
What are 'mass movements' (a.k.a. power propulsion), where and when does it occur?
Occurs in the large intestine. It is peristaltic movement that moves fecal content down towards the sigmoid colon. It occurs when there is a distension of the stomach or the duodenum
Which one secretes more lipases, the stomach or the pancreas?
Pancreas
What is the enzyme that breaks down fats? And what is the resulting composition?
Pancreatic lipase breaks down triglycerides into monoglyceride and fatty acids.
Explain what would happen to carbohydrate digestion in a patient with pancreatitis.
Pancreatitis is the inflammation of the pancreas. This causes a decrease pancreatic in amylase production. Therefore, starch found in the ingested food is not digested/broken down.
What role does the parasympathetic nerve do in pepsin and acid production?
Parasympathetic excites pepsin and acid production
What's the difference in response with the autonomic innervation on intestinal smooth muscle and the smooth muscles found at the sphincters?
Parasympathetic nerve activation on the intestinal smooth muscle results with an excitatory response and increase muslce contractility whilst parasympathetic nerve activation activation on the smooth muscles of the sphincters causes a relaxation response.
How does the parietal cell secrete H+ against the large H+ concentration gradient?
Parietal cells have a lot of mitochondria
Why are antibiotics frequently prescribed to patients with gastric ulcers?
Patients with gastric ulcers are treated with antibiotics as it rids of the bacterium 'Helicobacter pylori'. This is because gastric ulcers are commonly caused by the presence of this bacteria in the stomach.
Name three protein digesting enzymes and where they are found in the body.
Pepsin - Stomach (pepsinogen) Trypsin - Pancreas (trypsinogen) Chymotrypsin - Pancreas juice (chymotrypsinogen)
What is the role of lipase?
Role: helps the body to digest fats
What is the role of amylase?
Role: to convert starch into glucose
What stimulates the cephalic phase of gastric HCl secretion?
STIMULUS - 5 sense: smell, sight, taste, touch, sound therefore increase in ACh and gastrin and activation of parasympathetic nerves
What stimulates the intestinal phase of gastric HCl secretion?
STIMULUS - Distension of the duodenum - Presence of acidity in intestinal lumen - Protein digestion therefore decrease in ACh and gastrin secretions and inhibition of vagal nuclei
What stimulates the gastric phase of gastric HCl secretion?
STIMULUS - Stomach distension - Partially digested protein - PH increase therefore increase ACh and gastrin secretions and enteric and autonomic reflexes (vagus)
The amount of starch in the bolus between the oesophagus and stomach remains the same depsite having mixed with amylase previously. Why is that?
Saliva amylase gets chemically digested by the pepsin. Therefore, we no longer have an active starch or carbohydrate enzyme.
What do the ductal cells of the pancreas secrete?
Secret bicarbonate
What do the sensory afferent nerves of the gut trigger and where is the information sent to?
Sent to: CNS Trigger: Skeletal muscles to salivate and shit
Where are amino acids being absorbed into the bloodstream?
Small intestine (bottom of the duodenum to the ileum)
How are HCl secretions decreased?
Somatostatin -> can indrectly inhibit release of other hormones Antihistamines -> blocks the histamine receptors Proton pump inhibitors (PPIs) -> Most effective as no proton pumps can embed onto the luminal membrane -> blocks all the other pathways
What is heartburn?
The burning pain that occurs when acidic gastric juices regurgitates into the oesophagus caused by failure of the lower oesophageal sphincter
What is the enterohepatic circulation?
The enterohepatic circulation involves conservation of bile salts by its reabsorption into blood and recirculated through the hepatic portal vein back into the liver.
What are haustral contractions and where does it occur?
The large intestine has sacs called haustrum. Haustral contractions is the slow contraction of these haustrums and occurs when the chyme enters the large intestine. This causes distension which activates the local myenteric nerves to trigger contraction. This constricts the circular smooth muscles and moves the chyme through the large intestine.
Where is the myenteric plexus located and what is it's main role?
The myenteric plexus is located between the smooth muscle cells of the gut. It's main role is to control motility.
A healthy 21 year old woman eats a large meal and then takes a 3 hour ride on a bus without a toilet. 20 minutes after eating, the woman feels a strong urge to defecate, but manages to hold it. What is happening with the rectum, the internal and external anal sphincter?
The rectum is contracting The internal anal sphincter is contracting The external anal sphincter is contracting
What do the autonomic nerves innervate in the gut?
The smooth muscle and the glands
Where is the submucosal plexus located and what is it's main role?
The submucosal plexus is located below the muscularis in a layer made of connective tissue called the submucosal plexus. It's main role is to control glandular secretions and absorption of nutrients
Which of the 7 sphincters of the digestive system are skeletal?
The upper oesophageal and the external sphincter
Where do the fats of the ingested food absorb and in what form do they absorb in?
They are absorbed in the small intestine through the absorptive epithelial cells. They are arb
What is segmenting contractions (segmentation)? where does it predominantly occur? and what causes it?
They are localised constrictions of the gut muscle which moves the bolus back and forth, allowing mixture of secretions. Predominantly occurs in the small intestine Activation of the parasympathetic nerves and activation of the local myenteric neurons.
Where does the secretin and cholecystokinin get secreted from?
They are secreted from the duodenal wall enteroendocrine cells.
Why do we need bile for the digestion of fats?
This is to emulsify them so that lipase is able to enter the emulsion droplet in order to break down the triglycerides into monoglyceride and fatty acids. If we had a fatty droplet, the lipase can on act on the surface of it.
What are the main digestive enzymes of the pancreas and what are their function?
Trpysin, chypotrypsin -> protein digestion Lipase -> Fats Amylase -> Polysaccharides Ribonuclrease, deoxyribonuclease -> Nucleic acid
SHORT ANSWER: Q. Is the removal of gall bladder likely to have any significant effects on normal digestive function? Explain your answer.
We are still able to digest fat. In between meals when we have the bile going into the gallbladder, it becomes highly concentrated as we get absorption of water across the gallbladder. We will still have continuous secretion of bile from the liver in response to the level of bile salts returning from the bloodstream via the hepatic portal vessel. What we lack is the controlled secretion of the bolus of concentrated bile. Therefore, patients with their gall bladder removed should be careful of their dietary intake of fats where small fat intake is preferred.
Removal of the gall bladder (cholecystectomy) is a common medical procedure. Explain how the absence of this organ from the body would affect lipid digestion.
We are still able to digest fat. Inbetween meals when we have the bile going into the gallbladder, it becomes highly concentrated as we get absorpttion of water acrossthe gallbladder. We will still have continuous sedcretion of bile from the liver in response to the level of bile salts returning from the bloodstream via the hepatic portal vessel. What we lack is the controlled secretion of the bolus of concentrated bile. Therefore, patients with their gall bladder removed should be careful of their dietary intake of fats where small fat intake is preffered.
What is the pH of the gastric juice and why are they so acidic?
pH = 2-3 (~1 with maximal HCl secretions) REASON: 1. Need to activate pepsinogen into pepsin 2. To sterilise our meal 3. Helps with breakdown of cellulose
