Salivary secretion and function

Salivary secretion and function US Medical PG Question 1: A scientist is studying the mechanism by which the gastrointestinal system coordinates the process of food digestion. Specifically, she is interested in how distension of the lower esophagus by a bolus of food changes responses in the downstream segments of the digestive system. She observes that there is a resulting relaxation and opening of the lower esophageal (cardiac) sphincter after the introduction of a food bolus. She also observes a simultaneous relaxation of the orad stomach during this time. Which of the following substances is most likely involved in the process being observed here?

• A. Neuropeptide-Y
• B. Secretin
• C. Ghrelin
• D. Vasoactive intestinal polypeptide (Correct Answer)
• E. Motilin

Salivary secretion and function Explanation: ***Vasoactive intestinal polypeptide*** - **VIP (Vasoactive intestinal polypeptide)** is a neuropeptide that mediates **relaxation** of the **smooth muscle** in the gastrointestinal tract, including the **lower esophageal sphincter** and the **orad stomach**, facilitating the passage of food. - This relaxation is part of the **receptive relaxation** process, allowing the stomach to accommodate food without a significant increase in intragastric pressure. *Neuropeptide-Y* - **Neuropeptide-Y (NPY)** is primarily involved in stimulating **food intake** and **reducing energy expenditure**, acting as an orexigenic peptide. - It does not directly mediate the relaxation of the **lower esophageal sphincter** or **orad stomach** in response to food bolus distension. *Secretin* - **Secretin** is a hormone released in response to **acid in the duodenum** and primarily stimulates the pancreas to release **bicarbonate-rich fluid**. - Its main role is to neutralize stomach acid, not to mediate sphincter relaxation or stomach accommodation. *Ghrelin* - **Ghrelin** is known as the "**hunger hormone**" and primarily stimulates **appetite** and **growth hormone release**. - It does not play a direct role in the relaxation of the **lower esophageal sphincter** or **orad stomach** during swallowing. *Motilin* - **Motilin** promotes **gastric and intestinal motility** during the **interdigestive phase**, responsible for the migrating motor complex (MMC). - Its actions are generally prokinetic, rather than causing relaxation of the upper GI tract in response to a food bolus.

Salivary secretion and function US Medical PG Question 2: A 50-year-old woman with long-standing diabetes presents with severe, watery diarrhea that wakes her at night. Stool studies show normal osmotic gap and negative stool cultures. Colonoscopy is normal. Trial of fasting does not improve diarrhea. Gastric emptying study shows delayed emptying. What neurotransmitter deficiency in the enteric nervous system best explains both her gastric and colonic dysmotility?

• A. Acetylcholine
• B. Serotonin
• C. Nitric oxide (Correct Answer)
• D. Substance P
• E. Vasoactive intestinal peptide

Salivary secretion and function Explanation: ***Nitric oxide*** - **Nitric oxide (NO)** is the primary **inhibitory neurotransmitter** in the enteric nervous system responsible for mediating **receptive relaxation** of the stomach and smooth muscle inhibition during peristalsis. - In **diabetic autonomic neuropathy**, loss of nitrergic neurons leads to **gastroparesis** (failed pyloric relaxation) and **diabetic diarrhea** due to uncoordinated colonic motility and loss of descending inhibition. *Acetylcholine* - **Acetylcholine** is the primary **excitatory neurotransmitter** responsible for muscle contraction; a deficiency would likely lead to paralytic ileus rather than secretory-pattern diarrhea. - Though diabetic neuropathy affects cholinergic fibers, the hallmark of the inhibitory dysmotility seen in **gastroparesis** is specifically linked to the NO pathway. *Serotonin* - **Serotonin (5-HT)** is predominantly involved in the initiation of the **peristaltic reflex** and intestinal secretion via enterochromaffin cells. - While 5-HT levels can be altered in IBS, it is not the classic neurotransmitter deficiency cited for the specific combination of **delayed gastric emptying** and autonomic diarrhea in diabetics. *Substance P* - **Substance P** acts as a **co-transmitter** with acetylcholine to promote smooth muscle contraction and pain transmission. - Deficiency of Substance P would impair motor activity but does not explain the loss of **inhibitory control** and relaxation necessary for normal gastric emptying. *Vasoactive intestinal peptide* - **Vasoactive intestinal peptide (VIP)** is an inhibitory neurotransmitter that promotes intestinal secretion and smooth muscle relaxation. - While VIP is involved in relaxation, **Nitric oxide** is considered the more critical mediator for the **pyloric and fundal relaxation** that is specifically impaired in diabetic gastropathy.

Salivary secretion and function US Medical PG Question 3: A 25-year-old man undergoes extensive small bowel resection following trauma, leaving 40 cm of jejunum anastomosed to the ascending colon. The ileocecal valve is removed. Six months postoperatively, he requires total parenteral nutrition. Despite this, he develops calcium oxalate kidney stones. Evaluate the mechanism linking his intestinal resection to nephrolithiasis.

• A. Decreased urinary citrate from chronic metabolic acidosis
• B. Hyperparathyroidism from vitamin D malabsorption causing hypercalciuria
• C. Increased colonic absorption of oxalate due to enhanced permeability from bile salts
• D. Dehydration from chronic diarrhea concentrating urinary oxalate
• E. Increased enteric oxalate absorption from calcium binding to malabsorbed fatty acids (Correct Answer)

Salivary secretion and function Explanation: ***Increased enteric oxalate absorption from calcium binding to malabsorbed fatty acids*** - In **short bowel syndrome**, unabsorbed **fatty acids** in the gut compete with oxalate by binding to **calcium**, forming calcium soaps. - This leaves **oxalate** free and unbound in the intestinal lumen, significantly increasing its absorption in the **intact colon**, leading to **enteric hyperoxaluria**. *Decreased urinary citrate from chronic metabolic acidosis* - While **hypocitraturia** is a risk factor for stones due to diarrhea-induced metabolic acidosis, it is not the primary mechanism of **enteric hyperoxaluria** following fat malabsorption. - **Citrate** normally inhibits stone formation by binding calcium, but its absence does not explain the high levels of **urinary oxalate** seen in these patients. *Hyperparathyroidism from vitamin D malabsorption causing hypercalciuria* - Vitamin D malabsorption typically leads to **hypocalciuria**, as the body attempts to conserve calcium due to low levels and **secondary hyperparathyroidism**. - Stone formation in this context is driven by high **oxalate excretion**, not by high calcium levels in the urine. *Increased colonic absorption of oxalate due to enhanced permeability from bile salts* - While unabsorbed **bile salts** can irritate the colonic mucosa and slightly increase permeability, this is a secondary phenomenon. - The fundamental metabolic driver for stone formation is the **loss of calcium-oxalate binding** in the lumen due to fat malabsorption. *Dehydration from chronic diarrhea concentrating urinary oxalate* - Chronic diarrhea causes **low urine volume**, which concentrates all solutes, but this is a non-specific factor for all stone types. - The specific development of **calcium oxalate stones** in this patient is uniquely tied to the **malabsorptive biochemistry** resulting from the loss of the ileum.

Salivary secretion and function US Medical PG Question 4: A 68-year-old man with cirrhosis develops hepatic encephalopathy. He is started on lactulose and rifaximin. His mental status improves within 48 hours. Beyond decreasing ammonia-producing bacteria, lactulose exerts additional beneficial effects through altering colonic pH. Analyze how pH modification affects ammonia metabolism in the colon.

• A. Decreased pH enhances ammonia oxidation by colonocytes
• B. Low pH inhibits bacterial urease activity
• C. Low pH stimulates glutamine synthetase in colonic mucosa
• D. Acidic pH converts NH3 to NH4+ which is poorly absorbed (Correct Answer)
• E. Acidification promotes conversion of ammonia to urea

Salivary secretion and function Explanation: ***Acidic pH converts NH3 to NH4+ which is poorly absorbed*** - Lactulose is a non-absorbable disaccharide metabolized by colonic bacteria into **lactic and acetic acids**, which significantly lowers the **luminal pH**. - This acidic environment promotes the protonation of diffusible **ammonia (NH3)** into non-absorbable **ammonium (NH4+)**, a process known as **ion trapping**, which prevents its entry into the systemic circulation. *Decreased pH enhances ammonia oxidation by colonocytes* - Colonocytes do not primarily use oxidation as a mechanism for clearing ammonia; instead, they are often a source of ammonia via **glutamine metabolism**. - The primary benefit of low pH in **hepatic encephalopathy** is the physical trapping of ions in the lumen, not an alteration in cellular oxidative pathways. *Low pH inhibits bacterial urease activity* - While extreme pH changes can affect enzymes, the primary therapeutic mechanism of lactulose is the **acidification of the gut** and the resultant **protonation of ammonia**. - Rifaximin, not lactulose, is the agent primarily responsible for targeting and inhibiting the **ammonia-producing bacteria** themselves. *Low pH stimulates glutamine synthetase in colonic mucosa* - **Glutamine synthetase** is mainly active in the **liver, brain, and muscle** to neutralize ammonia; it is not the key mechanism of action for lactulose in the colon. - Low colonic pH focuses on the **extracellular conversion** of ammonia to ammonium rather than intracellular enzymatic synthesis. *Acidification promotes conversion of ammonia to urea* - The **urea cycle** occurs exclusively in the **liver** (hepatocytes) and is not stimulated by the acidification of the distal colon. - In patients with **cirrhosis**, the liver's ability to convert ammonia to urea is already severely compromised, which is why alternative clearance methods are required.

Salivary secretion and function US Medical PG Question 5: A 30-year-old woman presents with chronic diarrhea and a 15-pound weight loss. Stool studies show increased osmotic gap and pH of 5.0. Hydrogen breath test shows elevated hydrogen levels 30 minutes after lactose ingestion. Duodenal biopsy shows normal villous architecture with preserved brush border. What is the most likely enzyme deficiency?

• A. Maltase-glucoamylase
• B. Sucrase-isomaltase
• C. Trehalase
• D. Lactase (Correct Answer)
• E. Pancreatic amylase

Salivary secretion and function Explanation: ***Lactase*** - The positive **hydrogen breath test** following lactose ingestion is diagnostic of **lactase deficiency**, as undigested lactose is fermented by colonic bacteria to produce hydrogen gas. - The **increased osmotic gap** and **low stool pH (5.0)** reflect the presence of unabsorbed sugars and the production of **short-chain fatty acids** by intestinal flora. *Maltase-glucoamylase* - This enzyme deficiency is extremely rare and would primarily result in symptoms after consuming **starch or maltose**, rather than lactose. - A hydrogen breath test specifically using lactose would not be positive in isolated maltase deficiency. *Sucrase-isomaltase* - Deficiency in this enzyme causes intolerance to **sucrose (table sugar)** and starch, leading to symptoms early in childhood. - While it presents with similar osmotic diarrhea, it is not triggered by **lactose ingestion** or diagnosed via a lactose-specific breath test. *Trehalase* - Trehalase is an enzyme specifically required to digest **trehalose**, a sugar found in **mushrooms**. - Deficiency would cause symptoms only after mushroom consumption and would not explain a positive **lactose breath test**. *Pancreatic amylase* - Pancreatic amylase deficiency results in **maldigestion of complex carbohydrates** (starches) and is usually associated with **pancreatic insufficiency** and steatorrhea. - Unlike disaccharidase deficiencies, it would typically be accompanied by signs of **protein or fat malabsorption** and would not result in a normal duodenal biopsy appearance of the brush border.

Salivary secretion and function US Medical PG Question 6: A 42-year-old woman undergoes cholecystectomy. During surgery, the common bile duct is inadvertently ligated. Over the next week, she develops jaundice and pale stools. Laboratory studies show elevated direct bilirubin and alkaline phosphatase, but normal lipase. Despite complete biliary obstruction, she does not develop severe fat malabsorption. What physiologic mechanism best explains preserved fat digestion?

• A. Intestinal mucosal lipase compensates for bile salt deficiency
• B. Pancreatic lipase functions adequately without bile salts
• C. Salivary lipase becomes upregulated in biliary obstruction
• D. Gastric lipase can digest up to 30% of dietary fat (Correct Answer)
• E. Colonic bacteria produce enzymes that digest fat

Salivary secretion and function Explanation: ***Gastric lipase can digest up to 30% of dietary fat*** - **Gastric lipase** (secreted by chief cells) and **lingual lipase** are acid-stable enzymes that can hydrolyze dietary triglycerides into fatty acids and diglycerides without needing **bile salts** or **colipase**. - This secondary mechanism ensures that up to **10-30% of dietary fat** is digested even in cases of complete **biliary obstruction** or pancreatic insufficiency, preventing total malabsorption. *Intestinal mucosal lipase compensates for bile salt deficiency* - There is no significant "intestinal mucosal lipase" that acts on bulk dietary triglycerides to compensate for a lack of **bile-mediated emulsification**. - While enterocytes possess **intracellular lipases** for processing absorbed lipids, they do not facilitate significant **luminal digestion** of fat. *Pancreatic lipase functions adequately without bile salts* - **Pancreatic lipase** requires **bile salts** to create a large surface area via **emulsification** and for the formation of **micelles** to carry lipids to the brush border. - Without **colipase** and bile salts, pancreatic lipase is easily displaced from the fat droplet, leading to a significant decrease in its functional efficiency. *Salivary lipase becomes upregulated in biliary obstruction* - **Lingual lipase** does contribute to lipid digestion in the stomach, but there is no physiological evidence that it becomes significantly **upregulated** to compensate for biliary issues. - While it initiates digestion, its total capacity is limited compared to the primary role of the **pancreatic-biliary** axis. *Colonic bacteria produce enzymes that digest fat* - **Colonic bacteria** primarily digest undigested carbohydrates and proteins through **fermentation**, not lipids. - Excessive undigested fat reaching the colon usually results in **steatorrhea** and osmotic diarrhea rather than being utilized or digested by microflora.

Salivary secretion and function US Medical PG Question 7: A 55-year-old man with type 2 diabetes presents with early satiety, postprandial fullness, and nausea for 6 months. Gastric emptying scintigraphy shows 65% retention at 4 hours (normal <10%). His HbA1c is 9.2%. He takes metformin and glipizide. Endoscopy shows no mechanical obstruction. Which pharmacologic intervention addresses both his gastric dysmotility and glycemic control?

• A. Domperidone
• B. Erythromycin
• C. Bethanechol
• D. Metoclopramide (Correct Answer)
• E. GLP-1 receptor agonist

Salivary secretion and function Explanation: ***Metoclopramide*** - This is a **dopamine D2 receptor antagonist** and **5-HT4 agonist** that acts as a prokinetic agent, facilitating **gastric emptying** and providing antiemetic relief. - It addresses both symptoms and glycemic control indirectly, as improved motility ensures more predictable absorption of glucose and oral medications, reducing **postprandial glucose variability**. *Domperidone* - While it is an effective peripherally acting **D2 antagonist** for gastroparesis, it is generally considered second-line and is often restricted due to **QT prolongation** risks. - Unlike some other options, it lacks the broader systemic regulatory approval found in the primary choice for initial treatment of **diabetic gastroparesis**. *Erythromycin* - This **motilin receptor agonist** is a potent prokinetic usually reserved for **acute flares** or short-term use due to rapid development of **tachyphylaxis**. - It is not suitable for long-term management of chronic gastroparesis because its effectiveness diminishes significantly within weeks. *Bethanechol* - A **cholinergic agonist** that can stimulate gastrointestinal contraction; however, it lacks the specific **dopaminergic antagonism** required to effectively treat nausea and vomiting. - It is rarely used in clinical practice for gastroparesis due to frequent **parasympathetic side effects** like cramping, salivation, and blurred vision. *GLP-1 receptor agonist* - Although these agents are excellent for **glycemic control** and weight loss, they are known to **delay gastric emptying** as part of their mechanism. - Using a GLP-1 agonist in a patient with confirmed **gastroparesis (65% retention)** would likely exacerbate symptoms of early satiety and nausea.

Salivary secretion and function US Medical PG Question 8: A 28-year-old man presents with difficulty swallowing solids and liquids for 2 years. Esophageal manometry shows aperistalsis in the distal esophagus and incomplete lower esophageal sphincter (LES) relaxation with wet swallows. Resting LES pressure is 45 mmHg (normal 10-30 mmHg). What is the underlying pathophysiologic defect?

• A. Autoantibodies against nicotinic acetylcholine receptors
• B. Loss of inhibitory nitric oxide-producing neurons in myenteric plexus (Correct Answer)
• C. Fibrosis of LES smooth muscle from chronic inflammation
• D. Hyperactivity of excitatory cholinergic neurons
• E. Increased sensitivity of LES smooth muscle to gastrin

Salivary secretion and function Explanation: ***Loss of inhibitory nitric oxide-producing neurons in myenteric plexus*** - **Achalasia** is caused by the degeneration of high-threshold **inhibitory ganglion cells** in the **myenteric (Auerbach) plexus** of the distal esophagus. - The loss of **nitric oxide (NO)** and **vasoactive intestinal peptide (VIP)** mediators prevents the **lower esophageal sphincter (LES)** from relaxing during swallowing. *Autoantibodies against nicotinic acetylcholine receptors* - This is the primary mechanism of **Myasthenia Gravis**, which affects the **neuromuscular junction** of skeletal muscle. - While it can cause swallowing difficulties (dysphagia), it would not lead to **smooth muscle aperistalsis** or elevated **LES pressure**. *Fibrosis of LES smooth muscle from chronic inflammation* - This describes the process seen in **Systemic Sclerosis (Scleroderma)**, where collagen deposition leads to a weakened, **hypotensive LES**. - In contrast to achalasia, scleroderma typically presents with severe **gastroesophageal reflux** due to a lack of LES tone. *Hyperactivity of excitatory cholinergic neurons* - While there is relatively **unopposed cholinergic activity** in achalasia, the primary defect is the **loss of inhibition** rather than a primary hyperactive state of excitatory neurons. - Excessive cholinergic activity would typically present as **diffuse esophageal spasm**, characterized by high-pressure, non-peristaltic contractions. *Increased sensitivity of LES smooth muscle to gastrin* - While **gastrin** does increase LES tone, the manometric findings of **aperistalsis** and **incomplete relaxation** are not explained by hormonal sensitivity. - The manometric profile in this patient is pathognomonic for a **neuromuscular defect** in the enteric nervous system, not a metabolic or endocrine hypersensitivity.

Salivary secretion and function US Medical PG Question 9: A 35-year-old woman with Crohn disease involving the terminal ileum undergoes ileocecal resection. Six months later, she develops chronic watery diarrhea. Serum testing shows vitamin B12 deficiency and elevated 7α-hydroxy-4-cholesten-3-one (C4). Colonoscopy shows normal mucosa. What is the primary mechanism of her diarrhea?

• A. Lactose intolerance from mucosal injury
• B. Bacterial overgrowth from loss of ileocecal valve
• C. Short bowel syndrome with rapid transit
• D. Decreased bile salt reabsorption causing colonic secretion (Correct Answer)
• E. Pancreatic insufficiency from chronic inflammation

Salivary secretion and function Explanation: ***Decreased bile salt reabsorption causing colonic secretion*** - Resection of the **terminal ileum** eliminates the primary site for active **bile salt reabsorption**, leading to **bile acid malabsorption**. - Excess bile acids enter the **colon**, where they stimulate **secretory diarrhea** by inducing water and electrolyte secretion into the intestinal lumen. *Lactose intolerance from mucosal injury* - This typically presents with **osmotic diarrhea**, bloating, and flatulence following dairy consumption, usually due to **lactase deficiency**. - The colonoscopy in this patient showed **normal mucosa**, which makes secondary lactose intolerance from active inflammation unlikely. *Bacterial overgrowth from loss of ileocecal valve* - **Small Intestinal Bacterial Overgrowth (SIBO)** can occur after ileocecal resection, but usually presents with nutrient malabsorption and **steatorrhea**. - While it can cause Vitamin B12 deficiency, the high **C4 levels** (a marker of hepatic bile acid synthesis) specifically point toward compensative bile acid production due to malabsorption. *Short bowel syndrome with rapid transit* - **Short bowel syndrome** generally occurs after much more extensive resections (usually >200cm) rather than a focused **ileocecal resection**. - Diarrhea in this condition is often **fatty (steatorrhea)** and involves broader malnutrition beyond isolated B12 deficiency. *Pancreatic insufficiency from chronic inflammation* - **Pancreatic insufficiency** causes maldigestion of fats and proteins, leading to heavy, oily stools known as **steatorrhea**. - There is no clinical or laboratory evidence provided, such as low **fecal elastase**, to suggest a primary pancreatic pathology.

Salivary secretion and function US Medical PG Question 10: A 62-year-old woman undergoes truncal vagotomy and antrectomy for refractory peptic ulcer disease. Three months postoperatively, she reports explosive diarrhea and cramping 30 minutes after meals, particularly after consuming sweets. Her symptoms are temporarily relieved by lying down. What physiologic mechanism best explains her symptoms?

• A. Impaired pancreatic enzyme secretion from vagal denervation
• B. Rapid gastric emptying causing osmotic fluid shift into small intestine (Correct Answer)
• C. Bile salt malabsorption causing secretory diarrhea
• D. Decreased gastric acid production leading to bacterial overgrowth
• E. Loss of receptive relaxation causing increased intragastric pressure

Salivary secretion and function Explanation: ***Rapid gastric emptying causing osmotic fluid shift into small intestine*** - This patient is experiencing **early dumping syndrome**, a common complication following **gastric bypass** or **antrectomy**, where the loss of the pyloric sphincter allows food to enter the duodenum too quickly. - The presence of **hypertonicity** (especially from sweets) in the small bowel lumen draws fluid from the intravascular space, leading to **intestinal distention**, cramping, and **osmotic diarrhea**. *Impaired pancreatic enzyme secretion from vagal denervation* - While **vagal denervation** can reduce the stimulatory phase of pancreatic secretion, it typically results in **steatorrhea** (fatty stools) rather than explosive diarrhea 30 minutes after meals. - This mechanism does not explain the **vasomotor symptoms** or the immediate relief found by lying down often associated with osmotic shifts. *Bile salt malabsorption causing secretory diarrhea* - **Bile salt malabsorption** usually occurs following ileal resection or dysfunction, leading to **choleretic diarrhea** that is typically chronic and not specifically triggered by sweets. - Though vagotomy can affect gallbladder motility, it doesn't cause the rapid **osmotic fluid shift** seen within 30 minutes of a meal. *Decreased gastric acid production leading to bacterial overgrowth* - **Small Intestinal Bacterial Overgrowth (SIBO)** can happen due to hypochlorhydria, but it usually presents with **bloating** and **vitamin B12 deficiency** over a longer period. - It does not present with the acute, post-prandial "dumping" symptoms triggered by high-carbohydrate intake. *Loss of receptive relaxation causing increased intragastric pressure* - Although **vagotomy** does eliminate **receptive relaxation**, leading to increased pressure in the proximal stomach, this refers to the storage function rather than the primary mechanism of the diarrhea. - The symptomatic "explosive" nature of the diarrhea is specifically due to the **osmotic load** in the intestine, not just the pressure within the stomach itself.

Salivary secretion and function Flashcard 1 of 6

Question: satiety

Answer: ventromedial area of the hypothalamus

Salivary secretion and function Flashcard 2 of 6

Question: What is the function of the area postrema?

Answer: emetic center of brain

Salivary secretion and function Flashcard 3 of 6

Question: Give two cell types rich in rough endoplasmic reticulum (RER).

Answer: intestinal goblet cells, plasma cells

Salivary secretion and function Flashcard 4 of 6

Question: Is the plasma membrane symmetric or asymmetric?

Answer: asymmetric interior vs exterior asymmetric regionally on same side

Salivary secretion and function Flashcard 5 of 6

Question: What type of cytoskeletal element is most associated with adherens junctions?

Answer: microfilaments (actin and myosin)

Salivary secretion and function Flashcard 6 of 6

Question: What are the three classes of cytoskeletal elements?

Answer: microfilaments, intermediate filaments, microtubules