In the coronary steal phenomenon, vessel dilation is paradoxically harmful because blood is diverted from ischemic areas of the myocardium. Which of the following is responsible for the coronary steal phenomenon?
Q32
A 36-year-old woman is admitted to the hospital for the evaluation of progressive breathlessness. She has no history of major medical illness. Her temperature is 37°C (98.6°F), pulse is 110/min, and respirations are 22/min. Pulse oximetry on room air shows an oxygen saturation of 99%. Cardiac examination shows a loud S1 and S2. There is a grade 2/6 early systolic murmur best heard in the 2nd right intercostal space. Cardiac catheterization shows a mixed venous oxygen saturation of 55% (N= 65–70%). Which of the following is the most likely cause of this patient's breathlessness?
Q33
An investigator is studying the electrophysical properties of gastrointestinal smooth muscle cells using microelectrodes. He measures the resting membrane potential of a cell to be -70 mV. The equilibrium potentials of different ions involved in generating the membrane potential are shown.
ENa+ +65 mV
EK -85 mV
ECa2+ +120 mV
EMg2+ +10 mV
ECl- -85 mV
Which of the following is the most important contributor to the difference between the resting membrane potential and the equilibrium potential of potassium?
Q34
A 27-year-old woman presents to her doctor complaining of pain in her neck that radiates to her left ear. The pain has been more or less constant for the last 3 weeks and increases when she chews and swallows. She was in her normal state of health before the pain started. She also mentions that she has been experiencing palpitations, muscle weakness, and increased sweating for the last 2 weeks. Past medical history is significant for a flu-like illness 2 months ago. She currently takes no medication and neither consumes alcohol nor smokes cigarettes. Her pulse is 104/min and irregular with a blood pressure of 140/80 mm Hg. On examination, the physician notices that the patient is restless. There is a presence of fine tremors in both hands. The anterior neck is swollen, warm to the touch, and markedly tender on palpation. Thyroid function tests and a biopsy are ordered. Which of the following deviations from the normal is expected to be seen in her thyroid function tests?
Q35
A researcher measures action potential propagation velocity in various regions of the heart in a 42-year-old Caucasian female. Which of the following set of measurements corresponds to the velocities found in the atrial muscle, AV Node, Purkinje system, and ventricular muscle, respectively?
Q36
A 27-year-old man is running on the treadmill at his gym. His blood pressure prior to beginning his workout was 110/72. Which of the following changes in his cardiovascular system may be seen in this man now that he is exercising?
Q37
A group of investigators is performing a phase I trial of a novel drug among patients with chronic right upper quadrant pain. Iminodiacetic acid labeled with technetium 99m is administered intravenously and subjects are subsequently imaged with a gamma camera. It is found that administration of the experimental drug increases the amount of iminodiacetic acid in the intestines. The effect of this novel drug is most similar to that of a substance secreted by which of the following cells?
Q38
A 34-year-old woman comes to the emergency department because of decreased appetite, nausea, vomiting, and episodic abdominal pain for the past two months. The pain is sharp, colicky, and lasts about an hour after meals. Her stools are light in appearance and difficult to flush. Physical examination shows tenderness in the right upper quadrant. Without treatment, this patient is at greatest risk for developing which of the following?
Q39
A 48-year-old female visits your office complaining that she has trouble swallowing solids and liquids, has persistent bad breath, and sometimes wakes up with food on her pillow. Manometry studies show an absence of functional peristalsis and a failure of the lower esophageal sphincter to relax upon swallowing. The patient's disorder is associated with damage to which of the following?
Q40
A neurophysiology expert is teaching his students the physiology of the neuromuscular junction. While describing the sequence of events that takes place at the neuromuscular junction, he mentions that as the action potential travels down the motor neuron, it causes depolarization of the presynaptic membrane. This results in the opening of voltage-gated calcium channels, which leads to an influx of calcium into the synapse of the motor neuron. Consequently, the cytosolic concentration of Ca2+ ions increases. Which of the following occurs at the neuromuscular junction as a result of this increase in cytosolic Ca2+?
Cardiovascular US Medical PG Practice Questions and MCQs
Question 31: In the coronary steal phenomenon, vessel dilation is paradoxically harmful because blood is diverted from ischemic areas of the myocardium. Which of the following is responsible for the coronary steal phenomenon?
A. Venodilation
B. Microvessel dilation (Correct Answer)
C. Dilation of the large coronary arteries
D. Systemic arterial dilation
E. Volume loss of fluid in the periphery
Explanation: ***Microvessel dilation***
- The coronary steal phenomenon occurs when **vasodilators** are administered, causing dilation of **healthy coronary microvessels** and a decrease in resistance.
- This preferentially diverts blood flow away from already **ischemic areas** with maximally dilated intrinsic microvessels, worsening myocardial ischemia.
*Venodilation*
- **Venodilation** primarily reduces **preload** by increasing venous capacitance, not by directly altering coronary microcirculatory blood flow distribution in a way that causes "steal."
- While some vasodilators have venodilatory effects, this specific effect is not the mechanism behind coronary steal.
*Dilation of the large coronary arteries*
- Dilation of large coronary arteries alone doesn't cause the "steal" but rather improves overall blood flow. The critical issue is the differential response of **collateral** and **non-collateral microvessels**.
- **Stenoses** in large coronary arteries are the underlying pathology, but the steal phenomenon itself results from changes in **downstream microvascular resistance**.
*Systemic arterial dilation*
- **Systemic arterial dilation** primarily reduces afterload and can lower blood pressure, but it does not specifically explain the redistribution of coronary blood flow to the detriment of ischemic zones within the myocardium.
- The key to coronary steal is the **heterogeneity of response** at the microvascular level within the coronary circulation.
*Volume loss of fluid in the periphery*
- **Volume loss** in the periphery would influence overall circulatory dynamics and cardiac output but is not directly responsible for the **localized myocardial blood flow redistribution** characteristic of the coronary steal phenomenon.
- Coronary steal is a physiological process related to **vasoreactivity** and not hypovolemia.
Question 32: A 36-year-old woman is admitted to the hospital for the evaluation of progressive breathlessness. She has no history of major medical illness. Her temperature is 37°C (98.6°F), pulse is 110/min, and respirations are 22/min. Pulse oximetry on room air shows an oxygen saturation of 99%. Cardiac examination shows a loud S1 and S2. There is a grade 2/6 early systolic murmur best heard in the 2nd right intercostal space. Cardiac catheterization shows a mixed venous oxygen saturation of 55% (N= 65–70%). Which of the following is the most likely cause of this patient's breathlessness?
A. Increased peripheral shunting
B. Decreased hemoglobin concentration
C. Increased carbon dioxide retention
D. Increased pulmonary vascular resistance
E. Decreased left ventricular ejection fraction (Correct Answer)
Explanation: ***Decreased left ventricular ejection fraction***
- The key finding is a **mixed venous oxygen saturation of 55% (normal 65-70%)** with **normal arterial oxygen saturation (99%)**, which indicates **increased tissue oxygen extraction**
- Increased oxygen extraction occurs when **cardiac output is reduced** → tissues must extract more oxygen from each pass of blood to meet metabolic demands
- This is the classic physiologic compensation in **heart failure with reduced ejection fraction**
- The cardiac findings (loud heart sounds, systolic murmur) suggest underlying cardiac pathology causing reduced cardiac output and progressive breathlessness
*Increased peripheral shunting*
- Peripheral shunting (e.g., arteriovenous malformations) would cause **venous blood to bypass capillary beds**, resulting in **decreased oxygen extraction** and **higher mixed venous O2 saturation**, not lower
- Would typically cause **hypoxemia** with reduced pulse oximetry, but this patient has 99% oxygen saturation
*Decreased hemoglobin concentration*
- Anemia reduces oxygen-carrying capacity but would not explain the **low mixed venous oxygen saturation** to this degree
- The **pulse oximetry of 99%** indicates adequate oxygen saturation of available hemoglobin
- Anemia typically causes **high cardiac output** (compensatory) rather than the low cardiac output state indicated by the low mixed venous O2 saturation
*Increased carbon dioxide retention*
- **Hypercapnia** results from **hypoventilation** and impaired gas exchange, typically causing **respiratory acidosis**
- Would present with altered mental status, drowsiness, or signs of respiratory failure
- Does not explain the **low mixed venous oxygen saturation** with normal arterial oxygen saturation
- The cardiac findings point to a primary cardiac rather than respiratory problem
*Increased pulmonary vascular resistance*
- **Pulmonary hypertension** causes **right ventricular dysfunction** and can present with breathlessness and a loud P2 component of S2
- However, isolated pulmonary hypertension would not cause the same degree of **systemic oxygen extraction** increase
- The low mixed venous O2 saturation indicates **reduced systemic cardiac output**, which primarily reflects **left ventricular dysfunction** rather than isolated right-sided pathology
Question 33: An investigator is studying the electrophysical properties of gastrointestinal smooth muscle cells using microelectrodes. He measures the resting membrane potential of a cell to be -70 mV. The equilibrium potentials of different ions involved in generating the membrane potential are shown.
ENa+ +65 mV
EK -85 mV
ECa2+ +120 mV
EMg2+ +10 mV
ECl- -85 mV
Which of the following is the most important contributor to the difference between the resting membrane potential and the equilibrium potential of potassium?
A. Influx of Ca2+ ions
B. Influx of Cl- ions
C. Influx of Mg2+ ions
D. Influx of Na+ ions (Correct Answer)
E. Electrogenic effect of Na+/K+-ATPase
Explanation: ***Influx of Na+ ions***
- The resting membrane potential (-70 mV) is **more positive** than the potassium equilibrium potential (-85 mV) by approximately 15 mV. This difference exists because the membrane is **not exclusively permeable to K+**.
- At rest, there is a small but significant **Na+ permeability**, creating a continuous **Na+ leak** into the cell driven by both the concentration gradient and the electrical gradient (ENa = +65 mV is far more positive than RMP).
- This **depolarizing Na+ influx** pulls the resting membrane potential toward ENa, making it less negative than it would be if only K+ channels were open. This is the **primary contributor** to why RMP (-70 mV) is more positive than EK (-85 mV).
- The **Goldman-Hodgkin-Katz equation** demonstrates that RMP depends on the relative permeabilities and concentrations of all permeable ions, with Na+ leak being the key factor preventing RMP from equaling EK.
*Electrogenic effect of Na+/K+-ATPase*
- The **Na+/K+-ATPase** actively pumps **3 Na+ out** and **2 K+ in** per cycle, creating a net efflux of one positive charge with a small **hyperpolarizing effect** (approximately -3 to -5 mV).
- This would actually make the membrane potential **more negative**, moving it closer to or slightly below EK, not explaining why RMP is more positive than EK.
- While the pump is essential for maintaining ion gradients, its direct electrogenic contribution is small and acts in the opposite direction from what the question asks.
*Influx of Ca2+ ions*
- Influx of **Ca2+ ions** (ECa2+ = +120 mV) would cause **depolarization**, but Ca2+ permeability at rest is extremely low in most cells.
- While Ca2+ influx occurs during excitation in smooth muscle, it is not a significant contributor to the resting membrane potential difference from EK.
*Influx of Cl- ions*
- The equilibrium potential of **Cl- (-85 mV)** is essentially equal to EK. Chloride influx would tend to **stabilize** the membrane potential near -85 mV, not explain why it is more positive at -70 mV.
- In many smooth muscle cells, Cl- is near equilibrium at rest and does not significantly contribute to moving RMP away from EK.
*Influx of Mg2+ ions*
- The equilibrium potential of **Mg2+ (+10 mV)** is positive, but **Mg2+ permeability** at rest is negligible in most cell membranes.
- Mg2+ is primarily an intracellular ion that acts as a cofactor for enzymes and does not significantly contribute to resting membrane potential through membrane flux.
Question 34: A 27-year-old woman presents to her doctor complaining of pain in her neck that radiates to her left ear. The pain has been more or less constant for the last 3 weeks and increases when she chews and swallows. She was in her normal state of health before the pain started. She also mentions that she has been experiencing palpitations, muscle weakness, and increased sweating for the last 2 weeks. Past medical history is significant for a flu-like illness 2 months ago. She currently takes no medication and neither consumes alcohol nor smokes cigarettes. Her pulse is 104/min and irregular with a blood pressure of 140/80 mm Hg. On examination, the physician notices that the patient is restless. There is a presence of fine tremors in both hands. The anterior neck is swollen, warm to the touch, and markedly tender on palpation. Thyroid function tests and a biopsy are ordered. Which of the following deviations from the normal is expected to be seen in her thyroid function tests?
A. Normal Serum TSH, ↑ Total T4, Normal Free T4, Normal I131 Uptake
B. ↓ Serum TSH, ↑ Total T4, ↑ Free T4, ↑ I131 Uptake
C. Normal Serum TSH, ↓ Total T4, Normal Free T4, Normal I131 Uptake
D. ↑ Serum TSH, ↑ Total T4, ↑ Free T4, ↑ I 131 Uptake
E. ↓ Serum TSH, ↑ Total T4, ↑ Free T4, ↓ I131 Uptake (Correct Answer)
Explanation: ***↓ Serum TSH, ↑ Total T4, ↑ Free T4, ↓ I131 Uptake***
- This pattern is characteristic of **thyrotoxicosis** (increased thyroid hormone levels) combined with decreased thyroid gland activity, which is typical for conditions like **subacute thyroiditis** (as suggested by the flu-like illness and painful, tender thyroid).
- The elevated **T3/T4** (Total and Free) results from the release of preformed hormones from the inflamed thyroid, while the **low TSH** is due to negative feedback. The **decreased I-131 uptake** indicates that the thyroid gland is not actively synthesizing new hormones.
*Normal Serum TSH, ↑ Total T4, Normal Free T4, Normal I131 Uptake*
- This combination is inconsistent with the patient's symptoms of hyperthyroidism (palpitations, sweating, tremors) and a **tender, swollen thyroid**.
- **Normal Free T4** and **Normal TSH** would suggest euthyroid status, which is not what the clinical presentation indicates.
*↓ Serum TSH, ↑ Total T4, ↑ Free T4, ↑ I131 Uptake*
- This pattern suggests a hyperthyroid state where the thyroid gland is actively overproducing hormones, as seen in **Graves' disease** or toxic nodular goiter.
- However, subacute thyroiditis is characterized by **low I-131 uptake** because the thyroid gland is damaged and releases preformed hormones rather than actively synthesizing new ones.
*Normal Serum TSH, ↓ Total T4, Normal Free T4, Normal I131 Uptake*
- This combination is not indicative of the patient's hyperthyroid symptoms (- **palpitations, muscle weakness, increased sweating**).
- **Normal Free T4** and **Normal TSH** would suggest euthyroid status, which is inconsistent with the clinical picture.
*↑ Serum TSH, ↑ Total T4, ↑ Free T4, ↑ I 131 Uptake*
- An **elevated TSH** would indicate **primary hypothyroidism**, where the thyroid gland is underactive, and the pituitary tries to stimulate it.
- This contradicts the patient's clinical signs of **hyperthyroidism** (palpitations, sweating, tremors) and the elevated Total and Free T4.
Question 35: A researcher measures action potential propagation velocity in various regions of the heart in a 42-year-old Caucasian female. Which of the following set of measurements corresponds to the velocities found in the atrial muscle, AV Node, Purkinje system, and ventricular muscle, respectively?
A. 0.05 m/s, 1.1 m/s, 2.2 m/s, 3.3 m/s
B. 2.2 m/s, 0.3 m/s, 0.05 m/s, 1.1 m/s
C. 0.3 m/s, 2.2 m/s, 0.05 m/s, 1.1 m/s
D. 0.5 m/s, 1.1 m/s, 2.2 m/s, 3 m/s
E. 1.1 m/s, 0.05 m/s, 2.2 m/s, 0.3 m/s (Correct Answer)
Explanation: ***1.1 m/s, 0.05 m/s, 2.2 m/s, 0.3 m/s***
- This option correctly lists the approximate conduction velocities for the **atrial muscle (1.1 m/s)**, **AV node (0.05 m/s)**, **Purkinje system (2.2 m/s)**, and **ventricular muscle (0.3 m/s)**, respectively.
- The **AV node has the slowest conduction velocity (~0.05 m/s)**, which is crucial for delaying ventricular contraction and allowing complete ventricular filling.
- The **Purkinje system has the fastest conduction velocity (~2-4 m/s)**, ensuring rapid and coordinated ventricular depolarization.
- **Atrial muscle (~1 m/s)** and **ventricular muscle (~0.3-0.5 m/s)** have intermediate velocities.
*0.05 m/s, 1.1 m/s, 2.2 m/s, 3.3 m/s*
- This sequence is incorrect because it places the **AV node's velocity (0.05 m/s)** first (as atrial muscle) and significantly overestimates ventricular muscle velocity (3.3 m/s).
- Atrial muscle conducts faster than 0.05 m/s, and ventricular muscle velocity should be approximately 0.3-0.5 m/s, not 3.3 m/s.
*2.2 m/s, 0.3 m/s, 0.05 m/s, 1.1 m/s*
- This option incorrectly assigns the **highest velocity (2.2 m/s)** to atrial muscle, which is characteristic of the Purkinje system, and misplaces the **slowest velocity (0.05 m/s)** in the Purkinje system instead of the AV node.
- The values do not align with known physiological conduction speeds across cardiac tissues.
*0.3 m/s, 2.2 m/s, 0.05 m/s, 1.1 m/s*
- This sequence incorrectly places the **slowest velocity (0.05 m/s)** in the Purkinje system, which is known for the most rapid conduction, and assigns an unrealistically high velocity (2.2 m/s) to the AV node.
- The arrangement directly contradicts the physiological function and relative speeds within the cardiac conduction system.
*0.5 m/s, 1.1 m/s, 2.2 m/s, 3 m/s*
- This option underestimates the **atrial muscle velocity** (0.5 m/s instead of ~1 m/s) and significantly overestimates the **ventricular muscle velocity** (3 m/s instead of ~0.3-0.5 m/s).
- The provided values do not accurately represent the typical ranges of conduction velocities for each specified cardiac region.
Question 36: A 27-year-old man is running on the treadmill at his gym. His blood pressure prior to beginning his workout was 110/72. Which of the following changes in his cardiovascular system may be seen in this man now that he is exercising?
A. Decreased blood pressure
B. Decreased systemic vascular resistance (Correct Answer)
C. Increased systemic vascular resistance
D. Decreased stroke volume
E. Decreased heart rate
Explanation: ***Decreased systemic vascular resistance***
- During dynamic exercise, metabolic vasodilation in exercising muscles leads to a substantial **decrease in systemic vascular resistance (SVR)** to accommodate increased blood flow.
- This vasodilation overrides the systemic vasoconstriction driven by the sympathetic nervous system, resulting in a net decrease in overall SVR.
*Decreased blood pressure*
- While SVR decreases, **systolic blood pressure typically increases** during exercise due to increased cardiac output.
- **Diastolic blood pressure** usually remains stable or may slightly decrease, but overall blood pressure, specifically the mean arterial pressure, is generally maintained or elevated.
*Increased systemic vascular resistance*
- This is incorrect as **vasodilation in active muscles** causes a significant decrease in overall systemic vascular resistance.
- An increase in SVR would typically hinder blood flow to working muscles and is not a characteristic cardiovascular response to dynamic exercise.
*Decreased stroke volume*
- Stroke volume generally **increases significantly** during exercise due to enhanced venous return, increased contractility, and reduced afterload (from decreased SVR).
- A decreased stroke volume would limit cardiac output and exercise performance.
*Decreased heart rate*
- Heart rate **increases proportionally with exercise intensity** to boost cardiac output and oxygen delivery to active muscles.
- A decreased heart rate would counteract the body's physiological demand for increased blood flow during physical activity.
Question 37: A group of investigators is performing a phase I trial of a novel drug among patients with chronic right upper quadrant pain. Iminodiacetic acid labeled with technetium 99m is administered intravenously and subjects are subsequently imaged with a gamma camera. It is found that administration of the experimental drug increases the amount of iminodiacetic acid in the intestines. The effect of this novel drug is most similar to that of a substance secreted by which of the following cells?
A. Pancreatic D cells
B. Antral G cells
C. Jejunal I cells (Correct Answer)
D. Duodenal S cells
E. Duodenal K cells
Explanation: ***Jejunal I cells***
- The imaging study with **iminodiacetic acid (HIDA)** scan assesses **gallbladder function** and bile flow; increased intestinal HIDA suggests increased bile secretion into the duodenum.
- **I cells** in the jejunum secrete **cholecystokinin (CCK)**, which stimulates gallbladder contraction and the release of bile into the intestines, mimicking the drug's effect.
*Pancreatic D cells*
- **Pancreatic D cells** secrete **somatostatin**, a hormone that generally inhibits digestive processes, including bile secretion and gallbladder contraction.
- This effect is opposite to the experimental drug's action of increasing bile flow into the intestines.
*Antral G cells*
- **Antral G cells** secrete **gastrin**, which primarily stimulates gastric acid secretion by parietal cells and promotes gastric motility.
- Gastrin has a minimal direct effect on bile secretion or gallbladder contraction.
*Duodenal S cells*
- **S cells** in the duodenum secrete **secretin**, which primarily stimulates bicarbonate and water secretion from the pancreas and bile ducts, without a direct effect on gallbladder contraction or bile acid secretion.
- Secretin's main role is to neutralize gastric acid in the duodenum.
*Duodenal K cells*
- **K cells** in the duodenum and jejunum secrete **gastric inhibitory polypeptide (GIP)**, which primarily stimulates insulin release from pancreatic beta cells in response to glucose and inhibits gastric acid secretion.
- GIP does not directly stimulate bile flow or gallbladder contraction.
Question 38: A 34-year-old woman comes to the emergency department because of decreased appetite, nausea, vomiting, and episodic abdominal pain for the past two months. The pain is sharp, colicky, and lasts about an hour after meals. Her stools are light in appearance and difficult to flush. Physical examination shows tenderness in the right upper quadrant. Without treatment, this patient is at greatest risk for developing which of the following?
A. Steatohepatitis
B. Megaloblastic anemia
C. Delayed wound healing
D. Low bone mineral density (Correct Answer)
E. Glossitis
Explanation: ***Low bone mineral density***
- This patient presents with symptoms highly suggestive of **gallstones** (cholelithiasis) leading to **biliary obstruction** or **cholestasis**, evidenced by episodic postprandial colicky pain, nausea, vomiting, and light-colored, difficult-to-flush stools (steatorrhea due to **fat malabsorption**).
- Chronic fat malabsorption inhibits the absorption of **fat-soluble vitamins** (A, D, E, K). A deficiency in **vitamin D** is a significant risk factor for **low bone mineral density** and **osteoporosis** over time.
*Steatohepatitis*
- **Steatohepatitis** (fatty liver inflammation) is typically associated with **metabolic syndrome**, chronic alcohol use, or rapid weight loss, which are not directly indicated here.
- While gallstones can be linked to obesity, they do not directly cause steatohepatitis in the way described by the patient's acute symptoms.
*Megaloblastic anemia*
- **Megaloblastic anemia** is primarily caused by deficiencies in **vitamin B12** or **folate**.
- While malabsorption can lead to B12 deficiency (e.g., in pancreatic insufficiency affecting intrinsic factor activity), biliary obstruction directly impacts fat-soluble vitamin absorption, not primarily B12 or folate.
*Delayed wound healing*
- **Delayed wound healing** can be a symptom of **vitamin C** or **zinc deficiency**, or overall malnutrition.
- Although chronic malabsorption can lead to general malnutrition, vitamin D deficiency and its impact on bone health are a more specific and immediate long-term risk associated with the described symptoms.
*Glossitis*
- **Glossitis** (inflammation of the tongue) is typically associated with deficiencies in **vitamin B group** vitamins (e.g., B12, folate, niacin, riboflavin) or **iron deficiency**.
- While this patient might eventually develop general nutritional deficiencies, biliary obstruction leading to fat malabsorption does not directly or primarily cause these specific B vitamin deficiencies.
Question 39: A 48-year-old female visits your office complaining that she has trouble swallowing solids and liquids, has persistent bad breath, and sometimes wakes up with food on her pillow. Manometry studies show an absence of functional peristalsis and a failure of the lower esophageal sphincter to relax upon swallowing. The patient's disorder is associated with damage to which of the following?
A. Submucosal (Meissner’s) plexus
B. Muscularis mucosa
C. Myenteric (Auerbach’s) plexus (Correct Answer)
D. Lamina propria
E. Submucosa
Explanation: ***Myenteric (Auerbach’s) plexus***
- The clinical presentation, including **dysphagia for solids and liquids**, **regurgitation**, bad breath, and findings from manometry (absence of functional peristalsis, failure of LES relaxation), is classic for **achalasia**.
- Achalasia is characterized by the degeneration of ganglion cells in the **myenteric (Auerbach's) plexus** of the esophageal wall, leading to impaired esophageal motility and LES dysfunction.
*Submucosal (Meissner’s) plexus*
- The submucosal plexus is primarily involved in regulating **secretions and local blood flow** within the gastrointestinal tract.
- While important for GI function, damage to this plexus is not the primary cause of the severe motility disorders seen in achalasia.
*Muscularis mucosa*
- The muscularis mucosa is a thin layer of smooth muscle within the mucosa that causes **local movements of the mucosal folds**.
- It does not control the major peristaltic contractions of the esophagus or the relaxation of the lower esophageal sphincter.
*Lamina propria*
- The lamina propria is a layer of connective tissue within the mucosa, rich in **lymphoid tissue, blood vessels, and nerves**, providing structural support and immune function.
- It does not play a direct role in regulating esophageal peristalsis or LES function.
*Submucosa*
- The submucosa is a layer of connective tissue that contains the **submucosal plexus, blood vessels, and lymphatic vessels**.
- While it houses the submucosal plexus, the primary pathology in achalasia involves the myenteric plexus, which controls the muscularis propria responsible for peristalsis.
Question 40: A neurophysiology expert is teaching his students the physiology of the neuromuscular junction. While describing the sequence of events that takes place at the neuromuscular junction, he mentions that as the action potential travels down the motor neuron, it causes depolarization of the presynaptic membrane. This results in the opening of voltage-gated calcium channels, which leads to an influx of calcium into the synapse of the motor neuron. Consequently, the cytosolic concentration of Ca2+ ions increases. Which of the following occurs at the neuromuscular junction as a result of this increase in cytosolic Ca2+?
A. Generation of an end plate potential
B. Exocytosis of acetylcholine from the synaptic vesicles (Correct Answer)
C. Increased Na+ and K+ conductance of the motor end plate
D. Binding of Ca2+ ions to NM receptors
E. Release of Ca2+ ions into the synaptic cleft
Explanation: ***Exocytosis of acetylcholine from the synaptic vesicles***
- The increase in **cytosolic Ca2+** within the presynaptic terminal is the primary trigger for the fusion of **synaptic vesicles** filled with **acetylcholine (ACh)** with the presynaptic membrane.
- This fusion process, known as **exocytosis**, releases ACh into the **synaptic cleft**, initiating synaptic transmission.
*Generation of an end plate potential*
- The **end plate potential (EPP)** is generated *after* acetylcholine (ACh) is released into the synaptic cleft and binds to receptors on the motor end plate.
- This event occurs *following* the Ca2+-induced release of neurotransmitter, not as a direct result of the Ca2+ increase itself.
*Increased Na+ and K+ conductance of the motor end plate*
- Increased **Na+ and K+ conductance** across the motor end plate membrane is a direct consequence of acetylcholine binding to its receptors, which are **ligand-gated ion channels**.
- This change in conductance *generates the end plate potential*, occurring after ACh release.
*Binding of Ca2+ ions to NM receptors*
- **NM receptors** (nicotinic muscle receptors) are located on the **postsynaptic membrane** (motor end plate) and bind to **acetylcholine (ACh)**, not Ca2+ ions.
- Calcium's primary role in this context is presynaptic: triggering ACh release.
*Release of Ca2+ ions into the synaptic cleft*
- Calcium ions enter the **presynaptic terminal** from the synaptic cleft, and their increased cytosolic concentration within the presynaptic terminal drives neurotransmitter release.
- Calcium itself is not released *into* the synaptic cleft in this process; rather, it enters the presynaptic neuron from the cleft.
