General Physiology — MCQs

General Physiology Indian Medical PG Practice Questions and MCQs

Question 81: What is the mechanism of action of nitric oxide (NO)?

A. Through protein kinase C
B. Through the IP3-DAG system
C. Through cGMP (Correct Answer)
D. Through intracellular receptors

Explanation: **Explanation:** Nitric Oxide (NO), also known as Endothelium-Derived Relaxing Factor (EDRF), is a potent vasodilator that functions as a gaseous signaling molecule. **Why Option C is Correct:** The mechanism of action follows a specific sequence: 1. NO is synthesized from **L-arginine** by the enzyme Nitric Oxide Synthase (NOS). 2. Being lipid-soluble, it diffuses across cell membranes into target cells (e.g., vascular smooth muscle). 3. It binds to and activates the enzyme **Soluble Guanylyl Cyclase (sGC)**. 4. This activation increases the conversion of GTP to **cyclic GMP (cGMP)**. 5. cGMP activates **Protein Kinase G (PKG)**, which leads to a decrease in intracellular calcium and dephosphorylation of myosin light chains, resulting in **smooth muscle relaxation**. **Why Other Options are Incorrect:** * **Options A & B:** Protein Kinase C and the IP3-DAG system are secondary messengers for the Gq-protein coupled receptor pathway (e.g., used by Oxytocin or Alpha-1 receptors). NO bypasses surface receptors entirely. * **Option D:** While NO acts "intracellularly," it does not bind to classical "intracellular receptors" (like steroid or thyroid receptors that act as transcription factors). It acts directly on the enzyme guanylyl cyclase. **High-Yield NEET-PG Pearls:** * **Sildenafil (Viagra):** Inhibits **Phosphodiesterase-5 (PDE-5)**, the enzyme that breaks down cGMP, thereby prolonging the vasodilatory effect of NO. * **Nitroglycerin:** Acts as a prodrug that is denitrated to release NO, providing rapid relief in angina. * **Enzymes:** Note the three isoforms of NOS: nNOS (neuronal), eNOS (endothelial), and iNOS (inducible/macrophages).

Question 82: Which factor does not affect the diffusion of a substance through the cell membrane?

A. Molecular size of the substance
B. Lipid solubility of the membrane
C. ATP generating capacity on both sides of the membrane (Correct Answer)
D. Distribution of channels for the substance in the membrane

Explanation: **Explanation:** The core concept tested here is the difference between **Passive Diffusion** and **Active Transport**. **Why Option C is correct:** Diffusion is a passive process driven by the kinetic energy of molecules moving down a concentration or electrochemical gradient. According to **Fick’s Law of Diffusion**, this process does not require metabolic energy (ATP). Therefore, the ATP-generating capacity of the cell or its surroundings has no impact on the rate of simple or facilitated diffusion. ATP is only required for primary active transport (e.g., Na⁺-K⁺ ATPase pump). **Analysis of Incorrect Options:** * **A. Molecular size:** Smaller molecules diffuse more rapidly than larger ones. According to Graham’s Law, the diffusion rate is inversely proportional to the square root of the molecular weight. * **B. Lipid solubility:** The cell membrane is a phospholipid bilayer. Substances with high lipid solubility (e.g., O₂, CO₂, steroid hormones) can dissolve directly through the lipid matrix, significantly increasing their diffusion rate. * **C. Distribution of channels:** For non-lipid soluble substances (ions like Na⁺, K⁺), diffusion depends on the presence and density of specific protein channels or carriers (facilitated diffusion). **High-Yield NEET-PG Pearls:** * **Fick’s Law Equation:** $J = -DA (\Delta C / \Delta X)$. Diffusion is directly proportional to Surface Area (A), Diffusion Coefficient (D), and Concentration Gradient ($\Delta C$), but inversely proportional to Membrane Thickness ($\Delta X$). * **Rate-limiting factor:** For simple diffusion, the limit is the concentration gradient; for facilitated diffusion, the limit is the **Vmax** (saturation of carrier proteins). * **Gases:** Oxygen and Carbon dioxide diffusion is strictly perfusion-limited under normal physiological conditions.

Question 83: Osmotic pressure can be calculated by using which law?

A. Bernoulli's law
B. Van't Hoff law (Correct Answer)
C. Ohm's law
D. Poiseuille's law

Explanation: **Explanation:** **Correct Answer: B. Van't Hoff law** Osmotic pressure ($\pi$) is the pressure required to stop the net movement of water across a semipermeable membrane. According to **Van't Hoff’s Law**, osmotic pressure is directly proportional to the molar concentration of solutes and the absolute temperature. The formula is expressed as: **$\pi = nCRT$** *(Where $n$ = number of particles, $C$ = molar concentration, $R$ = gas constant, and $T$ = absolute temperature).* In physiology, this law helps determine the tonicity of body fluids and how solutes like albumin maintain oncotic pressure in the plasma. **Why other options are incorrect:** * **Bernoulli’s Law:** Relates to fluid dynamics, stating that as the speed of a moving fluid increases, the pressure within the fluid decreases. It explains phenomena like the *Venturi effect* in narrowed heart valves. * **Ohm’s Law:** Describes the relationship between voltage, current, and resistance ($V=IR$). In physiology, it is applied to blood flow: **Flow = Pressure Gradient / Resistance**. * **Poiseuille’s Law:** Determines the resistance to laminar flow in a vessel. It states that resistance is inversely proportional to the **fourth power of the radius** ($R \propto 1/r^4$), making vessel diameter the most significant factor in determining peripheral resistance. **High-Yield Clinical Pearls for NEET-PG:** 1. **Reflection Coefficient ($\sigma$):** A value between 0 and 1 that describes how easily a solute crosses a membrane. If $\sigma = 1$ (e.g., albumin), the solute is "impermeant" and exerts full osmotic pressure. 2. **Plasma Osmolality:** Normal range is **280–295 mOsm/kg**. It is primarily determined by Sodium ($Na^+$), Glucose, and BUN. 3. **Formula for Plasma Osmolality:** $2[Na^+] + \text{Glucose}/18 + \text{BUN}/2.8$.

Question 84: What is the approximate number of Golgi tendon organs per 100 muscle fibers?

A. 20-Jan (Correct Answer)
B. 200-400
C. 50-60
D. 80-100

Explanation: **Explanation:** The correct answer is **A (1–20)**. (Note: The option "20-Jan" in the prompt appears to be a typographical error for the range **1 to 20**). **1. Underlying Medical Concept:** Golgi Tendon Organs (GTOs) are encapsulated sensory receptors located at the junction of muscle fibers and tendons (musculotendinous junction). Unlike muscle spindles, which are arranged in parallel, GTOs are arranged **in series** with muscle fibers. Physiologically, GTOs are much less numerous than muscle fibers. On average, one GTO is associated with approximately **10 to 15 muscle fibers**, which translates to a ratio of roughly **5 to 10 GTOs per 100 muscle fibers** (falling within the 1–20 range). They function as force detectors, responding to increased muscle tension to prevent over-exertion via the **inverse stretch reflex** (Ib inhibition). **2. Analysis of Incorrect Options:** * **B (200-400):** This number is far too high. If this were true, there would be more GTOs than muscle spindles, which contradicts the anatomical distribution where spindles are the primary sensory apparatus. * **C (50-60) & D (80-100):** These options overestimate the density of GTOs. High densities are not required because GTOs are exquisitely sensitive to the tension produced by even a single motor unit. **3. NEET-PG High-Yield Pearls:** * **Arrangement:** Muscle Spindles = Parallel (detect length); GTOs = Series (detect tension). * **Afferent Fiber:** GTOs utilize **Type Ib** sensory fibers (fast-conducting). * **Reflex:** GTOs mediate the **autogenic inhibition** (inverse myotatic reflex), which causes the muscle to relax when tension becomes excessive. * **Location:** Specifically found in the tendons, stimulated by both passive stretch and active contraction (though more sensitive to active contraction).

Question 85: Parasympathetic stimulation of the heart accompanied by a withdrawal of sympathetic tone to most of the blood vessels of the body is characteristic of?

A. The fight-or-flight response
B. Vasovagal syncope (Correct Answer)
C. Exercise
D. The diving response

Explanation: **Explanation:** The correct answer is **Vasovagal syncope**. This condition is characterized by a sudden, transient loss of consciousness due to a paradoxical autonomic reflex. **1. Why Vasovagal Syncope is Correct:** The pathophysiology involves a "mismatch" in autonomic signaling, often triggered by emotional stress or prolonged standing. It involves two primary components: * **Cardioinhibitory response:** Increased **parasympathetic (vagal) tone** to the heart, leading to profound bradycardia. * **Vasodepressor response:** Sudden **withdrawal of sympathetic tone** to blood vessels, causing widespread vasodilation. Together, these result in a sharp drop in cardiac output and peripheral resistance, leading to cerebral hypoperfusion and fainting. **2. Why the other options are incorrect:** * **A. Fight-or-flight response:** This is characterized by massive **sympathetic activation** (increased heart rate and vasoconstriction), not parasympathetic stimulation. * **C. Exercise:** This involves a global increase in sympathetic activity to increase heart rate and redirect blood flow to muscles. * **D. The diving response:** While this involves parasympathetic-mediated bradycardia, it is accompanied by **intense sympathetic vasoconstriction** to peripheral limbs (to conserve oxygen for the brain/heart), not sympathetic withdrawal. **3. High-Yield Clinical Pearls for NEET-PG:** * **Bezold-Jarisch Reflex:** This is the specific inhibitory reflex involved in vasovagal syncope, where mechanoreceptors in the left ventricle (C-fibers) respond to low filling by triggering bradycardia and hypotension. * **Prodromal symptoms:** Patients often experience nausea, pallor, and diaphoresis (sweating) before the syncopal episode. * **Management:** The first-line treatment is the "Trendelenburg position" (elevating the legs) to increase venous return.

Question 86: Which of the following is a relaxation protein?

A. Actin
B. Myosin
C. Tropomyosin (Correct Answer)
D. Dystrophin

Explanation: ### Explanation **Why Tropomyosin is the Correct Answer:** Tropomyosin is referred to as a **relaxation protein** because of its regulatory role in the skeletal muscle contraction cycle. In a resting (relaxed) muscle, tropomyosin molecules lie in the grooves of the actin filament, physically masking the active binding sites for myosin. By preventing the interaction between actin and myosin, tropomyosin maintains the muscle in a state of relaxation. Contraction only occurs when calcium binds to Troponin C, causing a conformational change that shifts tropomyosin away from these binding sites. **Analysis of Incorrect Options:** * **Actin (Option A):** Known as the **thin filament**, actin is a contractile protein. It provides the binding sites for myosin heads to form cross-bridges. * **Myosin (Option B):** Known as the **thick filament**, myosin is the primary contractile protein. It possesses ATPase activity and acts as the "molecular motor" that pulls actin filaments during the power stroke. * **Dystrophin (Option D):** This is a **cytoskeletal/structural protein**. It anchors the actin cytoskeleton of the muscle fiber to the surrounding extracellular matrix through the cell membrane. It does not directly regulate the contraction-relaxation cycle. **High-Yield NEET-PG Pearls:** * **Regulatory Proteins:** Tropomyosin and Troponin (I, T, and C) are the two main regulatory proteins. * **Troponin I:** Inhibits the actin-myosin interaction. * **Troponin T:** Binds the troponin complex to tropomyosin. * **Clinical Correlation:** Mutations in the **Dystrophin** gene lead to Duchenne Muscular Dystrophy (DMD), the most common hereditary neuromuscular disease. * **LMM vs HMM:** Myosin can be cleaved into Light Meromyosin (LMM) and Heavy Meromyosin (HMM); the HMM contains the S1 head responsible for ATP hydrolysis.

Question 87: What is the action of the alpha subunit of a G protein?

A. Hydrolysis of GTP to GDP (Correct Answer)
B. Exchange of GDP for GTP
C. Receptor internalization
D. Agonist binding to the receptor

Explanation: **Explanation:** The G-protein complex is a heterotrimer consisting of **alpha (α), beta (β), and gamma (γ)** subunits. The **alpha subunit** is the functional engine of this complex and possesses intrinsic enzymatic activity. **1. Why Option A is Correct:** The alpha subunit acts as a **GTPase**. Once the G-protein is activated (bound to GTP), it triggers downstream effectors. To prevent continuous signaling, the alpha subunit eventually **hydrolyzes the bound GTP into GDP** and inorganic phosphate. This hydrolysis acts as a "molecular switch" that turns the protein off, allowing the alpha subunit to re-associate with the βγ complex and return to its inactive state. **2. Why Other Options are Incorrect:** * **Option B:** The exchange of GDP for GTP is facilitated by the **activated receptor** (acting as a Guanine Nucleotide Exchange Factor or GEF), not the alpha subunit itself. This step activates the G-protein. * **Option C:** Receptor internalization (downregulation) is typically mediated by **β-arrestins** and clathrin-coated pits following phosphorylation by G-protein-coupled receptor kinases (GRKs). * **Option D:** Agonist binding occurs at the **extracellular domain** of the G-Protein Coupled Receptor (GPCR), which then induces a conformational change to activate the intracellular G-protein. **High-Yield Clinical Pearls for NEET-PG:** * **Cholera Toxin:** Inhibits the GTPase activity of the **Gs alpha subunit**, leading to constitutive activation of Adenylyl Cyclase and permanent "on" state (causing secretory diarrhea). * **Pertussis Toxin:** Prevents the GDP-GTP exchange on the **Gi alpha subunit**, keeping it in the "off" state (inhibiting the inhibitor). * **Gs** stimulates Adenylyl Cyclase (increases cAMP); **Gi** inhibits it; **Gq** activates Phospholipase C (increases $IP_3/DAG$).

Question 88: If a region or compartment is in a steady state with respect to a particular substance, then:

A. The amount of the substance in the compartment is increasing
B. The amount of the substance in the compartment is decreasing
C. The amount of the substance in the compartment does not change with respect to time (Correct Answer)
D. There is no movement into or out of the compartment

Explanation: ### Explanation **1. Why Option C is Correct:** In physiology, a **Steady State** is defined as a condition where the variables of a system remain constant over time. For a specific substance in a compartment, this occurs when the **rate of influx (input) equals the rate of efflux (output)**. Because the net change is zero, the total amount or concentration of the substance remains stable. Unlike equilibrium (which is a passive state), a steady state often requires the continuous expenditure of energy (ATP) to maintain this stability (e.g., the Sodium-Potassium pump maintaining ionic gradients). **2. Why Incorrect Options are Wrong:** * **Options A & B:** If the amount were increasing or decreasing, the system would be in a **dynamic state of change** or "non-steady state." This implies an imbalance between input and output. * **Option D:** This is a common misconception. Steady state does **not** mean there is no movement. It means that the movement *into* the compartment is exactly balanced by the movement *out* of it. If there were no movement at all, the system would be static, which is rarely the case in living biological systems. **3. NEET-PG High-Yield Pearls:** * **Steady State vs. Equilibrium:** In *Equilibrium*, there are no gradients and no energy expenditure. In *Steady State*, gradients exist (e.g., high ECF $Na^+$, high ICF $K^+$), but they are kept constant through active processes. * **Homeostasis:** This is the overarching biological process that utilizes various steady-state mechanisms to maintain the "Milieu Intérieur" (Internal Environment). * **Pharmacokinetics Link:** In pharmacology, a drug reaches a steady state after approximately **4 to 5 half-lives**, where the rate of drug administration equals the rate of drug elimination.

Question 89: What is the primary mechanism by which catecholamines stabilize blood glucose concentration during hypoglycemia?

A. Stimulation of glycogen phosphorylase to release glucose from muscle.
B. Inhibition of glycogenolysis in the liver.
C. Stimulation of insulin release from the pancreas.
D. Stimulation of gluconeogenesis in the liver. (Correct Answer)

Explanation: ### Explanation **Correct Answer: D. Stimulation of gluconeogenesis in the liver.** **Mechanism:** Catecholamines (Epinephrine and Norepinephrine) act as counter-regulatory hormones during hypoglycemia. They primarily stabilize blood glucose by stimulating **hepatic gluconeogenesis** (the synthesis of glucose from non-carbohydrate precursors like lactate and amino acids) and **hepatic glycogenolysis**. Epinephrine activates $\beta_2$ receptors in the liver, increasing cAMP levels, which activates protein kinase A, leading to the induction of key gluconeogenic enzymes. **Analysis of Incorrect Options:** * **Option A:** While catecholamines do stimulate glycogen phosphorylase in muscles, muscle tissue lacks the enzyme **Glucose-6-Phosphatase**. Therefore, muscle glycogen is broken down into lactate (via glycolysis) rather than being released directly as free glucose into the blood. * **Option B:** Catecholamines **promote** glycogenolysis (breakdown of glycogen) to increase blood sugar; inhibiting it would worsen hypoglycemia. * **Option C:** Catecholamines actually **inhibit** insulin release (via $\alpha_2$ receptors) and stimulate glucagon release. Inhibiting insulin is crucial during hypoglycemia to prevent further glucose uptake by peripheral tissues. **High-Yield NEET-PG Pearls:** * **Dual Action:** Epinephrine increases glucose production (liver) and decreases glucose utilization (peripheral tissues). * **Receptor Specificity:** Hepatic glucose production is mediated by $\beta_2$ and $\alpha_1$ receptors, while the inhibition of insulin secretion is mediated by $\alpha_2$ receptors in pancreatic beta cells. * **Hierarchy of Response:** In hypoglycemia, the first line of defense is the suppression of insulin, followed by a rise in glucagon and epinephrine. Cortisol and Growth Hormone act later (4–6 hours) to maintain glucose levels.

Question 90: What is the resting membrane potential in a nerve?

A. +70 mV
B. -70 mV (Correct Answer)
C. +90 mV
D. -90 mV

Explanation: **Explanation:** The **Resting Membrane Potential (RMP)** is the electrical potential difference across the plasma membrane when a cell is at rest. In a typical large myelinated nerve fiber, this value is **-70 mV**. **Why -70 mV is correct:** The RMP is primarily determined by two factors: 1. **Selective Permeability:** The resting membrane is significantly more permeable to Potassium ($K^+$) than to Sodium ($Na^+$) due to "leak channels." 2. **Ionic Gradients:** The $Na^+$-$K^+$ ATPase pump actively transports 3 $Na^+$ out and 2 $K^+$ in, creating a net deficit of positive charges inside. While the equilibrium potential for $K^+$ is -94 mV, the slight inward leakage of $Na^+$ brings the actual nerve RMP to -70 mV. **Analysis of Incorrect Options:** * **+70 mV:** This is incorrect as the interior of a resting cell is always electronegative relative to the exterior. A positive potential occurs only during the peak of depolarization. * **+90 mV:** Incorrect; positive potentials of this magnitude are not physiological in human nerve cells. * **-90 mV:** While this is a common RMP for **skeletal muscle fibers** and **ventricular myocytes**, it is more negative than the standard RMP for a nerve fiber. **High-Yield NEET-PG Pearls:** * **Goldman-Hodgkin-Katz Equation:** Used to calculate RMP when multiple ions are involved. * **Nernst Equation:** Used to calculate the equilibrium potential for a *single* ion. * **Main Contributor:** The diffusion of $K^+$ through leak channels is the most important factor in establishing RMP. The $Na^+$-$K^+$ pump is "electrogenic" but contributes only about -4 mV directly to the RMP. * **Tissue Variations:** Nerve (-70 mV), Skeletal Muscle (-90 mV), RBC (-10 mV).

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