General Physiology — MCQs

General Physiology Indian Medical PG Practice Questions and MCQs

Question 201: Which fibres are most sensitive to local anesthesia?

A. A fibres
B. B fibres
C. C fibres (Correct Answer)
D. None of the above

Explanation: ### Explanation The sensitivity of nerve fibers to local anesthetics (LA) is determined by the **size of the fiber** and the **presence of myelin**. Local anesthetics work by blocking voltage-gated sodium channels. **Why C Fibers are the most sensitive:** According to the traditional physiological classification, **C fibers** are the most sensitive to local anesthesia because they are the **smallest in diameter** and **unmyelinated**. Because they lack a myelin sheath, the drug can easily access the axonal membrane along its entire length. In contrast, myelinated fibers require the anesthetic to reach a high enough concentration to block at least three successive Nodes of Ranvier to stop conduction. **Analysis of Options:** * **Option A (A fibers):** These are large, myelinated fibers. Due to their large diameter and thick myelin sheath, they are the **least sensitive** (most resistant) to local anesthesia. Within this group, A-alpha (motor) are the hardest to block, while A-delta (pain/temperature) are blocked earlier. * **Option B (B fibers):** These are small, preganglionic autonomic myelinated fibers. While they are very sensitive (often blocked even before C fibers in clinical practice due to their anatomical location), in standard physiological testing based on fiber diameter alone, C fibers are considered the most sensitive. **High-Yield Clinical Pearls for NEET-PG:** 1. **Order of Blockade (Clinical):** Autonomic (B) > Pain/Temperature (C & A-delta) > Touch/Pressure (A-beta) > Motor (A-alpha). 2. **Order of Recovery:** Exactly the reverse of the blockade. 3. **Sensitivity to Hypoxia:** B fibers > A fibers > C fibers (C fibers are most resistant to pressure and hypoxia). 4. **Sensitivity to Pressure:** A fibers > B fibers > C fibers. 5. **Rule of Thumb:** Smaller diameter and lack of myelin generally increase sensitivity to local anesthetics.

Question 202: In a 100-meter dash, most of the energy consumed by skeletal muscles to replenish ATP is derived from which source?

A. Phosphocreatine (Correct Answer)
B. Aerobic glycolysis
C. Oxidation of fatty acids
D. None of the above

Explanation: ### Explanation **Correct Option: A. Phosphocreatine** The primary determinant of the energy source used by skeletal muscle is the **intensity and duration** of the activity. A 100-meter dash is a high-intensity, short-duration explosive activity (typically lasting <10–12 seconds). 1. **Immediate Energy System (ATP-CP System):** At the onset of such exercise, the pre-existing stores of ATP are exhausted within 2–3 seconds. To rapidly replenish ATP without waiting for complex metabolic pathways, the muscle utilizes **Phosphocreatine (PCr)**. 2. **Mechanism:** The enzyme **Creatine Kinase** transfers a high-energy phosphate group from PCr to ADP to form ATP. This anaerobic process provides the highest rate of energy transfer, making it the dominant source for the first 10 seconds of maximal effort. --- ### Why Other Options are Incorrect: * **B. Aerobic Glycolysis:** This pathway requires oxygen and multiple enzymatic steps (Krebs cycle and Electron Transport Chain). It is too slow to meet the immediate, massive energy demands of a sprint. It becomes the primary source for activities lasting longer than 2 minutes (e.g., long-distance running). * **C. Oxidation of Fatty Acids:** Beta-oxidation is the slowest energy-producing pathway. While it yields the most ATP per molecule, it requires significant oxygen and time. It is the predominant energy source at **rest** and during low-intensity, prolonged endurance exercise. --- ### High-Yield Clinical Pearls for NEET-PG: * **Energy Sequence:** 1. **0–3 sec:** Stored ATP. 2. **3–10 sec:** Phosphocreatine (Phosphagen system). 3. **10–90 sec:** Anaerobic Glycolysis (Lactic acid system). 4. **>2 min:** Aerobic Metabolism. * **Respiratory Quotient (RQ):** During high-intensity exercise (carbohydrate use), the RQ approaches **1.0**, whereas during rest (fat use), it is approximately **0.7**. * **Type IIb (Fast-twitch) fibers** have high glycolytic capacity and high phosphocreatine content, making them the primary fibers used in a 100m dash.

Question 203: Extracellular fluid (ECF) volume can be measured using which of the following substances?

A. Inulin (Correct Answer)
B. Deuterium oxide (D2O)
C. Evan's blue
D. Tritiated water (3H2O)

Explanation: **Explanation:** The measurement of body fluid compartments is based on the **Indicator Dilution Principle** ($Volume = Amount / Concentration$). To measure a specific compartment, the substance used must be able to distribute evenly within that compartment without entering others or being metabolized rapidly. **Why Inulin is Correct:** **Inulin** is a polysaccharide that is small enough to pass through capillary endothelium into the interstitial space but is too large to cross cell membranes. Therefore, it distributes throughout the entire **Extracellular Fluid (ECF)**—which includes both plasma and interstitial fluid—making it the gold standard for ECF volume measurement. Other substances used for ECF include Mannitol, Sucrose, and Thiosulfate. **Why Other Options are Incorrect:** * **Deuterium oxide ($D_2O$) and Tritiated water ($^3H_2O$):** These are isotopes of water. They cross all cell membranes and distribute uniformly throughout the **Total Body Water (TBW)**. * **Evan’s Blue (T-1824):** This dye binds strongly to serum albumin. Since albumin is largely confined to the vascular system, Evan’s Blue is used to measure **Plasma Volume**. **High-Yield Clinical Pearls for NEET-PG:** * **Total Body Water (60% of body weight):** Measured by $D_2O$, Tritiated water, or Aminopyrine. * **ECF (20% of body weight):** Measured by Inulin, Mannitol, or $Na^{22}$. * **Plasma Volume (5% of body weight):** Measured by Evan’s Blue or Radio-iodinated Serum Albumin (RISA). * **Calculation Rule:** Intracellular Fluid (ICF) and Interstitial Fluid (ISF) cannot be measured directly. They are calculated: * $ICF = TBW - ECF$ * $ISF = ECF - Plasma\ Volume$

Question 204: What is the resting membrane potential of smooth muscle?

A. -50 mV (Correct Answer)
B. -75 mV
C. -90 mV
D. -35 mV

Explanation: The resting membrane potential (RMP) of a cell is determined by the selective permeability of the membrane to ions and the activity of the Na⁺/K⁺-ATPase pump. **Why -50 mV is correct:** Smooth muscle cells typically exhibit a less negative (more unstable) RMP compared to skeletal or cardiac muscles. The RMP of smooth muscle generally ranges from **-50 mV to -60 mV**. This higher (less negative) value is primarily due to a higher baseline permeability to sodium (Na⁺) ions compared to other excitable tissues, making the cell more excitable and prone to spontaneous rhythmic activity (slow waves). **Analysis of Incorrect Options:** * **-75 mV (Option B):** This is closer to the RMP of **neurons** (approx. -70 mV). * **-90 mV (Option C):** This is the characteristic RMP for **skeletal muscle fibers** and **ventricular cardiomyocytes**. These cells require a very stable, highly negative RMP to prevent involuntary contractions. * **-35 mV (Option D):** This value is too depolarized for a resting state; at this potential, most voltage-gated channels would already be activated or inactivated. **High-Yield NEET-PG Pearls:** 1. **Instability:** Unlike skeletal muscle, smooth muscle RMP is not constant and often shows "Slow Wave" potentials (Basic Electrical Rhythm), especially in the GI tract, mediated by **Interstitial Cells of Cajal**. 2. **Action Potential:** In many smooth muscles, the upstroke of the action potential is caused by the influx of **Calcium (Ca²⁺)** rather than Sodium. 3. **L-type Ca²⁺ channels:** These are the primary targets for Calcium Channel Blockers (CCBs) used in treating hypertension.

Question 205: Mutation of the gene coding for ryanodine receptors is implicated in malignant hyperthermia. Which of the following statements best explains the increased heat production in malignant hyperthermia?

A. Increased muscle metabolism due to excess calcium ions (Correct Answer)
B. Thermic effect of food
C. Increased sympathetic discharge
D. Mitochondrial thermogenesis

Explanation: ### Explanation **Correct Option: A. Increased muscle metabolism due to excess calcium ions** Malignant Hyperthermia (MH) is a pharmacogenetic disorder triggered by volatile anesthetics (e.g., halothane) or succinylcholine. The underlying pathology involves a mutation in the **RYR1 gene**, which codes for the **Ryanodine Receptor** (a calcium release channel in the sarcoplasmic reticulum). * **Mechanism:** The mutated receptor remains open for prolonged periods, leading to a massive efflux of $Ca^{2+}$ into the sarcoplasm. * **Heat Production:** To sequester this excess calcium, the **SERCA pump** (Sarcoplasmic/Endoplasmic Reticulum Calcium ATPase) works overactively. This process consumes massive amounts of ATP. The resulting hypermetabolic state increases oxygen consumption and $CO_2$ production, generating intense heat as a byproduct of accelerated ATP hydrolysis and sustained muscle contraction (rigidity). **Why other options are incorrect:** * **B. Thermic effect of food:** This refers to the energy expenditure associated with digestion and absorption (Specific Dynamic Action), which is unrelated to anesthetic-induced hyperthermia. * **C. Increased sympathetic discharge:** While tachycardia occurs in MH, it is a *secondary* response to hypercapnia and acidosis, not the primary source of the massive heat production. * **D. Mitochondrial thermogenesis:** While mitochondria are involved in cellular respiration, the primary "furnace" in MH is the uncontrolled ATP consumption by the contractile apparatus and calcium pumps in the cytoplasm. --- ### High-Yield Clinical Pearls for NEET-PG * **Inheritance:** Autosomal Dominant. * **Earliest Sign:** Increase in **End-Tidal $CO_2$ ($ETCO_2$)**. * **Clinical Triad:** Muscle rigidity (often masseter spasm), hyperthermia, and metabolic acidosis. * **Drug of Choice:** **Dantrolene** (Mechanism: Binds to RYR1 and inhibits calcium release). * **Associated Conditions:** Central Core Disease and King-Denborough Syndrome.

Question 206: Inhibitory postsynaptic potentials (IPSPs) commonly occur at which of the following sites?

A. The anterior gray horn of the cord, dorsal root ganglia, and muscle endplates
B. The anterior gray horn of the cord and dorsal root ganglia, but not at muscle endplates
C. The anterior gray horn of the cord, but not at dorsal root ganglia or muscle endplates (Correct Answer)
D. The anterior gray horn of the cord and muscle endplates, but not at dorsal root ganglia

Explanation: **Explanation:** The core concept behind this question is the location of **synaptic integration**. An Inhibitory Postsynaptic Potential (IPSP) is a local hyperpolarization of a postsynaptic membrane, typically caused by the opening of ligand-gated $Cl^-$ or $K^+$ channels. This requires a **synapse** between two neurons. 1. **Why Option C is Correct:** * **Anterior Gray Horn:** This is a major site of synaptic integration in the spinal cord. Alpha motor neurons receive thousands of synapses, including inhibitory inputs from **Renshaw cells** (glycinergic) and inhibitory interneurons involved in reciprocal inhibition. Thus, IPSPs are common here. * **Dorsal Root Ganglia (DRG):** These contain cell bodies of primary sensory neurons. Crucially, there are **no synapses** within the DRG; the cell bodies are pseudo-unipolar and do not receive inhibitory or excitatory synaptic input. * **Muscle Endplates:** The Neuromuscular Junction (NMJ) is an **obligatory excitatory** synapse. The neurotransmitter Acetylcholine (ACh) always produces an Excitatory Postsynaptic Potential (called an End Plate Potential or EPP). There is no inhibitory neurotransmitter or receptor at the human skeletal NMJ. 2. **Why Other Options are Wrong:** * **Options A, B, & D** are incorrect because they suggest IPSPs occur at either the DRG or the muscle endplate. As established, the DRG lacks synapses, and the muscle endplate is exclusively excitatory. Inhibition of skeletal muscle occurs centrally (in the spinal cord) by inhibiting the motor neuron, not peripherally at the muscle itself. **High-Yield NEET-PG Pearls:** * **Neurotransmitters:** The most common inhibitory neurotransmitters mediating IPSPs are **GABA** (brain) and **Glycine** (spinal cord). * **NMJ Fact:** Unlike the CNS, the NMJ has a high "safety factor," meaning every EPP normally reaches threshold to trigger an action potential. * **DRG Fact:** While DRG cells lack synapses, they can be modulated by "Presynaptic Inhibition," but this occurs at their axon terminals in the dorsal horn, not in the ganglion itself.

Question 207: Catabolism of H2O2 is carried out by which organelle?

A. Peroxisomes (Correct Answer)
B. Mitochondria
C. Endoplasmic reticulum
D. Lysosomes

Explanation: **Explanation:** The correct answer is **Peroxisomes** (Option A). **1. Why Peroxisomes are correct:** Peroxisomes (also known as microbodies) are membrane-bound organelles specialized for oxidative reactions. They contain high concentrations of **oxidases**, which produce hydrogen peroxide ($H_2O_2$) as a byproduct of metabolizing fatty acids and amino acids. Because $H_2O_2$ is a highly reactive and potentially toxic free radical, peroxisomes also contain the enzyme **catalase**. Catalase specifically catalyzes the catabolism (decomposition) of $H_2O_2$ into water and oxygen ($2H_2O_2 \rightarrow 2H_2O + O_2$), thereby protecting the cell from oxidative damage. **2. Why other options are incorrect:** * **Mitochondria:** While mitochondria are the primary site of ATP production and generate reactive oxygen species (ROS) during the electron transport chain, their primary antioxidant defense involves superoxide dismutase and glutathione peroxidase, rather than being the specialized site for $H_2O_2$ catabolism via catalase. * **Endoplasmic Reticulum (ER):** The ER is primarily involved in protein synthesis (RER), lipid synthesis, and calcium storage (SER), not the breakdown of peroxides. * **Lysosomes:** Known as the "suicidal bags" of the cell, lysosomes contain acid hydrolases for the degradation of macromolecules and cellular debris, not oxidative enzymes like catalase. **High-Yield Facts for NEET-PG:** * **Zellweger Syndrome:** A rare genetic disorder caused by the absence of functional peroxisomes, leading to the accumulation of very-long-chain fatty acids (VLCFA). * **Beta-oxidation:** Peroxisomes are the exclusive site for the oxidation of VLCFAs (chains >22 carbons). * **Marker Enzyme:** Catalase is the classic marker enzyme for identifying peroxisomes in histochemical studies.

Question 208: Which of the following transport processes does NOT follow saturation kinetics?

A. Facilitated diffusion
B. Na+ - Ca2+ exchanger
C. Simple diffusion (Correct Answer)
D. Na+ coupled active transport

Explanation: **Explanation:** The concept of **saturation kinetics** (Vmax) applies to any transport process that requires a **membrane protein (carrier or pump)**. Since there are a finite number of binding sites on these proteins, the rate of transport reaches a plateau once all carriers are occupied. **1. Why Simple Diffusion is the Correct Answer:** Simple diffusion occurs directly through the lipid bilayer or through non-gated protein channels. It is governed by **Fick’s Law**, which states that the rate of diffusion is directly proportional to the concentration gradient. Because it does not rely on a limited number of carrier binding sites, it **does not show saturation**. The graph of transport rate vs. concentration remains a straight line. **2. Why the Other Options are Incorrect:** * **Facilitated Diffusion (A):** Uses carrier proteins (e.g., GLUT transporters) to move substances down a gradient. Because carriers are involved, it follows Michaelis-Menten kinetics and exhibits saturation. * **Na+ - Ca2+ Exchanger (B):** This is a form of **Secondary Active Transport (Counter-transport)**. It relies on a specific membrane exchanger protein, which can be saturated. * **Na+ Coupled Active Transport (D):** This refers to **Secondary Active Transport (Co-transport)**, such as SGLT in the kidneys/intestines. These transporters have specific binding sites for Na+ and the solute, leading to a transport maximum (Tm) and saturation. **High-Yield Clinical Pearls for NEET-PG:** * **Transport Maximum (Tm):** The best clinical example of saturation is the **Tm of Glucose** in the proximal tubule (~375 mg/min). When blood glucose exceeds the renal threshold (~180 mg/dL), carriers saturate, leading to glucosuria. * **Stereospecificity and Competition:** These are two other hallmarks of carrier-mediated transport (Facilitated and Active) that are **absent** in simple diffusion. * **ATP Requirement:** Simple and Facilitated diffusion are passive; Primary and Secondary active transport require energy (directly or indirectly).

Question 209: Oxygen affinity decreases in which of the following conditions?

A. Hypoxia (Correct Answer)
B. Hypothermia
C. Hemoglobin F (HbF)
D. Increase in pH

Explanation: The affinity of hemoglobin for oxygen is represented by the **Oxygen-Dissociation Curve (ODC)**. A decrease in affinity means hemoglobin releases oxygen more easily to the tissues, which is represented by a **Rightward Shift** of the ODC. ### Why Hypoxia is Correct In conditions of **Hypoxia** (low oxygen levels), the body undergoes metabolic adaptations to ensure tissues receive adequate oxygen. One primary mechanism is the increased production of **2,3-Bisphosphoglycerate (2,3-BPG)** within red blood cells. 2,3-BPG binds to the beta chains of deoxyhemoglobin, stabilizing the "T" (Tense) state, which decreases oxygen affinity and shifts the curve to the **Right**. This facilitates oxygen unloading at the tissue level. ### Why Other Options are Incorrect * **Hypothermia:** A decrease in temperature stabilizes the "R" (Relaxed) state of hemoglobin, increasing oxygen affinity and shifting the curve to the **Left**. (Hyperthermia shifts it to the Right). * **Hemoglobin F (HbF):** Fetal hemoglobin lacks beta chains (it has gamma chains) and therefore cannot bind 2,3-BPG effectively. This results in a higher oxygen affinity (Left shift) to allow the fetus to "pull" oxygen from maternal blood. * **Increase in pH (Alkalosis):** According to the **Bohr Effect**, an increase in pH (or decrease in $H^+$ and $CO_2$) increases oxygen affinity, shifting the curve to the **Left**. ### High-Yield Clinical Pearls for NEET-PG * **Right Shift (CADET, face Right!):** **C**O2 increase, **A**cidosis ($H^+$), **D**-2,3-BPG, **E**xercise, and **T**emperature increase. * **P50:** The partial pressure of $O_2$ at which hemoglobin is 50% saturated. A **Right shift** increases the P50 (Normal P50 $\approx$ 26.7 mmHg). * **Carbon Monoxide (CO):** It shifts the curve to the **Left** and decreases the oxygen-carrying capacity (plateau), making it extremely lethal.

Question 210: Na+-K+ ATPase is a-

A. Extrinsic protein
B. Peripheral protein
C. Transmembrane protein (Correct Answer)
D. Intracellular protein

Explanation: ### Explanation **Correct Answer: C. Transmembrane protein** The **Na⁺-K⁺ ATPase** (Sodium-Potassium Pump) is a classic example of an **integral membrane protein**, specifically a **transmembrane protein**. For a protein to function as an active transporter or a pump, it must span the entire thickness of the lipid bilayer. This allows it to simultaneously access the intracellular space (to bind 3 Na⁺ ions) and the extracellular space (to bind 2 K⁺ ions), moving them against their respective concentration gradients using energy derived from ATP hydrolysis. #### Why other options are incorrect: * **A & B. Extrinsic/Peripheral proteins:** These terms are synonymous. Peripheral proteins are loosely attached to either the inner or outer surface of the membrane and do not penetrate the hydrophobic core. Since they do not span the membrane, they cannot transport ions from one side to the other. * **D. Intracellular protein:** These are proteins located entirely within the cytoplasm (e.g., metabolic enzymes like hexokinase). While the Na⁺-K⁺ ATPase has an intracellular catalytic domain, the protein itself is embedded in the plasma membrane. #### NEET-PG High-Yield Pearls: * **Stoichiometry:** It pumps **3 Na⁺ OUT** and **2 K⁺ IN** for every 1 ATP hydrolyzed. * **Electrogenic Nature:** Because it moves 3 positive charges out for every 2 in, it contributes to the negativity of the Resting Membrane Potential (RMP). * **Subunits:** It consists of an **α-subunit** (catalytic, contains binding sites for Na⁺, K⁺, and ATP) and a **β-subunit** (essential for membrane targeting). * **Inhibitors:** It is specifically inhibited by **Cardiac Glycosides** (e.g., Digoxin and Ouabain), which bind to the extracellular side of the α-subunit. * **Energy Consumption:** In a resting state, this pump accounts for approximately 30-40% of a cell's total energy expenditure.

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