General Physiology Practice Questions

Q: Which fibres are most sensitive to local anesthesia?

C fibres. ### 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.

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

Inulin. **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: * $ICF = TBW - ECF$ * $ISF = ECF - Plasma\ Volume$

Q: What is the resting membrane potential of smooth muscle?

-50 mV. 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.

Q: Catabolism of H2O2 is carried out by which organelle?

Peroxisomes. **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.

Q: Oxygen affinity decreases in which of the following conditions?

Hypoxia. 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.

Q: Na+-K+ ATPase is a-

Transmembrane protein. ### 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.

Question 1

Which fibres are most sensitive to local anesthesia?

Accepted Answer

C fibres

Answer

A fibres

Answer

B fibres

Answer

None of the above

Question 2

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

Accepted Answer

Phosphocreatine

Answer

Aerobic glycolysis

Answer

Oxidation of fatty acids

Answer

None of the above

Question 3

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

Accepted Answer

Inulin

Answer

Deuterium oxide (D2O)

Answer

Evan's blue

Answer

Tritiated water (3H2O)

Question 4

What is the resting membrane potential of smooth muscle?

Accepted Answer

-50 mV

Answer

-75 mV

Answer

-90 mV

Answer

-35 mV

Question 5

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?

Accepted Answer

Increased muscle metabolism due to excess calcium ions

Answer

Thermic effect of food

Answer

Increased sympathetic discharge

Answer

Mitochondrial thermogenesis

Question 6

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

Accepted Answer

The anterior gray horn of the cord, but not at dorsal root ganglia or muscle endplates

Answer

The anterior gray horn of the cord, dorsal root ganglia, and muscle endplates

Answer

The anterior gray horn of the cord and dorsal root ganglia, but not at muscle endplates

Answer

The anterior gray horn of the cord and muscle endplates, but not at dorsal root ganglia

Question 7

Catabolism of H2O2 is carried out by which organelle?

Accepted Answer

Peroxisomes

Answer

Mitochondria

Answer

Endoplasmic reticulum

Answer

Lysosomes

Question 8

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

Accepted Answer

Simple diffusion

Answer

Facilitated diffusion

Answer

Na+ - Ca2+ exchanger

Answer

Na+ coupled active transport

Question 9

Oxygen affinity decreases in which of the following conditions?

Accepted Answer

Hypoxia

Answer

Hypothermia

Answer

Hemoglobin F (HbF)

Answer

Increase in pH

Question 10

Na+-K+ ATPase is a-

Accepted Answer

Transmembrane protein

Answer

Extrinsic protein

Answer

Peripheral protein

Answer

Intracellular protein

General Physiology — MCQs

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