General Physiology — MCQs

General Physiology Indian Medical PG Practice Questions and MCQs

Question 161: What is the approximate protein to lipid ratio in the inner mitochondrial membrane?

A. 1:1
B. 2:1
C. 3:1 (Correct Answer)
D. 4:1

Explanation: **Explanation:** The composition of biological membranes varies significantly depending on their physiological function. The **inner mitochondrial membrane (IMM)** is unique because it is the site of the Electron Transport Chain (ETC) and ATP synthesis. **1. Why 3:1 is Correct:** The IMM has the highest protein concentration of any membrane in the cell, with a **protein-to-lipid ratio of approximately 3:1 (75% protein and 25% lipid)**. This high protein density is necessary to accommodate the massive complexes of the respiratory chain (Complexes I-IV), ATP synthase, and various transport proteins (like the ADP/ATP translocator). Unlike the outer membrane, the IMM is highly folded into **cristae** to increase the surface area for these metabolic reactions. **2. Analysis of Incorrect Options:** * **A. 1:1:** This is the approximate ratio for the **plasma membrane** of a typical human cell (e.g., an erythrocyte), where there is a roughly equal distribution of lipids and proteins. * **B. 2:1:** While higher than the plasma membrane, this ratio characterizes the **outer mitochondrial membrane**, which contains porins but lacks the dense metabolic machinery of the inner membrane. * **D. 4:1:** This ratio is excessively high and does not represent standard biological membranes. **High-Yield Clinical Pearls for NEET-PG:** * **Cardiolipin:** The IMM contains a unique phospholipid called cardiolipin, which makes the membrane impermeable to ions (especially H+), essential for maintaining the electrochemical gradient. * **Myelin:** In contrast to the IMM, myelin has a ratio of **1:4** (mostly lipid), reflecting its role as an electrical insulator. * **Permeability:** The IMM is impermeable to most polar molecules; specific transporters are required for pyruvate, fatty acids, and amino acids to enter the matrix.

Question 162: Steroids are transported inside the cell using which mechanism?

A. Simple diffusion (Correct Answer)
B. Facilitated diffusion
C. Active transport
D. Osmosis

Explanation: **Explanation:** **1. Why Simple Diffusion is Correct:** Steroid hormones (such as cortisol, aldosterone, estrogen, and testosterone) are derivatives of **cholesterol**, making them highly **lipophilic (lipid-soluble)**. The cell membrane is composed of a phospholipid bilayer. Because "like dissolves like," steroid hormones can easily dissolve in the lipid matrix of the plasma membrane and cross it without the need for carrier proteins or energy. They move down their concentration gradient directly into the cytosol or nucleus to bind with intracellular receptors. **2. Why Other Options are Incorrect:** * **Facilitated Diffusion:** This requires specific transmembrane carrier proteins or channels (e.g., GLUT transporters for glucose). Steroids do not require these because they are not blocked by the lipid bilayer. * **Active Transport:** This process moves molecules against a concentration gradient using ATP (e.g., Na+/K+ ATPase pump). Steroid transport is a passive process driven by concentration gradients. * **Osmosis:** This specifically refers to the movement of **water** molecules across a semi-permeable membrane. **3. NEET-PG High-Yield Pearls:** * **Intracellular Receptors:** Most steroid hormones bind to receptors in the cytoplasm (Type I) or nucleus (Type II), acting as transcription factors to alter gene expression. * **Exception:** While steroids cross the *cell membrane* via simple diffusion, they are poorly soluble in *blood* and must be transported in the plasma bound to specific carrier proteins (e.g., Sex Hormone Binding Globulin, Albumin). * **Thyroid Hormones (T3/T4):** Like steroids, they are lipophilic and have intracellular receptors, but they often utilize specialized membrane transporters (facilitated diffusion) to enter cells more efficiently.

Question 163: A person wakes up with pain, paresthesia, and tingling in the arms after sleeping with their arm below their head. Which nerve fibers are involved?

A. Type A fibers (Correct Answer)
B. Type B fibers
C. Type C fibers (pain)
D. Type C fibers (postganglionic)

Explanation: **Explanation:** The clinical scenario describes **"Saturday Night Palsy"** or **"Sleep Palsy,"** which results from prolonged mechanical **pressure** on a peripheral nerve. **Why Type A fibers are correct:** According to the **Gasser and Erlanger classification**, nerve fibers exhibit differential sensitivity to pressure, hypoxia, and local anesthetics. **Type A fibers** (large, myelinated) are the **most sensitive to pressure**. When a person sleeps on their arm, the mechanical compression leads to transient ischemia and conduction block in these fibers. Since Type A fibers include $A\alpha$ (motor), $A\beta$ (touch/pressure), and $A\delta$ (fast pain/temperature), their involvement results in the characteristic symptoms of motor weakness (heaviness), numbness, and tingling (paresthesia). **Why the other options are incorrect:** * **Type B fibers:** These are preganglionic autonomic fibers. They are the **most sensitive to hypoxia** but are less affected by direct mechanical pressure than Type A fibers. * **Type C fibers (Options C & D):** These are small, unmyelinated fibers. They are the **most sensitive to local anesthetics** but are the **least sensitive to pressure**. This is why, in cases of pressure-induced "pins and needles," slow/dull pain perception (carried by Type C) often remains intact even when touch and motor functions are lost. **High-Yield NEET-PG Pearls:** * **Sensitivity to Pressure:** A > B > C (Type A is most affected). * **Sensitivity to Hypoxia:** B > A > C (Type B is most affected). * **Sensitivity to Local Anesthesia:** C > B > A (Type C is most affected). * **Order of Loss in Spinal Anesthesia:** Pain > Temperature > Touch > Deep Pressure > Motor (C fibers are blocked first).

Question 164: What is the rate of migration in the motor- MIGRATING motor complex?

A. 5 cm/min at 60 minutes
B. 5 cm/min at 90 minutes (Correct Answer)
C. 10 cm/min at 60 minutes
D. 10 cm/min at 90 minutes

Explanation: **Explanation:** The **Migrating Motor Complex (MMC)** is a distinct pattern of electromechanical activity observed in the gastrointestinal smooth muscle during the fasting state (interdigestive period). Its primary function is the "housekeeping" of the gut—clearing residual undigested food, secretions, and bacteria from the stomach to the ileum. **1. Why Option B is Correct:** The MMC occurs in cycles that repeat every **90 to 120 minutes**. During the most active phase (Phase III), intense peristaltic contractions migrate down the small intestine. The velocity of this migration is approximately **5 cm/min** in the proximal small intestine. Therefore, the rate and periodicity align with 5 cm/min at 90-minute intervals. **2. Why Other Options are Incorrect:** * **Options A & C (60 minutes):** While the duration of an MMC cycle can vary, the standard physiological textbook value (e.g., Ganong, Guyton) for the interdigestive cycle is 90–120 minutes. 60 minutes is too frequent for a complete cycle. * **Options C & D (10 cm/min):** A velocity of 10 cm/min is too fast for the standard migration of the MMC. The wave moves slowly to ensure thorough clearing of the lumen; 5 cm/min is the established physiological rate. **High-Yield Clinical Pearls for NEET-PG:** * **Hormonal Control:** The hormone **Motilin**, secreted by M cells in the duodenum and jejunum, is the primary initiator of the MMC. * **Phases:** MMC has 4 phases; **Phase III** is the "activity front" characterized by the strongest contractions. * **Feeding Effect:** Ingestion of food immediately terminates the MMC, replacing it with the "fed pattern" (segmentation and peristalsis). * **Clinical Significance:** Absence or disruption of MMC can lead to **Small Intestinal Bacterial Overgrowth (SIBO)**. * **Pharmacology:** Erythromycin acts as a motilin agonist and can stimulate MMC-like activity.

Question 165: Depolarization occurs due to the entry of which ion?

A. Na+ (Correct Answer)
B. K+
C. Cl-
D. HCO3-

Explanation: **Explanation:** **1. Why Na+ is Correct:** Depolarization refers to the process where the resting membrane potential (RMP) becomes less negative (moves toward zero or becomes positive). In excitable cells like neurons and muscle fibers, the RMP is typically -70mV to -90mV. Upon stimulation, voltage-gated **Sodium (Na+) channels** open. Since Na+ concentration is much higher in the extracellular fluid (ECF) than the intracellular fluid (ICF), Na+ rushes into the cell following its electrochemical gradient. This influx of positive charge neutralizes the internal negativity, causing **depolarization**. **2. Why Other Options are Incorrect:** * **K+ (Potassium):** K+ is the primary intracellular cation. Its exit (efflux) from the cell makes the interior more negative, leading to **repolarization** or hyperpolarization, not depolarization. * **Cl- (Chloride):** Cl- is an anion. Its entry into the cell (influx) increases internal negativity, causing **Hyperpolarization** (Inhibitory Post-Synaptic Potential - IPSP). * **HCO3- (Bicarbonate):** While important for acid-base balance and CO2 transport, it does not play a primary role in the rapid phase of action potential generation. **3. Clinical Pearls & High-Yield Facts:** * **Threshold Potential:** Depolarization must reach a specific "threshold" (usually -55mV) to trigger an all-or-none action potential. * **Tetrodotoxin (TTX):** A potent toxin found in Pufferfish that blocks voltage-gated Na+ channels, preventing depolarization and causing paralysis. * **Exception:** In the **SA node (Pacemaker)** of the heart and smooth muscles, the upstroke of the action potential (depolarization) is primarily due to **Ca2+ influx**, not Na+. * **Hyperkalemia:** Increases membrane excitability initially by bringing the RMP closer to the threshold.

Question 166: The major source of calcium for contraction of skeletal muscle is:

A. Extracellular fluid (ECF)
B. Cytosol
C. Mitochondria
D. Sarcoplasmic reticulum (SR) (Correct Answer)

Explanation: **Explanation:** The correct answer is **Sarcoplasmic Reticulum (SR)**. In skeletal muscle, the process of excitation-contraction coupling relies almost exclusively on intracellular calcium stores. **1. Why Sarcoplasmic Reticulum is correct:** Skeletal muscle fibers contain an extensive network of Sarcoplasmic Reticulum (SR) that acts as a specialized reservoir for calcium ions ($Ca^{2+}$). When an action potential travels down the **T-tubules**, it activates **Dihydropyridine (DHP) receptors**. These receptors are mechanically linked to **Ryanodine receptors (RyR1)** on the SR membrane. This mechanical coupling triggers the massive release of $Ca^{2+}$ from the SR cisternae into the sarcoplasm, which then binds to **Troponin C** to initiate contraction. **2. Why other options are incorrect:** * **Extracellular Fluid (ECF):** Unlike cardiac or smooth muscle, skeletal muscle contraction is **not** dependent on ECF calcium. The DHP receptor acts as a voltage sensor, not a primary calcium channel for influx. * **Cytosol:** The cytosol is where calcium acts, but it is not the *source*. In a resting state, cytosolic calcium levels are kept extremely low to allow for muscle relaxation. * **Mitochondria:** While mitochondria can sequester some calcium, their primary role is ATP production; they do not provide the calcium required for the contractile cycle. **High-Yield NEET-PG Pearls:** * **Calsequestrin:** The protein inside the SR that binds and buffers calcium, allowing for high-capacity storage. * **SERCA Pump:** The $Ca^{2+}$-ATPase pump responsible for pumping calcium back into the SR to induce muscle relaxation. * **Malignant Hyperthermia:** A clinical condition caused by a mutation in the **Ryanodine receptor (RyR1)**, leading to excessive calcium release from the SR in response to certain anesthetics.

Question 167: Which ion has the greatest concentration in saliva under resting conditions?

A. Na+
B. K+
C. Cl-
D. HCO3- (Correct Answer)

Explanation: **Explanation:** The composition of saliva is unique because it is always **hypotonic** compared to plasma, and its ionic concentration varies significantly with the flow rate. **1. Why HCO3- is the correct answer:** At resting (low) flow rates, the contact time between the primary saliva and the ductal cells is maximal. While Na+ and Cl- are extensively reabsorbed, **HCO3- is actively secreted** into the ductal lumen in exchange for Cl- (via the Cl-/HCO3- exchanger). Under resting conditions, the concentration of HCO3- (approx. 25-30 mEq/L) is higher than that of Na+ and Cl-, making it the most abundant ion relative to its plasma concentration and other cations/anions in the final saliva. It serves as a critical buffer to maintain oral pH. **2. Why the other options are incorrect:** * **Na+ and Cl-:** In the salivary ducts, Na+ is actively reabsorbed (via Na+/H+ exchange) and Cl- follows passively or via exchangers. At resting states, reabsorption is so efficient that their concentrations drop to very low levels (Na+ ~15 mEq/L; Cl- ~10 mEq/L), much lower than HCO3-. * **K+:** Although K+ is actively secreted into saliva and its concentration is higher than in plasma (approx. 20 mEq/L), it typically remains lower than the concentration of HCO3- at resting states. **High-Yield Clinical Pearls for NEET-PG:** * **Flow Rate Relationship:** As salivary flow rate **increases**, the concentration of Na+, Cl-, and HCO3- increases, while K+ decreases slightly or stays constant. * **Aldosterone Effect:** Aldosterone acts on salivary ducts just like the renal tubules, increasing Na+ reabsorption and K+ secretion. * **Always Hypotonic:** Saliva is always hypotonic because the ductal epithelium is impermeable to water, preventing water from following the reabsorbed NaCl.

Question 168: Nitric oxide (NO) is secreted by which of the following structures?

A. Endothelium (Correct Answer)
B. Endoderm
C. Ectoderm
D. Bones

Explanation: **Explanation:** **Nitric Oxide (NO)**, formerly known as Endothelium-Derived Relaxing Factor (EDRF), is a potent vasodilator gas synthesized primarily by the **vascular endothelium**. It is produced from the amino acid **L-arginine** by the enzyme **Endothelial Nitric Oxide Synthase (eNOS)** in the presence of oxygen and NADPH. Once released, NO diffuses into adjacent vascular smooth muscle cells, activating soluble guanylyl cyclase, which increases cGMP levels, leading to smooth muscle relaxation and vasodilation. **Analysis of Options:** * **Endothelium (Correct):** As the primary site of eNOS expression, the endothelium regulates vascular tone, inhibits platelet aggregation, and prevents leukocyte adhesion through NO secretion. * **Endoderm & Ectoderm (Incorrect):** These are primary germ layers formed during gastrulation. While certain derivatives (like neurons from ectoderm) can produce NO via neuronal NOS (nNOS), the germ layers themselves are not secretory structures for NO in the context of physiological regulation. * **Bones (Incorrect):** While bone cells (osteoblasts/osteocytes) may produce minor amounts of NO to modulate bone remodeling, they are not a primary or systemic source of the molecule. **High-Yield Clinical Pearls for NEET-PG:** * **Isoforms of NOS:** There are three types: **nNOS** (Type 1, Neuronal), **iNOS** (Type 2, Inducible - found in macrophages during inflammation), and **eNOS** (Type 3, Endothelial). * **Mechanism:** NO → ↑ cGMP → Protein Kinase G activation → Dephosphorylation of Myosin Light Chain → **Vasodilation**. * **Therapeutic Link:** Nitroglycerin works by being converted into NO, providing rapid relief in Angina Pectoris. * **Septic Shock:** Overproduction of NO by **iNOS** in response to bacterial toxins leads to the profound systemic vasodilation seen in sepsis.

Question 169: What happens to the resting membrane potential when extracellular potassium is increased from 4 meq/L to 10 meq/L?

A. Resting membrane potential becomes more negative.
B. Increase in sodium conductance.
C. Increase in potassium conductance. (Correct Answer)
D. Na+/K+ ATPase activity will be inhibited.

Explanation: **Explanation:** The resting membrane potential (RMP) is primarily determined by the permeability of the cell membrane to potassium ions ($K^+$) via **inward rectifier potassium channels ($K_{ir}$)**. **Why Option C is Correct:** According to the **Goldman-Hodgkin-Katz equation**, membrane conductance (permeability) for an ion is influenced by its extracellular concentration. When extracellular $K^+$ increases (hyperkalemia), it causes a conformational change in potassium channels that **increases their conductance**. Additionally, the increased extracellular $K^+$ reduces the concentration gradient, causing the RMP to become **less negative (depolarization)**. This depolarization moves the membrane potential closer to the threshold, initially increasing excitability. **Analysis of Incorrect Options:** * **Option A:** Increasing extracellular $K^+$ decreases the efflux of $K^+$, making the RMP **less negative** (depolarization), not more negative (hyperpolarization). * **Option B:** Sodium conductance is primarily regulated by voltage-gated sodium channels during the action potential, not by changes in extracellular potassium at rest. * **Option D:** $Na^+/K^+$ ATPase activity is actually **stimulated** by an increase in extracellular potassium, as it works harder to pump the excess $K^+$ back into the cell to maintain homeostasis. **High-Yield Clinical Pearls for NEET-PG:** * **Hyperkalemia Paradox:** While mild hyperkalemia increases excitability (depolarization), severe hyperkalemia leads to **"depolarization block"** because sodium channels remain in an inactivated state, leading to cardiac arrest in diastole. * **ECG Changes:** Tall tented T-waves, PR prolongation, and widened QRS complexes are classic signs of hyperkalemia. * **Nernst Equation:** Used to calculate the equilibrium potential for a single ion; for $K^+$, it is approximately -94 mV.

Question 170: A 2-year-old girl passes stool 10 minutes after the intake of food. What physiological reflex is responsible for this?

A. Enterogastric reflex
B. Intestino-intestinal reflex
C. Gastrocolic reflex (Correct Answer)
D. Recto-sphincteric reflex

Explanation: **Explanation:** The correct answer is **C. Gastrocolic reflex**. **Mechanism of the Correct Answer:** The gastrocolic reflex is a physiological reflex where the distension of the stomach by food (or the presence of breakdown products in the small intestine) increases the motility of the colon. This often triggers **mass movements**, which propel fecal matter into the rectum, leading to the urge to defecate. This reflex is mediated by the autonomic nervous system (parasympathetic) and gastrointestinal hormones like gastrin and cholecystokinin (CCK). It is particularly prominent in infants and young children, which explains why they often pass stool shortly after a meal. **Why the other options are incorrect:** * **A. Enterogastric reflex:** This reflex is initiated in the duodenum in response to acid or distension; it **inhibits** gastric motility and secretions to slow down stomach emptying. * **B. Intestino-intestinal reflex:** This occurs when over-distension of one segment of the intestine causes the **relaxation** of the rest of the intestine to prevent further movement and potential injury. * **C. Recto-sphincteric reflex (Defecation reflex):** This is triggered by the distension of the **rectum** (not the stomach), leading to the relaxation of the internal anal sphincter. **High-Yield Facts for NEET-PG:** * **Gastrocolic Reflex:** Primarily responsible for the "post-prandial urge" to defecate. * **Clinical Correlation:** This reflex is often exaggerated in patients with **Irritable Bowel Syndrome (IBS)**, leading to immediate diarrhea after meals. * **Mediators:** Gastrin and CCK are the primary hormonal mediators; the neural component is mediated via the pelvic nerves (parasympathetic).

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