Nerve and Muscle Physiology — MCQs

Nerve and Muscle Physiology Indian Medical PG Practice Questions and MCQs

Question 251: A 10-year-old boy presents with increasing muscle weakness and elevated creatine phosphokinase (CPK) levels. What is the most likely cellular defect?

A. Neurons
B. Muscle fibers (Correct Answer)
C. Basement membrane
D. All body cells

Explanation: ### Explanation The clinical presentation of progressive muscle weakness in a young boy, combined with significantly elevated **Creatine Phosphokinase (CPK)** levels, is a classic hallmark of **Duchenne Muscular Dystrophy (DMD)**. **Why Muscle Fibers are the Correct Answer:** CPK is an enzyme primarily located in the sarcoplasm of muscle fibers. In muscular dystrophies, the defect lies in the **dystrophin protein**, which links the cytoskeleton of the muscle fiber to the extracellular matrix. When this structural link is defective, the muscle fiber membrane (sarcolemma) becomes fragile and ruptures during contraction. This leads to the leakage of intracellular enzymes like CPK into the bloodstream and eventual necrosis of the **muscle fibers**. **Analysis of Incorrect Options:** * **A. Neurons:** While motor neurons control muscle contraction, their dysfunction (as seen in SMA or Polio) typically presents with neurogenic atrophy and normal or only mildly elevated CPK. * **C. Basement membrane:** While the dystrophin-glycoprotein complex interacts with the basement membrane (specifically laminin), the primary pathology and the source of elevated CPK is the muscle cell itself, not the membrane scaffold. * **D. All body cells:** The defect in DMD is specific to the expression of dystrophin, which is primarily found in skeletal, cardiac, and smooth muscle, as well as certain brain neurons—not every cell in the body. **High-Yield Clinical Pearls for NEET-PG:** * **DMD Inheritance:** X-linked recessive; caused by a **deletion** mutation in the *DMD* gene (the largest known human gene). * **Gowers’ Sign:** A classic physical exam finding where the child uses their hands to "climb up" their own thighs to stand, indicating proximal muscle weakness. * **Pseudohypertrophy:** The calves appear large but are actually composed of fibrofatty tissue, not muscle. * **Gold Standard Diagnosis:** Genetic testing (MLPA); Muscle biopsy shows absent dystrophin staining.

Question 252: Which of the following nerve fibers is unmyelinated?

A. A-alpha
B. A-beta
C. A-delta
D. C fiber (Correct Answer)

Explanation: ### Explanation The classification of nerve fibers is a high-yield topic in NEET-PG, primarily based on the **Erlanger-Gasser classification**, which categorizes fibers according to their diameter, conduction velocity, and presence of myelin. **Why C fiber is the correct answer:** C fibers are the only nerve fibers in the human body that are **completely unmyelinated**. Because they lack a myelin sheath, they exhibit the smallest diameter and the slowest conduction velocity (0.5–2.0 m/s). They primarily transmit "slow" or "second" pain (dull, aching), temperature, and postganglionic autonomic signals. **Analysis of Incorrect Options:** * **A-alpha (Aα):** These are the thickest and most heavily myelinated fibers. They have the fastest conduction velocity and are responsible for proprioception and somatic motor function. * **A-beta (Aβ):** These are also myelinated fibers with a medium diameter. They primarily transmit sensations of touch and pressure. * **A-delta (Aδ):** While thinner than Aα and Aβ, these are still **myelinated**. They are responsible for "fast" or "first" pain (sharp, localized) and cold temperature. **High-Yield Clinical Pearls for NEET-PG:** * **Susceptibility to Blockade:** * **Local Anesthetics:** Block **Type C fibers first** (smallest diameter) and Type A-alpha last. * **Pressure:** Affects **Type A fibers first** (e.g., "limb falling asleep"). * **Hypoxia:** Affects **Type B fibers first**, followed by Type A and then C. * **Type B fibers:** These are preganglionic autonomic fibers; they are myelinated but have a smaller diameter than Type A fibers. * **Rule of Thumb:** Conduction velocity in myelinated fibers (m/s) is approximately **6 × diameter (μm)**.

Question 253: Which of the following is NOT involved in fast axonal transport?

A. Kinesin
B. Dynein
C. Lysosomes
D. Neurofilaments (Correct Answer)

Explanation: **Explanation:** Axonal transport is the process by which organelles and proteins are moved along the axon. It is categorized into **Fast** and **Slow** transport based on speed and the machinery involved. **Why Neurofilaments is the correct answer:** Neurofilaments, along with microtubules and microfilaments, form the structural cytoskeleton of the neuron. They are transported from the cell body to the nerve terminals via **Slow Axonal Transport** (specifically Slow Component b). Unlike fast transport, slow transport does not utilize membrane-bound vesicles and moves at a rate of 0.1 to 10 mm/day. **Analysis of Incorrect Options:** * **Kinesin:** This is a molecular motor protein responsible for **Fast Anterograde Transport** (away from the cell body). It moves organelles and mitochondria along microtubules. * **Dynein:** This motor protein is responsible for **Fast Retrograde Transport** (toward the cell body). It carries endosomes and "used" materials for degradation. * **Lysosomes:** These are membrane-bound organelles. All membrane-bound structures (including mitochondria, secretory vesicles, and lysosomes) are moved via **Fast Axonal Transport** (200–400 mm/day). **High-Yield Clinical Pearls for NEET-PG:** * **Retrograde Transport & Pathology:** Certain neurotropic viruses (Rabies, Herpes Simplex, Polio) and the Tetanus toxin reach the CNS via **fast retrograde transport** (Dynein-mediated). * **Mechanism:** Fast transport requires **ATP** and **Microtubules** (the "tracks"). * **Slow Transport:** Always **anterograde**; it moves structural proteins (neurofilaments, tubulin) and soluble enzymes. It is the rate-limiting factor for nerve regeneration.

Question 254: What occurs when sodium enters cells?

A. There is a spike in action potential (Correct Answer)
B. There is a plateau in action potential
C. There is repolarization
D. There is hyperpolarization

Explanation: ### Explanation **Correct Answer: A. There is a spike in action potential** The fundamental mechanism behind the generation of an action potential is the rapid change in membrane permeability. When a stimulus reaches the threshold potential, **voltage-gated sodium (Na⁺) channels** open rapidly. Sodium ions, being higher in concentration in the extracellular fluid, rush into the cell following their electrochemical gradient. This influx of positive charge causes **depolarization**, leading to the rapid upstroke or "spike" of the action potential. **Why other options are incorrect:** * **B. Plateau in action potential:** This is characteristic of cardiac ventricular muscle fibers. It is caused by the slow prolonged opening of **L-type Calcium channels** (Ca²⁺ influx) balancing the efflux of Potassium (K⁺), not by sodium entry alone. * **C. Repolarization:** This phase occurs after the spike and is primarily due to the **closure of Na⁺ channels** and the **opening of voltage-gated K⁺ channels**, leading to an efflux of K⁺ ions from the cell. * **D. Hyperpolarization:** This occurs when the membrane potential becomes more negative than the resting membrane potential, usually due to an **excessive efflux of K⁺ ions** or an **influx of Cl⁻ ions**. **High-Yield Clinical Pearls for NEET-PG:** * **Tetrodotoxin (Pufferfish) & Saxitoxin:** Block voltage-gated Na⁺ channels, preventing the action potential spike and causing paralysis. * **Local Anesthetics (e.g., Lidocaine):** Work by blocking voltage-gated Na⁺ channels from the inside, preventing signal conduction. * **Overshoot:** The portion of the action potential where the membrane potential becomes positive (above 0 mV) due to continued Na⁺ influx. * **Na⁺-K⁺ ATPase:** Does not participate in the action potential itself; it works to restore the ionic gradients *after* the electrical activity is complete.

Question 255: All of the following structures undergo mass contraction except?

A. Ureter (Correct Answer)
B. Uterus
C. Urinary bladder
D. Gall bladder

Explanation: ### Explanation The concept of **mass contraction** refers to the synchronous contraction of an entire organ as a single unit. This is a characteristic feature of **Single-unit (Visceral) Smooth Muscle**, where cells are electrically coupled via **gap junctions**, allowing action potentials to spread rapidly across the entire tissue. **Why Ureter is the Correct Answer:** The **ureter** does not undergo mass contraction; instead, it exhibits **peristaltic contractions**. The smooth muscle of the ureter is organized to contract in a progressive, wave-like fashion to propel urine from the renal pelvis to the bladder. If the entire ureter contracted simultaneously (mass contraction), it would impede the forward flow of urine and cause reflux. **Analysis of Incorrect Options:** * **Uterus:** A classic example of a single-unit smooth muscle organ. During labor, the entire myometrium undergoes coordinated mass contractions (stimulated by oxytocin) to expel the fetus. * **Urinary Bladder:** The **detrusor muscle** acts as a single unit. During micturition, the entire bladder wall contracts simultaneously to increase intravesical pressure and void urine. * **Gall Bladder:** Undergoes mass contraction in response to **Cholecystokinin (CCK)** to squeeze bile into the cystic duct and eventually the duodenum. **High-Yield NEET-PG Pearls:** * **Single-unit Smooth Muscle:** Found in the GI tract, uterus, bladder, and small blood vessels. They show "pacemaker activity" and "syncytial" behavior. * **Multi-unit Smooth Muscle:** Found in the **Iris, Ciliary body, and Piloerector muscles**. These act independently, lack gap junctions, and do not show mass contraction or spontaneous rhythmicity. * **Ureteral Peristalsis:** The rate is typically 2–5 times per minute, initiated by pacemaker cells in the renal pelvis.

Question 256: Which of the following best describes the subunit composition of voltage-gated sodium channels?

A. Multimeric
B. Pentameric
C. Heterotrimer (Correct Answer)
D. Monomeric

Explanation: **Explanation:** The voltage-gated sodium channel (VGSC) is a complex transmembrane protein essential for the generation and propagation of action potentials in excitable tissues. **Why Heterotrimer is correct:** The functional voltage-gated sodium channel is a **heterotrimer** consisting of three distinct subunits: 1. **One Alpha ($\alpha$) subunit:** This is the large, pore-forming subunit (approx. 260 kDa) that contains the voltage sensor and the selectivity filter. It is sufficient for ion conduction on its own. 2. **Two Beta ($\beta$) subunits ($\beta1$ and $\beta2$):** These are smaller auxiliary proteins (approx. 30-40 kDa) that modulate the kinetics of channel gating, assist in membrane localization, and link the channel to the cytoskeleton. **Analysis of Incorrect Options:** * **Monomeric:** While the $\alpha$-subunit can function alone in experimental settings, the physiological channel in vivo requires the $\beta$-subunits for stability and proper signaling. * **Pentameric:** This structure is characteristic of **Ligand-gated ion channels**, such as the Nicotinic Acetylcholine Receptor (nAChR). * **Multimeric:** This is a generic term. "Heterotrimer" is the specific and more accurate description of the sodium channel's composition. **High-Yield NEET-PG Pearls:** * **The $\alpha$-subunit** consists of 4 homologous domains (I-IV), each containing 6 transmembrane segments (S1-S6). * **S4 Segment:** Acts as the **voltage sensor** (rich in positively charged Arginine and Lysine). * **P-loop (between S5-S6):** Forms the **selectivity filter**. * **Clinical Correlation:** Local anesthetics (like Lidocaine) and Tetrodotoxin (Pufferfish toxin) bind to the $\alpha$-subunit to block sodium influx, preventing depolarization. * **Channelopathies:** Mutations in sodium channel subunits are linked to conditions like **Hyperkalemic Periodic Paralysis** and **Dravet Syndrome**.

Question 257: When measuring skeletal muscle tension that develops during isometric contractions, what is observed regarding the relationship between active tension and muscle fiber length?

A. Total tension is inversely proportional to the length of the fiber.
B. Total tension increases monotonically with the length of the fiber.
C. Active tension increases monotonically with the length of the fiber.
D. Active tension first increases then decreases with the length of the fiber. (Correct Answer)

Explanation: ### Explanation The relationship between muscle fiber length and tension is governed by the **Sliding Filament Theory** and the **Length-Tension Curve**. **1. Why Option D is Correct:** Active tension is the force generated specifically by the cross-bridge cycling between actin and myosin filaments. According to the **Frank-Starling mechanism** (applied to skeletal muscle), there is an **optimal resting length ($L_0$)** where the overlap between actin and myosin is maximal. * **At short lengths:** Filaments overlap too much, causing mechanical interference and reducing tension. * **At optimal length ($L_0$):** Maximum cross-bridge formation occurs, resulting in peak active tension. * **At long lengths:** Filaments are pulled apart, reducing the number of available cross-bridges, causing active tension to decrease. Thus, active tension follows an **inverted U-shaped curve** (increases then decreases). **2. Why Other Options are Incorrect:** * **Option A:** Total tension is the sum of active and passive tension. It generally increases at longer lengths due to the massive contribution of passive elastic elements (like titin), not an inverse relationship. * **Option B:** While total tension does eventually increase at very long lengths due to passive stretch, it does not increase "monotonically" (constantly) because the active component drops off first. * **Option C:** Active tension does not increase indefinitely; it drops to zero once the muscle is stretched beyond the point where actin and myosin can touch. **3. High-Yield Facts for NEET-PG:** * **Passive Tension:** Developed by stretching the connective tissue and the protein **Titin** (the "molecular spring"). It increases exponentially with length. * **Total Tension:** Active Tension + Passive Tension. * **Optimal Length ($L_0$):** In humans, this is usually about 2.0 to 2.2 micrometers per sarcomere. * **Clinical Correlation:** In **Heart Failure**, the cardiac muscle is overstretched beyond $L_0$, leading to a decrease in contractile force (moving down the right limb of the Starling curve).

Question 258: In malignant hyperthermia, excess heat production is due to:

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

Explanation: **Explanation:** **Malignant Hyperthermia (MH)** is a life-threatening pharmacogenetic hypermetabolic disorder of skeletal muscle. **1. Why Option A is Correct:** The pathophysiology involves a mutation in the **Ryanodine Receptor (RYR1)** located on the sarcoplasmic reticulum. When triggered by volatile anesthetics (e.g., Halothane) or depolarizing muscle relaxants (e.g., Succinylcholine), there is an **uncontrolled release of Calcium ions ($Ca^{2+}$)** into the sarcoplasm. This massive calcium surge causes: * Continuous muscle contraction (rigidity). * Overactivation of $Ca^{2+}$-ATPase pumps to sequester calcium back into the reticulum. * Increased ATP consumption, leading to accelerated aerobic and anaerobic metabolism, which generates **excessive heat**, $CO_2$, and lactic acid. **2. Why Other Options are Incorrect:** * **B. Thermic effect of food:** This refers to the energy expenditure for digestion and absorption (SDA), which is unrelated to anesthetic-induced hyperthermia. * **C. Increased sympathetic discharge:** While MH causes tachycardia and hypertension due to hypermetabolism, the primary source of heat is the skeletal muscle itself, not the autonomic nervous system. * **D. Mitochondrial thermogenesis:** While mitochondria are involved in cellular respiration, the "trigger" and primary driver of heat in MH is the calcium-induced mechanical and biochemical activity in the sarcoplasm, not a primary mitochondrial defect. **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** (acts by inhibiting the Ryanodine receptor and preventing $Ca^{2+}$ release). * **Associated Conditions:** Central Core Disease and King-Denborough Syndrome.

Question 259: What is the mechanism of action of curare?

A. Reducing end-plate potential (Correct Answer)
B. Reducing presynaptic potential
C. Inhibits K+ channels
D. Inhibits Na+ channels

Explanation: **Mechanism of Action of Curare** **Explanation of the Correct Answer:** Curare (specifically d-tubocurarine) acts as a **competitive antagonist** at the nicotinic acetylcholine receptors (nAChR) located on the motor end-plate of the neuromuscular junction. By binding to these receptors, it prevents acetylcholine (ACh) from opening the ligand-gated cation channels. This results in a significant decrease in the magnitude of the **End-Plate Potential (EPP)**. When the EPP fails to reach the threshold required to trigger an action potential in the muscle fiber, muscle contraction is inhibited, leading to flaccid paralysis. **Analysis of Incorrect Options:** * **B. Reducing presynaptic potential:** Curare acts post-synaptically. It does not interfere with the nerve action potential or the release of ACh from the presynaptic terminal (unlike Botulinum toxin). * **C. Inhibits K+ channels:** Curare does not target voltage-gated potassium channels. Drugs like Tetraethylammonium (TEA) are classic K+ channel blockers. * **D. Inhibits Na+ channels:** Curare does not block voltage-gated sodium channels. Sodium channel inhibition is the mechanism of local anesthetics (like Lidocaine) or toxins like Tetrodotoxin (TTX). **High-Yield Clinical Pearls for NEET-PG:** * **Reversibility:** Since curare is a competitive inhibitor, its effects can be reversed by increasing the concentration of ACh using **Acetylcholinesterase inhibitors** (e.g., Neostigmine). * **Safety Factor:** Curare reduces the "safety factor" of neuromuscular transmission (the margin by which EPP exceeds the threshold). * **Clinical Use:** Non-depolarizing neuromuscular blockers (derivatives of curare like Atracurium or Vecuronium) are used in anesthesia to provide muscle relaxation. * **Contrast:** Unlike Succinylcholine (a depolarizing blocker), curare does not cause initial fasciculations.

Question 260: Which nerve fiber exhibits the slowest conduction velocity?

A. Pre-ganglionic autonomic fibers
B. Post-ganglionic autonomic fibers (Correct Answer)
C. Somatic motor fibers
D. Fibers carrying proprioception

Explanation: **Explanation:** The conduction velocity of a nerve fiber is primarily determined by two factors: **myelination** and **fiber diameter**. According to the **Erlanger-Gasser classification**, nerve fibers are categorized into Types A, B, and C. **1. Why Post-ganglionic autonomic fibers are correct:** Post-ganglionic autonomic fibers are classified as **Type C fibers**. These are the only nerve fibers in the human body that are **unmyelinated** and have the smallest diameter (0.4–1.2 μm). Because they lack the insulating myelin sheath required for saltatory conduction, they exhibit the slowest conduction velocity, typically ranging from **0.5 to 2.0 m/s**. **2. Analysis of Incorrect Options:** * **Pre-ganglionic autonomic fibers (Option A):** These are **Type B fibers**. They are myelinated (though thinly) and have a larger diameter than Type C fibers, resulting in intermediate conduction speeds (3–15 m/s). * **Somatic motor fibers (Option C):** These are **Type A-alpha (Aα) fibers**. They are heavily myelinated with large diameters, making them among the fastest conducting fibers (70–120 m/s). * **Fibers carrying proprioception (Option D):** These are also **Type Aα (from muscle spindles/GTOs)** or **Aβ fibers**. They are highly myelinated and designed for rapid transmission of sensory information. **High-Yield Clinical Pearls for NEET-PG:** * **Fastest Fibers:** Type A-alpha (Proprioception and Somatic Motor). * **Slowest Fibers:** Type C (Chronic pain, Temperature, Post-ganglionic autonomics). * **Susceptibility to Blockade:** * **Local Anesthetics:** Block Type C fibers first (smallest diameter). * **Pressure:** Blocks Type A fibers first (e.g., "foot falling asleep"). * **Hypoxia:** Blocks Type B fibers first. * **Pain Transmission:** Fast/Sharp pain is carried by **A-delta (Aδ)** fibers, while Slow/Dull-aching pain is carried by **Type C** fibers.

Practice by Chapter

Resting Membrane Potential

Practice Questions

Action Potential Generation and Propagation

Practice Questions

Neuromuscular Junction

Practice Questions

Skeletal Muscle Contraction

Practice Questions

Smooth Muscle Physiology

Practice Questions

Cardiac Muscle Properties

Practice Questions

Muscle Metabolism and Fatigue

Practice Questions

Motor Unit Function

Practice Questions

Neurotransmitters and Receptors

Practice Questions

Electrophysiological Measurements

Practice Questions

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