Nerve and Muscle Physiology Practice Questions

Q: Which of the following nerve fibers is unmyelinated?

C fiber. ### 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)**.

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

Neurofilaments. **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.

Q: What occurs when sodium enters cells?

There is a spike in action potential. ### 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.

Q: All of the following structures undergo mass contraction except?

Ureter. ### 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.

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

Heterotrimer. **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**.

Q: In malignant hyperthermia, excess heat production is due to:

Increased muscle metabolism by excess of calcium ions. **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.

Q: What is the mechanism of action of curare?

Reducing end-plate potential. **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.

Q: Which nerve fiber exhibits the slowest conduction velocity?

Post-ganglionic autonomic fibers. **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.

Question 1

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

Accepted Answer

Muscle fibers

Answer

Neurons

Answer

Basement membrane

Answer

All body cells

Question 2

Which of the following nerve fibers is unmyelinated?

Accepted Answer

C fiber

Answer

A-alpha

Answer

A-beta

Answer

A-delta

Question 3

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

Accepted Answer

Neurofilaments

Answer

Kinesin

Answer

Dynein

Answer

Lysosomes

Question 4

What occurs when sodium enters cells?

Accepted Answer

There is a spike in action potential

Answer

There is a plateau in action potential

Answer

There is repolarization

Answer

There is hyperpolarization

Question 5

All of the following structures undergo mass contraction except?

Accepted Answer

Ureter

Answer

Uterus

Answer

Urinary bladder

Answer

Gall bladder

Question 6

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

Accepted Answer

Heterotrimer

Answer

Multimeric

Answer

Pentameric

Answer

Monomeric

Question 7

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

Accepted Answer

Active tension first increases then decreases with the length of the fiber.

Answer

Total tension is inversely proportional to the length of the fiber.

Answer

Total tension increases monotonically with the length of the fiber.

Answer

Active tension increases monotonically with the length of the fiber.

Question 8

In malignant hyperthermia, excess heat production is due to:

Accepted Answer

Increased muscle metabolism by excess of calcium ions

Answer

Thermic effect of food

Answer

Increased sympathetic discharge

Answer

Mitochondrial thermogenesis

Question 9

What is the mechanism of action of curare?

Accepted Answer

Reducing end-plate potential

Answer

Reducing presynaptic potential

Answer

Inhibits K+ channels

Answer

Inhibits Na+ channels

Question 10

Which nerve fiber exhibits the slowest conduction velocity?

Accepted Answer

Post-ganglionic autonomic fibers

Answer

Pre-ganglionic autonomic fibers

Answer

Somatic motor fibers

Answer

Fibers carrying proprioception

Nerve and Muscle Physiology — MCQs

Nerve and Muscle Physiology — MCQs

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