Nerve and Muscle Physiology Practice Questions

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: Afferent of Golgi tendon organ arises from which structure?

Tendon. **Explanation:** The **Golgi Tendon Organ (GTO)** is a specialized sensory receptor located at the **musculotendinous junction**, where muscle fibers insert into the **tendon**. 1. **Why Option B is Correct:** The GTO is arranged in **series** with the extrafusal muscle fibers. When a muscle contracts, it pulls on the tendon, stretching the GTO. This mechanical deformation activates **Ib afferent nerve fibers**. Unlike the muscle spindle (which detects change in length), the GTO primarily functions as a **force/tension sensor**. It mediates the **inverse stretch reflex** (autogenic inhibition), causing the muscle to relax when tension becomes excessive to prevent injury. 2. **Why Other Options are Incorrect:** * **Option A (Muscle Spindle):** These are located in the muscle belly, arranged in **parallel** with extrafusal fibers. They detect muscle **length** and mediate the stretch reflex via **Ia and II afferents**. * **Option C (Periosteum):** The periosteum contains free nerve endings and Pacinian corpuscles primarily sensitive to pressure and pain, not muscle tension. * **Option D (Joint):** Joint receptors (like Ruffini endings or Paciniform corpuscles) provide information regarding joint position and angle (proprioception), not the tension within a specific muscle-tendon unit. **High-Yield NEET-PG Pearls:** * **Afferent Fiber Type:** GTO = **Ib** fibers; Muscle Spindle = **Ia** (primary) and **II** (secondary) fibers. * **Arrangement:** GTO is in **Series**; Muscle Spindle is in **Parallel**. * **Function:** GTO prevents over-contraction (Tension); Spindle prevents over-stretching (Length). * **Reflex:** GTO mediates the **Inverse Stretch Reflex** (polysynaptic/inhibitory).

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: Which of the following best describes the trigger for muscle contraction?

Calcium binding to troponin C. **Explanation:** The initiation of muscle contraction follows the **Sliding Filament Theory**, specifically the process of excitation-contraction coupling. **Why Option B is Correct:** In a resting muscle, the binding sites on actin are covered by the **troponin-tropomyosin complex**, preventing interaction with myosin. When an action potential triggers the release of Calcium ($Ca^{2+}$) from the sarcoplasmic reticulum, the $Ca^{2+}$ ions bind specifically to **Troponin C** (the calcium-binding subunit). This binding induces a conformational change in the entire troponin complex, which pulls the tropomyosin away from the active sites on actin, allowing the myosin head to form a cross-bridge and initiate contraction. **Why Other Options are Incorrect:** * **Option A:** Calcium does not bind directly to tropomyosin; it binds to Troponin C, which then moves the tropomyosin. * **Option C:** ATP breakdown (hydrolysis) provides the energy for the "power stroke" and the detachment of the myosin head, but it is not the initial *trigger* for the contraction process. * **Option D:** Troponin I is the **inhibitory** subunit that binds to actin to prevent contraction; it does not bind calcium. **High-Yield NEET-PG Pearls:** * **Troponin Subunits:** Remember **T-I-C**: **T** (binds to **T**ropomyosin), **I** (**I**nhibits actin-myosin interaction), **C** (binds **C**alcium). * **Cardiac Biomarkers:** Troponin I and T are specific markers for myocardial infarction (Troponin C is not used as it is identical in both skeletal and cardiac muscle). * **Rigor Mortis:** Occurs due to the lack of ATP, which is required for the *detachment* of myosin from actin.

Q: Which type of nerve fiber innervates the extrafusal muscle fibers?

Aα. ### Explanation The correct answer is **Aα (Alpha motor neurons)**. **1. Why Aα is correct:** In skeletal muscle physiology, muscle fibers are divided into two types: **extrafusal** and **intrafusal**. Extrafusal fibers constitute the bulk of the muscle and are responsible for generating the force required for contraction and movement. These fibers are innervated by **Alpha (Aα) motor neurons**, which are the largest, fastest-conducting myelinated fibers. When an action potential reaches the neuromuscular junction of an extrafusal fiber via an Aα neuron, it triggers the release of acetylcholine, leading to muscle contraction. **2. Why the other options are incorrect:** * **Ia (Primary Afferents):** These are **sensory** (afferent) fibers, not motor. They wrap around the central portion of the muscle spindle (intrafusal fibers) and detect the *rate of change* in muscle length. * **Ib (Golgi Tendon Organ Afferents):** These are also **sensory** fibers. They originate from the Golgi Tendon Organs (GTO) and monitor muscle *tension* to prevent over-contraction. * **Aδ (Delta):** These are small, myelinated fibers primarily involved in transmitting "fast pain" and temperature sensations from the skin. They do not innervate muscle fibers. **3. High-Yield Clinical Pearls for NEET-PG:** * **Aγ (Gamma motor neurons):** These innervate the **intrafusal fibers** of the muscle spindle. They maintain spindle sensitivity during contraction (Alpha-Gamma co-activation). * **Size Principle (Henneman's):** Smaller motor units are recruited before larger ones. * **Conduction Velocity:** Aα fibers have the highest conduction velocity (70–120 m/s) due to their large diameter and heavy myelination. * **Reflex Arc:** In the stretch reflex (e.g., knee jerk), the afferent limb is **Ia** and the efferent limb is **Aα**.

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.

Question 1

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

Accepted Answer

Neurofilaments

Answer

Kinesin

Answer

Dynein

Answer

Lysosomes

Question 2

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 3

All of the following structures undergo mass contraction except?

Accepted Answer

Ureter

Answer

Uterus

Answer

Urinary bladder

Answer

Gall bladder

Question 4

Afferent of Golgi tendon organ arises from which structure?

Accepted Answer

Tendon

Answer

Muscle spindle

Answer

Periosteum

Answer

Joint

Question 5

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

Accepted Answer

Heterotrimer

Answer

Multimeric

Answer

Pentameric

Answer

Monomeric

Question 6

Which of the following best describes the trigger for muscle contraction?

Accepted Answer

Calcium binding to troponin C

Answer

Calcium binding to tropomyosin

Answer

ATP breakdown

Answer

Calcium binding to troponin I

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

What is the source of calcium for smooth muscle contraction?

Accepted Answer

Combination of influx of Ca2+ through voltage-gated and ligand-gated Ca2+ channels

Answer

Influx into intracellular stores through IP3 receptor Ca2+ channels

Answer

Efflux through voltage-gated Ca2+ channels

Answer

Efflux through ligand-gated Ca2+ channels

Question 9

Which type of nerve fiber innervates the extrafusal muscle fibers?

Accepted Answer

Aα

Answer

Ia

Answer

Ib

Answer

Aδ

Question 10

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

Nerve and Muscle Physiology — MCQs

Nerve and Muscle Physiology — MCQs

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