Nerve and Muscle Physiology Practice Questions

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.

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.

Q: Which component possesses ATPase activity?

Myosin head. **Explanation:** The correct answer is **A. Myosin head**. **Why Myosin head is correct:** Muscle contraction is an energy-dependent process requiring the hydrolysis of ATP. The **Myosin molecule** consists of a tail and a globular head (Cross-bridge). The myosin head functions as a **magnesium-dependent ATPase enzyme**. It contains a specific binding site where it binds to ATP and hydrolyzes it into ADP and inorganic phosphate (Pi). This hydrolysis "cocks" the myosin head into a high-energy state, allowing it to bind to actin and perform the "power stroke," which is the fundamental mechanism of the Sliding Filament Theory. **Why the other options are incorrect:** * **B. Actin:** Actin is a globular protein (G-actin) that polymerizes into filaments (F-actin). Its primary role is to provide the binding sites for myosin heads; it does not possess enzymatic/ATPase activity. * **C. Troponin:** This is a regulatory protein complex consisting of three subunits: Troponin C (binds Calcium), Troponin I (inhibitory), and Troponin T (binds tropomyosin). It acts as a switch but does not hydrolyze ATP. * **D. Tropomyosin:** This is a fibrous protein that covers the active sites on actin during the resting state, preventing interaction with myosin. It lacks enzymatic activity. **High-Yield NEET-PG Pearls:** * **Rate-limiting step:** The rate of ATP hydrolysis by Myosin ATPase determines the speed of muscle contraction (Fast-twitch vs. Slow-twitch fibers). * **Rigor Mortis:** Occurs because ATP is required for the *detachment* of the myosin head from actin. Without ATP, the cross-bridge remains permanently locked. * **Calcium Source:** In skeletal muscle, calcium is released from the Sarcoplasmic Reticulum (SR) via **Ryanodine receptors (RyR1)**.

Q: In the action potential of a nerve, what is the 'overshoot'?

Above the iso-potential line. ### Explanation **1. Why Option A is Correct:** In the context of an action potential, the **overshoot** refers to the portion of the depolarization phase where the membrane potential becomes **positive**. * The **iso-potential line** (or zero potential line) represents 0 mV. * During the rapid influx of $Na^+$ ions, the membrane potential moves from its negative resting state, crosses the zero mark, and reaches a peak (usually around +35 mV). * Therefore, any value **above the iso-potential line** is termed the overshoot. It represents the reversal of membrane polarity. **2. Why the Other Options are Incorrect:** * **Option B:** Values below the iso-potential line are simply part of the depolarization or repolarization phases within the negative range. * **Option C:** Values below the resting membrane potential (RMP) are referred to as **After-hyperpolarization** (or undershoot), caused by the slow closure of $K^+$ channels. * **Option D:** While the overshoot is technically above the RMP, this is an incomplete definition. The region between the RMP (-70 mV) and the iso-potential line (0 mV) is simply "depolarization." The specific term "overshoot" is reserved only for the positive excursion beyond 0 mV. **3. High-Yield Clinical Pearls for NEET-PG:** * **Ionic Basis:** The overshoot is primarily due to the opening of voltage-gated $Na^+$ channels. It never reaches the $Na^+$ equilibrium potential (+60 mV) because $Na^+$ channels begin to inactivate and $K^+$ channels begin to open. * **Threshold Stimulus:** For an action potential to occur, the stimulus must reach the **firing level** (usually -55 mV). * **Tetrodotoxin (TTX):** A potent toxin found in Pufferfish that blocks voltage-gated $Na^+$ channels, preventing the overshoot and the action potential entirely. * **After-hyperpolarization:** This phase is responsible for the **relative refractory period**.

Q: Most of the ATP generated in nerve cells is utilized to energize which of the following pumps or processes?

Na-K ATPase. **Explanation:** In nerve cells, the maintenance of the resting membrane potential (RMP) and the restoration of ionic gradients following action potentials are the most energy-demanding processes. The **Na-K ATPase (Sodium-Potassium Pump)** is responsible for this, pumping 3 Na⁺ ions out and 2 K⁺ ions into the cell against their concentration gradients. In the central nervous system, it is estimated that approximately **50% to 70% of the total ATP** generated is consumed by this pump alone to maintain cellular excitability and osmotic balance. **Analysis of Options:** * **Na-Ca Exchanger (Option A):** This is primarily a secondary active transporter (driven by the Na⁺ gradient created by Na-K ATPase) and does not consume the majority of cellular ATP directly. * **H-ATPase (Option B):** While present in certain organelles (like synaptic vesicles) and specific cell types (like renal intercalated cells), it is not the primary consumer of energy in a standard nerve cell. * **Synthesis of Proteins (Option D):** While protein synthesis is energy-intensive, it accounts for a much smaller fraction of the total energy budget in a mature neuron compared to the continuous demands of ion transport. **High-Yield Clinical Pearls for NEET-PG:** * **Electrogenic Nature:** The Na-K ATPase is electrogenic because it moves 3 positive charges out for every 2 in, contributing roughly -4 to -10 mV to the RMP. * **Digitalis/Ouabain:** These drugs specifically inhibit the Na-K ATPase by binding to the extracellular alpha subunit. * **Metabolic Rate:** The brain represents only 2% of body weight but consumes 20% of the body's total oxygen, largely to fuel the Na-K ATPase.

Question 1

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 2

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 3

Which type of nerve fiber innervates the extrafusal muscle fibers?

Accepted Answer

Aα

Answer

Ia

Answer

Ib

Answer

Aδ

Question 4

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 5

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

Question 6

Which component possesses ATPase activity?

Accepted Answer

Myosin head

Answer

Actin

Answer

Troponin

Answer

Tropomyosin

Question 7

In the action potential of a nerve, what is the 'overshoot'?

Accepted Answer

Above the iso-potential line

Answer

Below the iso-potential line

Answer

Below the resting membrane potential

Answer

Above the resting membrane potential

Question 8

Tetanizing stimulation of a muscle causes a sustained forceful contraction due to which of the following?

Accepted Answer

Failure of Ca++ removal from the sarcoplasm

Answer

Recruitment phenomenon

Answer

Summation of the stimuli

Answer

"Beneficial effect"

Question 9

Most of the ATP generated in nerve cells is utilized to energize which of the following pumps or processes?

Accepted Answer

Na-K ATPase

Answer

Na-Ca exchanger

Answer

H-ATPase in the cell membrane

Answer

Synthesis of proteins

Question 10

Refractory period of a myelinated nerve is due to:

Accepted Answer

Voltage-dependence of Na+ channels

Answer

Time-dependence of Na+ channels

Answer

Voltage-dependence of K+ channels

Answer

Time-dependence of K+ channels

Nerve and Muscle Physiology — MCQs

Nerve and Muscle Physiology — MCQs

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