Nerve and Muscle Physiology Practice Questions

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.

Q: How will you assess the function of a nerve after injury?

Nerve conduction study. ### Explanation **1. Why Nerve Conduction Study (NCS) is the Correct Answer:** Nerve Conduction Study is the gold standard for assessing the **functional integrity** of a peripheral nerve. It involves stimulating a nerve at one point and recording the action potential at another. By measuring parameters like **Conduction Velocity (CV)** and **Compound Muscle Action Potential (CMAP)**, clinicians can determine the site, severity, and type of nerve injury (e.g., Neuropraxia vs. Axonotmesis). It specifically evaluates the nerve's ability to transmit electrical impulses, making it the most direct functional assessment. **2. Why Other Options are Incorrect:** * **Electromyography (EMG):** While often performed alongside NCS, EMG records the electrical activity of **muscles**, not the nerve itself. It detects "denervation potentials" (like fibrillations) that occur in a muscle *secondary* to nerve injury. It is better for assessing the timing of injury and reinnervation rather than the immediate functional status of the nerve trunk. * **Open Dissection:** This is an invasive surgical procedure used to visualize the **anatomical continuity** of a nerve (e.g., checking for a physical transection). It does not provide information on the physiological or functional conduction capacity of the nerve fibers. **3. NEET-PG High-Yield Clinical Pearls:** * **Timing:** NCS/EMG changes are not immediate. It takes roughly **7–14 days** for Wallerian degeneration to occur; testing done too early may yield false negatives. * **Neuropraxia:** Characterized by a "conduction block" on NCS, but the nerve remains excitable distal to the lesion. * **Axonotmesis/Neurotmesis:** Shows a loss of distal excitability once Wallerian degeneration is complete. * **Conduction Velocity:** Primarily reflects the state of the **myelin sheath** (slowed in demyelinating disorders like GBS).

Q: A 20-year-old man presents with fatigue and severe muscle weakness in his limbs, typically occurring after large meals. He is diagnosed with familial periodic paralysis. What is the most likely electrolyte abnormality associated with this condition?

Hypokalemia. **Explanation:** The clinical presentation of muscle weakness following a large meal in a young patient is a classic description of **Hypokalemic Periodic Paralysis (HOKPP)**. **1. Why Hypokalemia is correct:** HOKPP is most commonly caused by mutations in the **voltage-gated calcium channels (Cav1.1)** or sodium channels in skeletal muscle. Large meals, especially those high in carbohydrates, trigger a significant release of **insulin**. Insulin stimulates the **Na⁺-K⁺ ATPase pump**, causing a massive shift of potassium from the extracellular fluid into the intracellular compartment. This results in acute hypokalemia, which leads to hyperpolarization of the muscle membrane, making it less excitable and resulting in flaccid paralysis. **2. Analysis of Incorrect Options:** * **A. Hyperkalemia:** While Hyperkalemic Periodic Paralysis exists, it is typically triggered by fasting, cold exposure, or potassium intake, rather than large carbohydrate-rich meals. * **B. Weakness and atrophy of the hands:** This describes a chronic neurogenic or myogenic pattern (like ALS or distal myopathy). Periodic paralysis is characterized by transient, episodic weakness without significant atrophy between attacks. * **D. Hypercalcemia:** Hypercalcemia typically presents with "stones, bones, abdominal groans, and psychic overtones," but it is not the primary electrolyte driver of meal-induced periodic paralysis. **High-Yield Pearls for NEET-PG:** * **Triggers for HOKPP:** High-carb meals, strenuous exercise followed by rest, and administration of insulin or steroids. * **Channelopathy:** Most common mutation is in the **CACNA1S gene** (L-type Calcium channel). * **Management:** Acute attacks are treated with potassium replacement; prophylaxis involves **Acetazolamide** (carbonic anhydrase inhibitor) or potassium-sparing diuretics. * **Thyrotoxic Periodic Paralysis:** A similar clinical picture seen in hyperthyroidism, particularly in Asian males.

Q: What does a muscle spindle primarily detect?

Length. **Explanation:** The **muscle spindle** is a specialized sensory receptor (proprioceptor) located within the belly of skeletal muscles. Its primary physiological function is to detect **changes in muscle length** and the rate of change in length. **1. Why "Length" is the correct answer:** Muscle spindles are arranged in **parallel** with extrafusal muscle fibers. When a muscle is stretched, the spindle is also stretched, triggering sensory signals via Type Ia (primary) and Type II (secondary) afferent fibers. This information is crucial for the **stretch reflex** (myotatic reflex), which helps maintain muscle tone and posture by causing the muscle to contract when it is lengthened. **2. Why other options are incorrect:** * **Tension (Option A):** Tension is primarily detected by the **Golgi Tendon Organ (GTO)**. Unlike spindles, GTOs are arranged in **series** with muscle fibers and respond to the force of contraction to prevent tendon avulsion. * **Proprioception (Option C):** This is a broad term referring to the sense of self-movement and body position. While muscle spindles *contribute* to proprioception, they are specific receptors for length. In exams, always choose the most specific physiological parameter. * **Stretch (Option D):** While "stretch" is the stimulus, "length" is the physical parameter being measured. In physiological nomenclature, spindles are defined as **length detectors**, whereas GTOs are tension detectors. **High-Yield Clinical Pearls for NEET-PG:** * **Innervation:** Intrafusal fibers are innervated by **Gamma ($\gamma$) motor neurons**, which maintain spindle sensitivity during muscle contraction (Alpha-Gamma co-activation). * **Nuclear Bag vs. Chain:** Nuclear bag fibers detect **dynamic** changes (velocity), while nuclear chain fibers detect **static** changes (length). * **Reflex Arc:** The muscle spindle is the afferent limb of the **monosynaptic** stretch reflex (e.g., Knee-jerk reflex).

Question 1

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 2

Which component possesses ATPase activity?

Accepted Answer

Myosin head

Answer

Actin

Answer

Troponin

Answer

Tropomyosin

Question 3

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 4

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 5

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 6

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

Question 7

How will you assess the function of a nerve after injury?

Accepted Answer

Nerve conduction study

Answer

Open dissection

Answer

Electromyography (EMG)

Answer

None of the above

Question 8

A 20-year-old man presents with fatigue and severe muscle weakness in his limbs, typically occurring after large meals. He is diagnosed with familial periodic paralysis. What is the most likely electrolyte abnormality associated with this condition?

Accepted Answer

Hypokalemia

Answer

Hyperkalemia

Answer

Weakness and atrophy of the hands

Answer

Hypercalcemia

Question 9

Anterograde axonal transport involves all except?

Accepted Answer

Nerve growth factor, toxins, and viruses are transported using kinesin

Answer

Kinesin and dynein molecular motors

Answer

Anterograde transport moves from the cell body toward the axon terminals

Answer

Mitochondria and vesicles containing neurotransmitters are transported using kinesin

Question 10

What does a muscle spindle primarily detect?

Accepted Answer

Length

Answer

Tension

Answer

Proprioception

Answer

Stretch

Nerve and Muscle Physiology — MCQs

Nerve and Muscle Physiology — MCQs

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