Nerve and Muscle Physiology — MCQs

Nerve and Muscle Physiology Indian Medical PG Practice Questions and MCQs

Question 261: Peripheral nerves can withstand ischemia for approximately how long?

A. 30 minutes
B. 1 hour
C. 2 hours
D. None of the above (Correct Answer)

Explanation: **Explanation:** Peripheral nerves are remarkably resilient to ischemia compared to central nervous system tissue. The correct answer is **None of the above** because peripheral nerves can typically withstand ischemia for **up to 6 to 8 hours** before irreversible damage occurs. **1. Why "None of the above" is correct:** Peripheral nerves possess a robust, redundant blood supply (the *vasa nervorum*) and have lower metabolic demands than the brain or spinal cord. While functional conduction (physiological block) may cease within 30–60 minutes of ischemia, the structural integrity of the nerve remains intact for several hours. Irreversible histological damage and permanent loss of function generally do not occur until the 6–8 hour mark. **2. Why other options are incorrect:** * **30 minutes / 1 hour:** These timeframes are too short. While a patient may experience "pins and needles" (paresthesia) or numbness within this window, the nerve fibers remain viable and will recover immediately upon reperfusion. * **2 hours:** This is the standard safe limit for a surgical tourniquet to prevent nerve compression and ischemia, but it is not the limit of nerve *viability*. Nerves can survive significantly longer than this before undergoing necrosis. **High-Yield Clinical Pearls for NEET-PG:** * **Order of Susceptibility:** In a peripheral nerve, **Large myelinated fibers** (Motor, Touch, Pressure) are affected by ischemia *before* small unmyelinated fibers (Pain, Temperature). * **Saturday Night Palsy:** A classic example of neuropraxia caused by prolonged ischemia/compression of the radial nerve. * **Critical Time Window:** For limb reattachment or compartment syndrome, the "golden period" is often cited as 6 hours, correlating with the onset of irreversible nerve and muscle death.

Question 262: Tetanizing stimulation of a muscle causes a sustained forceful contraction due to which of the following?

A. Recruitment phenomenon
B. Failure of Ca++ removal from the sarcoplasm (Correct Answer)
C. Summation of the stimuli
D. "Beneficial effect"

Explanation: ### Explanation **Correct Option: B. Failure of Ca++ removal from the sarcoplasm** Tetanus occurs when a muscle is stimulated at a high frequency, such that individual twitches fuse into a single, sustained contraction. The physiological basis for this is the **persistent elevation of cytosolic Ca++ levels**. Normally, after a single stimulus, Ca++ is pumped back into the Sarcoplasmic Reticulum (SR) via **SERCA** (Sarcoplasmic/Endoplasmic Reticulum Ca++ ATPase). However, during rapid repetitive stimulation, the time between stimuli is too short for the SR to resequester the Ca++. Consequently, Ca++ remains bound to **Troponin C**, keeping the actin-binding sites exposed and allowing continuous cross-bridge cycling. This results in a sustained contraction rather than a series of twitches. **Analysis of Incorrect Options:** * **A. Recruitment phenomenon:** This refers to the activation of additional motor units to increase the force of contraction. While it increases strength, it is not the mechanism behind the fusion of twitches into tetanus. * **C. Summation of the stimuli:** While "Summation of Contractions" leads to tetanus, the question asks for the *cause* of the sustained force. Summation is the *process*, but the underlying molecular cause is the failure of Ca++ removal. * **D. "Beneficial effect":** Also known as the **Treppe (Staircase) phenomenon**, this refers to an increase in contraction strength during the first few stimuli of a rested muscle. Unlike tetanus, the muscle fully relaxes between these stimuli. **High-Yield NEET-PG Pearls:** * **Critical Fusion Frequency:** The minimum frequency of stimulation at which individual twitches fuse into a smooth, sustained contraction (tetanus). * **SERCA Pump:** The primary protein responsible for muscle relaxation by removing Ca++ from the sarcoplasm. * **Rigor Mortis:** Occurs due to the total absence of ATP, preventing the dissociation of myosin heads from actin; distinct from tetanus, which is an active physiological state.

Question 263: Refractory period of a myelinated nerve is due to:

A. Voltage-dependence of Na+ channels (Correct Answer)
B. Time-dependence of Na+ channels
C. Voltage-dependence of K+ channels
D. Time-dependence of K+ channels

Explanation: ### Explanation The **Refractory Period** is the duration following an action potential during which a nerve fiber cannot be re-excited. This phenomenon is primarily governed by the state of **Voltage-Gated Sodium (Na+) Channels**. **Why Option A is Correct:** Voltage-gated Na+ channels have two gates: an outer **activation gate** and an inner **inactivation gate**. During depolarization, the activation gate opens. However, as the membrane potential reaches its peak, the **voltage-sensitive inactivation gate** closes (the "ball and chain" mechanism). This gate will not reopen until the membrane repolarizes to a sufficiently negative threshold. Because this transition between states is triggered by changes in membrane potential, the refractory period is fundamentally due to the **voltage-dependence** of these channels. **Why Other Options are Incorrect:** * **Option B:** While Na+ channels do close after a certain duration, the primary trigger for the transition from the "inactive" to the "closed/rest" state (allowing for a new action potential) is the return to a specific **voltage level**, not merely the passage of time. * **Options C & D:** K+ channels are responsible for repolarization and the "after-hyperpolarization" phase. While they influence the *Relative* Refractory Period by making the cell more negative, they do not dictate the *Absolute* Refractory Period, which is strictly an Na+ channel event. ### High-Yield Clinical Pearls for NEET-PG: * **Absolute Refractory Period (ARP):** Corresponds to the period from the firing level until repolarization is about 1/3rd complete. No stimulus, regardless of strength, can excite the nerve. * **Relative Refractory Period (RRP):** Corresponds to the remaining period of repolarization. A suprathreshold stimulus can trigger a response. * **Function:** The refractory period ensures **one-way propagation** of action potentials and limits the **maximum frequency** of nerve impulses. * **Nerve vs. Heart:** The refractory period in nerves is very short (approx. 1 ms), whereas in cardiac muscle, it is long (250-300 ms), preventing tetanization of the heart.

Question 264: How will you assess the function of a nerve after injury?

A. Nerve conduction study (Correct Answer)
B. Open dissection
C. Electromyography (EMG)
D. None of the above

Explanation: ### 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).

Question 265: 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?

A. Hyperkalemia
B. Weakness and atrophy of the hands
C. Hypokalemia (Correct Answer)
D. Hypercalcemia

Explanation: **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.

Question 266: What does a muscle spindle primarily detect?

A. Tension
B. Length (Correct Answer)
C. Proprioception
D. Stretch

Explanation: **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 267: Orthodromic conduction is:

A. An axon conducting an impulse in one direction only. (Correct Answer)
B. An axon conducting an impulse in both directions.
C. The jumping of depolarization from node to node.
D. The point at which a runaway spike potential occurs.

Explanation: ### Explanation **1. Why Option A is Correct:** In a physiological setting, an action potential travels along an axon in one specific direction: from the **cell body (soma) toward the axon terminal**. This is known as **Orthodromic conduction**. This unidirectional flow is maintained by the **refractory period** of the sodium channels; once a segment of the membrane has depolarized, it enters an absolute refractory state, preventing the impulse from traveling backward. **2. Analysis of Incorrect Options:** * **Option B:** While an axon *can* experimentally conduct in both directions (if stimulated in the middle), this is not the definition of orthodromic. Conduction toward the cell body is termed **Antidromic conduction**. * **Option C:** This describes **Saltatory conduction**, which occurs in myelinated fibers where the impulse "jumps" between the Nodes of Ranvier, significantly increasing conduction velocity. * **Option D:** This refers to the **Threshold potential** or the "firing level," where the inward sodium current exceeds the outward potassium current, triggering an all-or-none action potential. **3. High-Yield Facts for NEET-PG:** * **Synaptic Unidirectionality:** Conduction is strictly one-way across a synapse (from pre-synaptic to post-synaptic) due to the location of neurotransmitter vesicles and receptors. * **Antidromic Conduction:** In clinical practice, this is seen in the **Axon Reflex** (e.g., the flare response in Lewis’s triple response), where sensory nerve impulses travel backward to cause vasodilation. * **Conduction Velocity:** Directly proportional to fiber diameter and myelination. Type A-alpha fibers are the fastest; Type C fibers are the slowest and unmyelinated.

Question 268: What is the mechanism responsible for sustained vascular smooth muscle contraction?

A. Sustained calcium release from SERCA pump
B. Vascular smooth muscle tone
C. Latch bridge mechanism (Correct Answer)
D. Henneman principle

Explanation: **Explanation:** The **Latch bridge mechanism** is the physiological basis for sustained, low-energy contraction in vascular smooth muscle. Unlike skeletal muscle, smooth muscle can maintain high tension for long periods with minimal ATP consumption. **1. Why the Correct Answer is Right:** In smooth muscle, contraction is initiated when Calcium-Calmodulin activates **Myosin Light Chain Kinase (MLCK)**, which phosphorylates the myosin head. Dephosphorylation by **Myosin Light Chain Phosphatase (MLCP)** usually leads to relaxation. However, if dephosphorylation occurs while the myosin head is still attached to actin, the cross-bridge enters a "latch state." In this state, the dissociation of myosin from actin becomes extremely slow, allowing the muscle to maintain tension (tone) without further ATP hydrolysis. **2. Why Other Options are Wrong:** * **Sustained calcium release from SERCA pump:** The SERCA pump is responsible for the **reuptake** of calcium into the sarcoplasmic reticulum (relaxation), not sustained release. * **Vascular smooth muscle tone:** This is a descriptive term for the state of partial contraction, not the *mechanism* that causes it. * **Henneman principle (Size Principle):** This relates to the recruitment order of motor units in **skeletal muscle** (small motor units are recruited before large ones), not smooth muscle contraction. **High-Yield Facts for NEET-PG:** * **Energy Efficiency:** The latch mechanism allows smooth muscle to maintain contraction using 1/300th the energy required by skeletal muscle. * **Calmodulin vs. Troponin:** Smooth muscle lacks Troponin; Calcium binds to **Calmodulin** instead. * **Multi-unit vs. Unitary:** Vascular smooth muscle is typically **Unitary** (visceral), acting as a syncytium via gap junctions.

Question 269: Smooth muscle contraction, which is triggered by the release of calcium, is primarily mediated by which of the following mechanisms?

A. Increased cyclic AMP (cAMP) levels
B. Calcium binding to troponin C
C. Increased cyclic GMP (cGMP) levels
D. Calmodulin-dependent myosin light chain kinase activation (Correct Answer)

Explanation: **Explanation:** In smooth muscle, the contraction mechanism differs significantly from skeletal muscle because smooth muscle **lacks troponin**. The process is primarily "thick-filament regulated." When intracellular calcium ($Ca^{2+}$) increases, it binds to a regulatory protein called **Calmodulin**. This $Ca^{2+}$-Calmodulin complex then activates the enzyme **Myosin Light Chain Kinase (MLCK)**. MLCK phosphorylates the myosin light chain, increasing myosin ATPase activity, which allows myosin to bind to actin and initiate the cross-bridge cycle. **Analysis of Incorrect Options:** * **Option A (cAMP):** Increased cAMP levels in smooth muscle actually lead to **relaxation** (e.g., in bronchial smooth muscle) by inhibiting MLCK and promoting calcium sequestration. * **Option B (Troponin C):** This is the mechanism for **skeletal and cardiac muscle**. Smooth muscle does not contain the troponin complex; it uses Calmodulin as its calcium sensor. * **Option C (cGMP):** Increased cGMP (mediated by Nitric Oxide) leads to **relaxation** of vascular smooth muscle by activating Protein Kinase G, which dephosphorylates myosin light chains via Myosin Light Chain Phosphatase (MLCP). **High-Yield Clinical Pearls for NEET-PG:** * **Latch-bridge mechanism:** Allows smooth muscle to maintain prolonged tension with low ATP consumption (important for vascular tone). * **Relaxation:** Requires **Myosin Light Chain Phosphatase (MLCP)** to remove the phosphate group from the myosin light chain. * **Unitary vs. Multi-unit:** Unitary smooth muscle (e.g., GI tract) acts as a syncytium via gap junctions, whereas multi-unit (e.g., Iris) acts independently. * **Caldesmon and Calponin:** These are other regulatory proteins in smooth muscle that inhibit the actin-myosin interaction.

Question 270: Which statement is TRUE regarding nerve conduction?

A. It follows the all-or-none phenomenon. (Correct Answer)
B. Conduction is dependent on the amplitude.
C. A propagated action potential is generated in the axon hillock.
D. Conduction is faster in myelinated fibers than in unmyelinated fibers.

Explanation: ### Explanation **Correct Option: A. It follows the all-or-none phenomenon.** Nerve conduction is governed by the **All-or-None Law**. This principle states that if a stimulus reaches the threshold potential, an action potential of constant amplitude and duration is generated and conducted. If the stimulus is sub-threshold, no action potential occurs. Once triggered, the magnitude of the impulse does not depend on the strength of the stimulus. **Analysis of Incorrect Options:** * **B. Conduction is dependent on the amplitude:** This is incorrect because the amplitude of an action potential remains constant (stereotyped) regardless of stimulus intensity. Intensity is coded by the **frequency** of firing, not the amplitude. * **C. A propagated action potential is generated in the axon hillock:** While the axon hillock is the site of summation, the action potential is specifically initiated at the **Initial Segment** of the axon (the area between the hillock and the first myelin sheath), which has the highest density of voltage-gated Na+ channels. * **D. Conduction is faster in myelinated fibers:** While this statement is physiologically true, in the context of this specific question (often sourced from standard texts like Guyton), the "All-or-None Law" is considered the most fundamental defining characteristic of nerve fiber conduction. *Note: In many exams, if multiple statements are factually true, the one defining the core physiological principle is preferred.* **High-Yield Clinical Pearls for NEET-PG:** * **Saltatory Conduction:** Occurs in myelinated fibers where the impulse "jumps" from one **Node of Ranvier** to the next, significantly increasing velocity. * **Erlanger-Gasser Classification:** Type A-alpha fibers are the fastest (proprioception/motor), while Type C fibers are the slowest (unmyelinated, pain/temperature). * **Local Anesthetics:** These block nerve conduction by inhibiting voltage-gated Na+ channels, preventing the threshold from being reached (abolishing the all-or-none response).

Practice by Chapter

Resting Membrane Potential

Practice Questions

Action Potential Generation and Propagation

Practice Questions

Neuromuscular Junction

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Skeletal Muscle Contraction

Practice Questions

Smooth Muscle Physiology

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Cardiac Muscle Properties

Practice Questions

Muscle Metabolism and Fatigue

Practice Questions

Motor Unit Function

Practice Questions

Neurotransmitters and Receptors

Practice Questions

Electrophysiological Measurements

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