Nerve and Muscle Physiology — MCQs

Nerve and Muscle Physiology Indian Medical PG Practice Questions and MCQs

Question 191: Which of the following is true about Duchenne's muscular dystrophy?

A. It is an X-linked recessive disorder. (Correct Answer)
B. Skeletal muscle wasting occurs due to abnormalities of the contractile filaments.
C. Muscle wasting occurs in the pelvic region only.
D. It is a form of titanopathy.

Explanation: **Explanation:** **Duchenne Muscular Dystrophy (DMD)** is the most common and severe form of muscular dystrophy. 1. **Why Option A is correct:** DMD is an **X-linked recessive** disorder caused by a mutation in the *DMD* gene located on the short arm of the X chromosome (Xp21). This gene is one of the largest in the human body, making it highly susceptible to spontaneous mutations. Because it is X-linked, it primarily affects males, while females are typically asymptomatic carriers. 2. **Why the other options are incorrect:** * **Option B:** The defect is not in the contractile filaments (actin/myosin) but in **Dystrophin**, a structural protein. Dystrophin links the intracellular cytoskeleton (F-actin) to the extracellular matrix via the dystroglycan complex. Its absence leads to membrane instability and muscle fiber necrosis. * **Option C:** While muscle weakness begins in the **pelvic girdle** (proximal muscles), it is progressive and eventually involves the shoulder girdle, respiratory muscles, and the heart (cardiomyopathy). It is not restricted to the pelvic region. * **Option D:** DMD is a **dystrophinopathy**, not a titanopathy. Titanopathies (like certain forms of Limb-Girdle Muscular Dystrophy) involve mutations in the protein Titin, which handles muscle elasticity. **High-Yield Clinical Pearls for NEET-PG:** * **Gower’s Sign:** The child uses their hands to "climb up" their own body to stand, indicating proximal muscle weakness. * **Pseudohypertrophy:** The calves appear enlarged due to the replacement of muscle tissue with fat and connective tissue (fibrosis). * **Diagnosis:** Elevated **Serum Creatine Kinase (CK)** levels (often 10–100x normal) are seen from birth. Genetic testing is the gold standard; muscle biopsy shows absent dystrophin. * **Becker Muscular Dystrophy (BMD):** A milder form caused by *truncated* (functional but abnormal) dystrophin, whereas DMD involves a total *absence* of dystrophin.

Question 192: Which of the following factors regulate the rate of firing of axons during nerve conduction?

A. K+ channels
B. Refractory period
C. Na+ channels opening (Correct Answer)
D. Na+ channels closing

Explanation: **Explanation:** The **rate of firing** (frequency of action potentials) in an axon is primarily determined by the speed and efficiency with which the membrane reaches the threshold potential to trigger an action potential. **Why Option C is Correct:** The initiation of an action potential depends on the **opening of voltage-gated Na+ channels**. Once the threshold potential is reached, these channels open rapidly (positive feedback loop), leading to a massive influx of Na+ ions and depolarization. The rate at which these channels transition from a closed/resting state to an open state directly dictates how quickly subsequent action potentials can be generated. Therefore, the kinetics of Na+ channel opening is the fundamental regulator of the firing rate. **Analysis of Incorrect Options:** * **Refractory Period (Option B):** While the refractory period determines the *maximum* theoretical limit of firing frequency (by preventing immediate re-excitation), it does not "regulate" the active rate of firing under physiological conditions as much as the initial trigger mechanism does. * **K+ channels (Option A):** These are primarily responsible for repolarization and maintaining the resting membrane potential, not the initiation or rate of firing. * **Na+ channels closing (Option D):** This refers to inactivation, which contributes to the absolute refractory period but is a restorative process rather than a regulatory one for firing frequency. **High-Yield Clinical Pearls for NEET-PG:** * **Accommodation:** If a nerve is subjected to a slowly rising current, the threshold for firing increases because Na+ channels have time to inactivate before an action potential triggers. * **Local Anesthetics (e.g., Lidocaine):** These drugs work by blocking voltage-gated Na+ channels from the inside, preventing the initiation and propagation of action potentials. * **Tetrodotoxin (Pufferfish):** Specifically blocks voltage-gated Na+ channels, leading to respiratory paralysis. * **Batrachotoxin (Poison Dart Frog):** Keeps Na+ channels open, preventing repolarization and causing permanent depolarization.

Question 193: What type of fibers are postganglionic fibers?

A. Type C fibers (Correct Answer)
B. Type B fibers
C. Both
D. None

Explanation: **Explanation:** The classification of nerve fibers is based on the **Erlanger-Gasser classification**, which categorizes fibers according to their diameter, myelination, and conduction velocity. **1. Why Type C fibers is the correct answer:** Postganglionic autonomic fibers (both sympathetic and parasympathetic) are **Type C fibers**. These are the smallest in diameter (0.4–1.2 μm) and are **unmyelinated**. Due to the lack of myelin and small size, they have the slowest conduction velocity (0.5–2.0 m/s). Their primary function is to carry autonomic signals to effector organs (smooth muscle, cardiac muscle, and glands) and transmit slow pain/temperature sensations. **2. Why other options are incorrect:** * **Type B fibers:** These are small, **myelinated** fibers. In the autonomic nervous system, **preganglionic fibers** are Type B. They conduct impulses faster than Type C fibers but slower than Type A. * **Type A fibers:** These are large, myelinated fibers subdivided into Alpha (proprioception/somatic motor), Beta (touch/pressure), Gamma (muscle spindles), and Delta (fast pain/cold). They are not involved in postganglionic autonomic transmission. **High-Yield Clinical Pearls for NEET-PG:** * **Myelination Rule:** Preganglionic = Myelinated (Type B); Postganglionic = Unmyelinated (Type C). * **Sensitivity to Local Anesthetics:** Type C fibers are highly sensitive to local anesthetics (like Lidocaine), which is why pain and autonomic function are blocked before motor function (Type A). * **Sensitivity to Pressure/Hypoxia:** Type A fibers are most sensitive to pressure; Type B fibers are most sensitive to hypoxia. * **Fast vs. Slow Pain:** Type A-delta fibers carry "fast/sharp" pain, while Type C fibers carry "slow/dull/aching" pain.

Question 194: Increase in threshold level on applying a subthreshold, slowly rising stimulus is known as?

A. Adaptation
B. Accommodation (Correct Answer)
C. Refractoriness
D. Electrotonus

Explanation: **Explanation:** The correct answer is **Accommodation**. **Why Accommodation is correct:** Accommodation is a property of excitable tissues (nerve and muscle) where the threshold for excitation increases when a stimulus is applied **slowly**. When a subthreshold stimulus rises gradually, the membrane has time to undergo minor changes that prevent an action potential. * **Mechanism:** The slow depolarization allows time for the **inactivation gates (h-gates) of voltage-gated Na+ channels** to close and for **voltage-gated K+ channels** to open. This counteracts the depolarizing effect, requiring a much stronger stimulus (higher threshold) to trigger an action potential compared to a sudden stimulus. **Why other options are incorrect:** * **Adaptation:** This refers to a decrease in the **frequency of action potentials** fired by a sensory receptor over time while exposed to a constant, maintained stimulus (e.g., smelling a scent until you no longer notice it). It is a property of receptors, not the nerve fiber threshold itself. * **Refractoriness:** This is the period following an action potential during which a second stimulus cannot elicit a new response (Absolute) or requires a suprathreshold stimulus (Relative). It is due to the state of Na+ channels after an impulse, not the rate of stimulus rise. * **Electrotonus:** This refers to the local, non-propagated changes in membrane potential (depolarizing or hyperpolarizing) that occur when subthreshold current flows through the membrane. **High-Yield Clinical Pearls for NEET-PG:** * **Nerve vs. Muscle:** Nerve fibers accommodate much more rapidly than skeletal muscle fibers. * **Hypocalcemia Connection:** Low extracellular calcium levels decrease the threshold for excitation (making the nerve "hyperexcitable"), effectively reducing the degree of accommodation and leading to tetany. * **Clinical Test:** The "Rheobase" and "Chronaxie" are related concepts measuring excitability, but accommodation specifically describes the response to the *gradient* (slope) of the stimulus.

Question 195: Duchenne's muscular dystrophy affects which group of muscles commonly?

A. Calf muscles (Correct Answer)
B. Shoulder muscles
C. Forearm muscles
D. Respiratory muscles

Explanation: **Explanation:** **Duchenne Muscular Dystrophy (DMD)** is an X-linked recessive disorder caused by a mutation in the **dystrophin gene**, leading to the absence of the dystrophin protein. This protein is essential for anchoring the muscle cytoskeleton to the extracellular matrix; its absence leads to progressive muscle fiber necrosis. **Why Calf Muscles are the correct answer:** DMD characteristically presents with **pseudohypertrophy of the calf muscles**. While the muscle fibers are actually wasting (atrophy), the calf appears enlarged because the necrotic muscle tissue is replaced by **fatty and connective tissue (fibrofatty infiltration)**. This is one of the earliest and most classic clinical signs of the disease. **Analysis of Incorrect Options:** * **Shoulder muscles:** While the pelvic girdle is affected first, followed by the shoulder girdle (proximal muscles), they typically show visible atrophy rather than the characteristic "pseudohypertrophy" seen in the calves. * **Forearm muscles:** DMD primarily affects **proximal muscles** (thighs, pelvis, shoulders) before distal muscles. The forearm is usually spared until the very late stages of the disease. * **Respiratory muscles:** These are involved in the terminal stages of the disease. While respiratory failure is a common cause of death, it is not the "commonly affected group" used for initial clinical diagnosis or the most characteristic early finding. **High-Yield Clinical Pearls for NEET-PG:** * **Gower’s Sign:** The child uses their hands to "climb up" their own thighs to stand up, indicating proximal muscle weakness (specifically the gluteus maximus). * **Laboratory Marker:** Significantly elevated **Serum Creatine Kinase (CK)** levels (often 10–100x normal) are seen from birth. * **Becker’s MD:** A milder form where dystrophin is truncated/mutated but present (unlike DMD where it is absent). * **Inheritance:** X-linked recessive (affects males; females are carriers).

Question 196: Which of the following substances is produced by osteoblasts?

A. Collagen (Correct Answer)
B. Calcium
C. Pyrophosphate
D. Monosodium urate

Explanation: **Explanation:** Osteoblasts are the bone-forming cells derived from mesenchymal stem cells. Their primary function is to synthesize and secrete the organic matrix of bone, known as **osteoid**. * **Why Collagen is Correct:** Approximately 90-95% of the organic matrix (osteoid) produced by osteoblasts consists of **Type I Collagen**. This collagen provides the structural framework and tensile strength of the bone. Osteoblasts also secrete non-collagenous proteins like osteocalcin and osteopontin. * **Why other options are incorrect:** * **Calcium:** Calcium is a mineral obtained from the diet and transported via the blood. While osteoblasts facilitate the *deposition* of calcium hydroxyapatite into the matrix, they do not "produce" the element itself. * **Pyrophosphate:** This is a potent **inhibitor** of bone mineralization. It is present in the extracellular fluid to prevent hydroxyapatite precipitation in soft tissues. Osteoblasts actually produce **Alkaline Phosphatase (ALP)** to break down pyrophosphate, thereby allowing mineralization to occur in the bone matrix. * **Monosodium urate:** These are crystals formed by the precipitation of uric acid. They are the causative agents of **Gout** and are not a physiological product of bone cells. **High-Yield NEET-PG Pearls:** * **Marker of Osteoblastic Activity:** Serum **Alkaline Phosphatase (ALP)** and **Osteocalcin** are clinical markers used to assess bone formation rates. * **Collagen Type:** Remember "Type **One** for B**one**." (Type II is for cartilage). * **Mineralization:** Osteoblasts secrete membrane-bound **matrix vesicles** containing ALP and calcium-binding proteins, which serve as the initial sites for hydroxyapatite crystal formation.

Question 197: The latch bridge mechanism in smooth muscles primarily aids in which of the following?

A. Initiation of contraction
B. Sustained contraction (Correct Answer)
C. Early dephosphorylation
D. None of the above

Explanation: **Explanation:** The **Latch Bridge Mechanism** is a unique physiological feature of smooth muscle that allows for **sustained contraction** (tonus) with minimal energy expenditure. **1. Why "Sustained Contraction" is correct:** In smooth muscle, contraction is regulated by the phosphorylation of the myosin light chain (MLC) by Myosin Light Chain Kinase (MLCK). When the enzyme **Myosin Light Chain Phosphatase (MLCP)** dephosphorylates the myosin while it is still attached to actin, the detachment rate of the myosin heads decreases significantly. These "latch bridges" remain attached for a prolonged period, maintaining tension (tone) without requiring additional ATP hydrolysis. This is crucial for organs like blood vessels and the bladder, which must maintain pressure for long durations. **2. Why the other options are incorrect:** * **Option A (Initiation of contraction):** Initiation is dependent on the increase in intracellular Calcium and the activation of MLCK. The latch mechanism occurs at the *end* of the cycle or during prolonged phases, not at the start. * **Option C (Early dephosphorylation):** While dephosphorylation is part of the process, the latch mechanism is defined by the *result* of that dephosphorylation (prolonged attachment), not the speed of the enzyme itself. **High-Yield NEET-PG Pearls:** * **Energy Efficiency:** Smooth muscle can maintain the same tension as skeletal muscle using **1/10th to 1/300th** of the energy. * **Calmodulin:** Smooth muscle lacks troponin; Calcium binds to **Calmodulin** to initiate the bridge cycle. * **Reverse Latch:** The mechanism is reversed when a new ATP molecule eventually binds or when intracellular calcium levels drop significantly, allowing for relaxation.

Question 198: What is the role of creatine phosphate in muscle?

A. Aids in gluconeogenesis
B. Provides instant energy (Correct Answer)
C. Is involved in action-contraction coupling
D. Aids in the stretch reflex

Explanation: **Explanation:** **1. Why Option B is Correct:** Creatine phosphate (also known as phosphocreatine) acts as a **high-energy phosphate reservoir** in muscle cells. During the first few seconds of intense muscular activity, the demand for ATP exceeds the rate at which oxidative phosphorylation or glycolysis can produce it. The enzyme **Creatine Kinase (CK)** catalyzes the transfer of a phosphate group from creatine phosphate to ADP, regenerating ATP almost instantaneously. This provides the "instant energy" required for short bursts of maximal effort (e.g., a 100m sprint). **2. Why Other Options are Incorrect:** * **Option A:** Gluconeogenesis (synthesis of glucose from non-carbohydrate sources) occurs primarily in the liver and kidneys, not through creatine phosphate. * **Option C:** Excitation-contraction coupling is mediated by calcium release from the sarcoplasmic reticulum and its binding to Troponin C; creatine phosphate does not play a structural or signaling role in this process. * **Option D:** The stretch reflex is a neurological feedback loop involving muscle spindles and alpha-motor neurons; it is not dependent on the biochemical energy stores of the muscle fiber itself. **Clinical Pearls & High-Yield Facts for NEET-PG:** * **Lohmann’s Reaction:** The reversible chemical reaction: $ADP + \text{Creatine Phosphate} \rightleftharpoons ATP + \text{Creatine}$. * **Creatine Kinase (CK) Isoenzymes:** CK-MM (Skeletal muscle), CK-MB (Cardiac muscle - marker for MI), and CK-BB (Brain). * **Creatinine:** A waste product formed by the non-enzymatic breakdown of creatine phosphate. Its excretion rate is relatively constant and used as a marker for GFR. * **Energy Sequence:** ATP stores (1-2 sec) $\rightarrow$ Creatine Phosphate (5-8 sec) $\rightarrow$ Anaerobic Glycolysis $\rightarrow$ Aerobic Metabolism.

Question 199: What is true about neuropraxia?

A. Blockage of axon
B. Physiological block (Correct Answer)
C. Incomplete transection of nerve
D. Complete transection of nerve

Explanation: **Explanation:** **Neuropraxia** is the mildest form of nerve injury according to **Seddon’s classification**. It is characterized by a **physiological block** of nerve conduction without any structural damage to the axon or the connective tissue sheaths (epineurium, perineurium, and endoneurium). 1. **Why the correct answer is right:** In neuropraxia, the nerve remains anatomically intact, but conduction is temporarily halted, usually due to focal demyelination or ischemia caused by sustained pressure (e.g., Saturday Night Palsy). Since the axon is preserved, there is **no Wallerian degeneration** distal to the site of injury. Recovery is typically spontaneous and complete within days to weeks once the pressure is relieved. 2. **Why the incorrect options are wrong:** * **Blockage of axon:** This is misleading. While conduction is blocked, the physical continuity of the axon is not interrupted. * **Incomplete/Complete transection:** These describe higher grades of injury. **Axonotmesis** involves axonal disruption with preservation of the sheath, while **Neurotmesis** (Option D) involves complete transection of both the axon and the connective tissue, requiring surgical intervention. **High-Yield Facts for NEET-PG:** * **Seddon’s Classification:** Neuropraxia (Mildest) → Axonotmesis → Neurotmesis (Most severe). * **Sunderland’s Classification:** Neuropraxia corresponds to **First-degree** injury. * **Clinical Feature:** Motor fibers are more susceptible than sensory fibers; autonomic functions are usually preserved. * **EMG Finding:** No denervation potentials (fibrillations) are seen because the axon is intact.

Question 200: The inverse stretch reflex is mediated by which of the following structures?

A. Trail fibre ending
B. Golgi tendon organ (Correct Answer)
C. Tail fibre ending
D. Muscle spindle

Explanation: **Explanation:** The **Inverse Stretch Reflex** (also known as the autogenic inhibition reflex) is a protective mechanism that prevents muscle damage due to excessive tension. **1. Why Golgi Tendon Organ (GTO) is correct:** The GTO is the sensory receptor for this reflex, located at the junction of muscle fibers and tendons. Unlike the muscle spindle, which responds to changes in muscle *length*, the GTO is arranged in **series** with muscle fibers and responds primarily to **muscle tension** (force). When a muscle undergoes heavy contraction, the GTO is stimulated and sends impulses via **Type Ib afferent fibers** to the spinal cord. These fibers synapse on inhibitory interneurons, which then inhibit the alpha motor neurons of the same muscle, causing it to relax. **2. Why other options are incorrect:** * **Muscle Spindle:** This is the receptor for the **Stretch Reflex** (Myotatic reflex). It is arranged in **parallel** with extrafusal fibers and responds to changes in muscle **length**. * **Trail and Tail fibre endings:** These terms refer to the types of motor nerve endings on intrafusal muscle fibers. **Trail endings** are typically found on nuclear chain fibers, while **Plate endings** (often confused with "tail") are on nuclear bag fibers. They are involved in the efferent (motor) gamma system, not the afferent limb of the inverse stretch reflex. **Clinical Pearls for NEET-PG:** * **Receptor:** Golgi Tendon Organ (Tension sensor). * **Afferent Nerve:** Type Ib (Fast conducting). * **Synapse:** Polysynaptic (involves an inhibitory interneuron). * **Function:** Prevents tendon avulsion and muscle tearing during extreme exertion. * **Clasp-knife response:** In upper motor neuron (UMN) lesions, the sudden relaxation of a spastic muscle upon passive stretching is attributed to the activation of the inverse stretch reflex.

Practice by Chapter

Resting Membrane Potential

Practice Questions

Action Potential Generation and Propagation

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Neuromuscular Junction

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Skeletal Muscle Contraction

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Smooth Muscle Physiology

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Cardiac Muscle Properties

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Muscle Metabolism and Fatigue

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Motor Unit Function

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Neurotransmitters and Receptors

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Electrophysiological Measurements

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