Nerve and Muscle Physiology — MCQs

Nerve and Muscle Physiology Indian Medical PG Practice Questions and MCQs

Question 81: What is the primary difference between skeletal and smooth muscle contraction and relaxation?

A. Troponin is present in skeletal muscle but absent in smooth muscle. (Correct Answer)
B. Myosin light chain kinase is involved in smooth muscle contraction.
C. Calcium ion regulation differs between skeletal and smooth muscle.
D. Actin filaments are structured differently in skeletal and smooth muscle.

Explanation: ### Explanation The fundamental difference in the regulatory mechanism of contraction between skeletal and smooth muscle lies in the **regulatory proteins** associated with actin. **1. Why Option A is Correct:** In **skeletal muscle**, the contraction is "actin-regulated." The troponin complex (specifically Troponin C, I, and T) is present on the thin filaments. Calcium binds to Troponin C, causing a conformational change that moves tropomyosin away from the myosin-binding sites on actin. In contrast, **smooth muscle lacks troponin**. Instead, it utilizes **Calmodulin** to bind calcium, making it "myosin-regulated." **2. Analysis of Incorrect Options:** * **Option B:** While it is true that Myosin Light Chain Kinase (MLCK) is essential for smooth muscle contraction (it phosphorylates the myosin head), this is a *mechanism* of action rather than the primary structural/regulatory protein difference defined by the presence or absence of troponin. * **Option C:** Both muscle types rely on calcium ions for contraction. While the *source* of calcium differs (skeletal relies on the sarcoplasmic reticulum; smooth muscle relies on both ECF and SR), the question asks for the primary regulatory protein difference. * **Option D:** Both muscle types contain actin filaments. While smooth muscle lacks the organized sarcomere structure (striations) and uses **dense bodies** instead of Z-lines, the actin filaments themselves are biochemically similar. ### NEET-PG High-Yield Pearls: * **Caldesmon & Calponin:** These are unique regulatory proteins in smooth muscle that inhibit the ATPase activity of myosin; they are the functional equivalents of troponin in terms of inhibition. * **Latch-bridge Mechanism:** Unique to smooth muscle, allowing it to maintain prolonged tension with minimal ATP consumption. * **Caveolae:** Smooth muscles lack a well-developed T-tubule system; instead, they have rudimentary invaginations called caveolae. * **Multi-unit vs. Unitary:** Unitary (visceral) smooth muscle has **gap junctions** and acts as a syncytium (e.g., GI tract, uterus).

Question 82: Which sensory modalities are primarily transmitted by large axons?

A. Proprioception
B. Somatic motor function
C. Conscious touch and pressure
D. All of the above (Correct Answer)

Explanation: The speed of nerve conduction is directly proportional to the diameter of the axon and the presence of a myelin sheath. According to the **Erlanger-Gasser classification**, large-diameter, heavily myelinated fibers (Type A) conduct impulses the fastest. **Explanation of the Correct Answer:** The correct answer is **D (All of the above)** because all listed modalities rely on **Type A fibers**, which are the largest and fastest in the body: * **Proprioception:** Transmitted by **Type Aα** fibers (Ia and Ib). These are the largest (12–20 µm) and fastest (70–120 m/s) fibers, essential for real-time spatial awareness. * **Somatic Motor Function:** Alpha motor neurons, which innervate extrafusal muscle fibers for contraction, are also **Type Aα** fibers. * **Conscious Touch and Pressure:** Transmitted by **Type Aβ** fibers. These are slightly smaller than Aα but still classified as large, fast-conducting myelinated axons. **Why individual options are part of the whole:** While A, B, and C are individually correct, selecting only one would be incomplete. In the NEET-PG pattern, when multiple modalities belong to the "Type A" group, "All of the above" is the most accurate choice. **High-Yield Clinical Pearls for NEET-PG:** * **Smallest/Slowest Fibers:** Type C fibers (unmyelinated) carry slow pain, temperature, and post-ganglionic autonomic signals. * **Susceptibility:** * **Local Anesthetics:** Block small **Type C** fibers first (Pain goes first). * **Pressure/Hypoxia:** Affects large **Type A** fibers first (Motor/Proprioception goes first). * **Order of conduction velocity:** Aα > Aβ > Aγ > Aδ > B > C.

Question 83: Muscle tone is maintained by?

A. Golgi tendon organ
B. Renshaw cells
C. Muscle spindle (Correct Answer)
D. None of the above

Explanation: **Explanation:** Muscle tone is defined as the continuous and passive partial contraction of the muscles, or the muscle's resistance to passive stretch during resting state. It is primarily maintained by the **Stretch Reflex** (Myotatic Reflex). **Why Muscle Spindle is Correct:** The muscle spindle acts as the sensory receptor for the stretch reflex. When a muscle is passively stretched, the muscle spindles (located in parallel with extrafusal fibers) are activated. They send afferent impulses via **Type Ia fibers** to the spinal cord, where they synapse directly with **Alpha Motor Neurons**. This results in a compensatory contraction of the same muscle, thereby maintaining tone. **Analysis of Incorrect Options:** * **Golgi Tendon Organ (GTO):** These are located in series with muscle fibers and respond to **muscle tension** (contraction), not stretch. They mediate the *Inverse Stretch Reflex*, which causes muscle relaxation to prevent injury from excessive force. * **Renshaw Cells:** These are inhibitory interneurons in the anterior horn of the spinal cord. They provide **recurrent inhibition** to alpha motor neurons to prevent over-activity and "sharpen" motor signals; they do not maintain tone. **High-Yield Clinical Pearls for NEET-PG:** * **Gamma Motor Neurons:** These innervate the intrafusal fibers of the muscle spindle. They regulate the sensitivity of the spindle, thereby "setting" the level of muscle tone. * **Supraspinal Control:** The Pontine Reticular Formation and Vestibular nuclei facilitate tone, while the Medullary Reticular Formation inhibits it. * **Clinical Correlation:** Lesions of the Upper Motor Neuron (UMN) lead to **Hypertonia** (Spasticity), while Lower Motor Neuron (LMN) lesions lead to **Hypotonia** (Flaccidity).

Question 84: What are the key molecules involved in the differences between skeletal and smooth muscle contraction and relaxation?

A. Troponin and Myosin light chain kinase
B. Troponin
C. Troponin and Myosin (Correct Answer)
D. Myosin light chain kinase and Calcium ions

Explanation: The fundamental difference between skeletal and smooth muscle contraction lies in the **regulatory proteins** and the **state of the myosin head**. ### 1. Why "Troponin and Myosin" is Correct * **Skeletal Muscle (Troponin-regulated):** Contraction is "actin-linked." Calcium binds to **Troponin C**, causing a conformational change in tropomyosin that uncovers the active sites on actin. * **Smooth Muscle (Myosin-regulated):** Contraction is "myosin-linked." Smooth muscle lacks troponin. Instead, Calcium binds to **Calmodulin**, which activates **Myosin Light Chain Kinase (MLCK)**. This enzyme phosphorylates the myosin head, enabling it to bind to actin. * **Relaxation:** In skeletal muscle, relaxation occurs via simple calcium sequestration. In smooth muscle, it requires **Myosin Light Chain Phosphatase** to dephosphorylate the myosin head. ### 2. Analysis of Incorrect Options * **Option A & D:** While MLCK is vital for smooth muscle, these options are incomplete. They focus only on the activation enzyme or ions, ignoring the structural protein differences (Troponin vs. Myosin state) that define the two systems. * **Option B:** Troponin is exclusive to skeletal and cardiac muscle; it does not explain the mechanism in smooth muscle. ### 3. High-Yield NEET-PG Pearls * **Calmodulin** in smooth muscle is functionally analogous to **Troponin C** in skeletal muscle. * **Latch-bridge mechanism:** Unique to smooth muscle; allows for prolonged tone with minimal ATP consumption. * **Caveolae:** Smooth muscle lacks a well-developed T-tubule system; instead, it uses these membrane invaginations. * **Multi-unit vs. Unitary:** Unitary (visceral) smooth muscle uses **gap junctions** for syncytial contraction (e.g., GI tract, uterus).

Question 85: In a sarcomere, which band or zone is NOT characterized by the presence of actin filaments?

A. H band (Correct Answer)
B. I band
C. M band
D. Z band

Explanation: **Explanation:** The sarcomere is the functional unit of skeletal muscle, extending from one Z-line to the next. Its appearance is defined by the arrangement of thick (myosin) and thin (actin) filaments. **Why the H band is correct:** The **H band** (from German *heller*, meaning brighter) is the central region of the A band. It contains **only thick (myosin) filaments**. During muscle contraction, actin filaments slide over myosin toward the center, narrowing the H band, but at rest, it is characterized specifically by the absence of actin. **Analysis of incorrect options:** * **I band:** This is the isotropic band which contains **only thin (actin) filaments**. It spans across two adjacent sarcomeres, bisected by the Z disc. * **M band:** Located in the dead center of the H zone, the M line (M band) contains proteins (like myomesin) that anchor the thick filaments. While it is within the H zone, the question asks for the "band or zone" defined by the absence of actin; the H zone is the primary anatomical region meeting this criteria. * **Z band (Z disc):** This is the boundary of the sarcomere where **actin filaments are anchored** via the protein alpha-actinin. **High-Yield NEET-PG Pearls:** 1. **"A" is for All:** The **A band** (Anisotropic) contains the entire length of the thick filament, including the area where actin and myosin overlap. Its length **remains constant** during contraction. 2. **Contraction Dynamics:** During contraction, the **H zone and I band shorten**, while the Z lines move closer together. 3. **Titans of the Sarcomere:** **Titin** is the largest protein in the body; it acts as a spring, connecting the Z disc to the M line, providing passive elasticity.

Question 86: Which of the following is NOT true about the tendon organ?

A. Inhibitory
B. Negative feedback
C. Protective
D. Contracting (Correct Answer)

Explanation: The **Golgi Tendon Organ (GTO)** is a high-threshold mechanoreceptor located in the tendons of skeletal muscles. It functions as a tension sensor, monitoring the force of muscle contraction. ### Why "Contracting" is the Correct Answer The GTO is stimulated by **tension**, not by the act of contraction itself (though contraction creates tension). When tension becomes excessive, the GTO triggers the **Inverse Stretch Reflex** (Autogenic Inhibition). This reflex causes the muscle to **relax** rather than contract. Therefore, "Contracting" is the opposite of its physiological effect. ### Analysis of Incorrect Options * **A. Inhibitory:** The GTO sends impulses via **Ib afferent fibers** to the spinal cord, where they synapse with inhibitory interneurons. These interneurons inhibit the alpha motor neurons of the agonist muscle, leading to relaxation. * **B. Negative Feedback:** It operates on a negative feedback loop to regulate muscle tension. If tension increases, the GTO reduces motor output to prevent injury, maintaining tension within a physiological range. * **C. Protective:** Its primary role is to prevent tendon avulsion or muscle tearing during extreme exertion by forcing the muscle to "give way" (the **Clasp-knife response**). ### NEET-PG High-Yield Pearls * **Afferent Fiber:** Ib (Fast-conducting). * **Reflex:** Inverse Stretch Reflex (Disynaptic). * **GTO vs. Muscle Spindle:** The Muscle Spindle (Ia/II fibers) responds to **length/stretch** and causes contraction; the GTO responds to **tension** and causes relaxation. * **Clinical Sign:** The **Clasp-knife phenomenon** seen in upper motor neuron lesions is mediated by the GTO.

Question 87: What is the contractile unit of a muscle?

A. Sarcomere (Correct Answer)
B. Sarcolemma
C. Myofibril
D. Sarcotubular System

Explanation: ### Explanation **Correct Answer: A. Sarcomere** The **sarcomere** is defined as the functional and structural unit of contraction in skeletal muscle. It is the segment of a myofibril located between two successive **Z-lines**. During muscle contraction (based on the Sliding Filament Theory), the distance between the Z-lines decreases as actin (thin) filaments slide over myosin (thick) filaments, effectively shortening the sarcomere. Because this is the smallest component of the muscle capable of independent contraction, it is designated the "contractile unit." **Analysis of Incorrect Options:** * **B. Sarcolemma:** This is the cell membrane of a muscle fiber. While it conducts action potentials via voltage-gated channels, it does not perform the mechanical act of contraction. * **C. Myofibril:** These are long, cylindrical bundles of myofilaments found within the muscle cell. A myofibril is composed of many sarcomeres arranged in series; thus, it contains the contractile units but is not the unit itself. * **D. Sarcotubular System:** This consists of the T-tubules and the Sarcoplasmic Reticulum (SR). Its primary role is **Excitation-Contraction Coupling** (releasing $Ca^{2+}$), not the mechanical contraction. **High-Yield NEET-PG Pearls:** * **Sarcomere Composition:** Contains one full **A-band** (anisotropic) and two **half I-bands** (isotropic). * **Contraction Dynamics:** During contraction, the **H-zone** and **I-band** shorten/disappear, but the **A-band width remains constant**. * **Titin:** The largest protein in the human body; it acts as a molecular spring, connecting the Z-line to the M-line, providing passive elasticity to the sarcomere. * **L-Tubules:** The longitudinal part of the sarcotubular system (SR) specifically stores calcium via the protein **calsequestrin**.

Question 88: Saltatory conduction refers to which process?

A. Propagation of action potential along the myelin sheath.
B. Propagation of action potential between the nodes of Ranvier. (Correct Answer)
C. Propagation of action potential along the axon proper.
D. Propagation of action potential through dendrites.

Explanation: **Explanation:** **Saltatory conduction** (from the Latin *saltare*, meaning "to leap") is the process by which nerve impulses are transmitted along myelinated axons. **Why Option B is Correct:** In myelinated nerves, the lipid-rich **myelin sheath** acts as an electrical insulator, preventing ion flow across the axonal membrane. However, the sheath is interrupted at regular intervals by **Nodes of Ranvier**, which contain a high density of voltage-gated sodium channels. Consequently, the action potential cannot flow continuously; instead, it "jumps" from one node to the next. This mechanism significantly increases the velocity of nerve conduction while conserving energy (ATP), as ionic exchange is limited only to the nodal regions. **Why Other Options are Incorrect:** * **Option A:** Action potentials do not travel *through* the myelin sheath; the sheath serves as an insulator to prevent leakage. * **Option C:** Propagation along the axon proper (continuous conduction) occurs in **unmyelinated fibers**. This is much slower as the entire membrane must undergo depolarization. * **Option D:** Dendrites typically conduct graded potentials toward the cell body, not saltatory action potentials. **NEET-PG High-Yield Pearls:** * **Velocity:** Saltatory conduction is approximately 50–100 times faster than unmyelinated conduction. * **Clinical Correlation:** In **Multiple Sclerosis** (CNS) and **Guillain-Barré Syndrome** (PNS), demyelination occurs. This disrupts saltatory conduction, leading to "conduction block" or slowed signal transmission. * **Energy Efficiency:** It is more energy-efficient because the **Na+-K+ ATPase pump** needs to work less to restore ionic gradients, as depolarization is localized to the nodes.

Question 89: Optic nerve fibres, once cut, do not regenerate because they are not covered by which of the following structures?

A. Myelin sheath
B. Neurilemma (Correct Answer)
C. Both myelin sheath and neurilemma
D. Neither myelin sheath nor neurilemma

Explanation: The correct answer is **B. Neurilemma**. ### **Explanation** The **neurilemma** (also known as the sheath of Schwann) is the outermost nucleated cytoplasmic layer of Schwann cells that surrounds the axon of a neuron. Its primary function in the Peripheral Nervous System (PNS) is to facilitate nerve regeneration by forming **Bungner bands**, which guide the regenerating axonal sprouts toward their target. The **optic nerve** is unique because it is embryologically an outgrowth of the diencephalon; therefore, it is considered a part of the **Central Nervous System (CNS)**, not a peripheral nerve. In the CNS, axons are myelinated by **oligodendrocytes** rather than Schwann cells. Unlike Schwann cells, oligodendrocytes do not form a neurilemma. Without this neurilemmal sheath to provide a scaffold and growth factors, and due to the presence of inhibitory molecules (like Nogo-A) produced by oligodendrocytes, regeneration in the optic nerve is impossible once it is severed. ### **Analysis of Incorrect Options** * **A. Myelin sheath:** The optic nerve **is** myelinated. However, its myelin is produced by oligodendrocytes. The presence of myelin itself does not prevent regeneration; it is the lack of the neurilemmal regenerative framework that is the limiting factor. * **C & D:** These are incorrect because the optic nerve possesses a myelin sheath but lacks a neurilemma. ### **High-Yield NEET-PG Pearls** * **PNS vs. CNS Myelination:** 1 Schwann cell myelinates **one** internode of a single axon, whereas 1 Oligodendrocyte can myelinate up to **50** different axons. * **Regeneration:** Regeneration is possible in the PNS (e.g., Sciatic nerve) due to the neurilemma and Schwann cells, but not in the CNS (e.g., Optic nerve, Spinal cord). * **Clinical Correlation:** In Multiple Sclerosis (MS), the CNS myelin (oligodendrocytes) is attacked, affecting the optic nerve (optic neuritis). In Guillain-Barré Syndrome (GBS), the PNS myelin (Schwann cells) is attacked.

Question 90: Which type of nerve fibers are least susceptible to pressure?

A. Type A fibers
B. Type B fibers
C. Type C fibers (Correct Answer)
D. Type D fibers

Explanation: **Explanation:** The susceptibility of nerve fibers to various insults depends on their diameter and myelination. According to the **Gasser-Erlanger classification**, nerve fibers are categorized into Types A, B, and C. **Why Type C is correct:** Type C fibers are the **least susceptible to pressure**. They are the smallest in diameter and are unmyelinated. Because they lack a bulky myelin sheath and have a smaller surface area, they are mechanically more resilient to compression compared to the larger, myelinated fibers. **Why the other options are incorrect:** * **Type A fibers:** These are the **most susceptible to pressure**. They are large-diameter, heavily myelinated fibers (e.g., alpha motor neurons, sensory fibers for touch/pressure). Their large size and complex myelin structure make them highly vulnerable to mechanical deformation and ischemia caused by pressure. * **Type B fibers:** These are intermediate-sized, preganglionic autonomic fibers. While more resistant than Type A, they are more susceptible to pressure than Type C. * **Type D fibers:** This is not a standard category in the Gasser-Erlanger classification of nerve fibers. **High-Yield Clinical Pearls for NEET-PG:** To remember the susceptibility patterns, use the mnemonic **"P-A-I-N"**: 1. **P**ressure: Most susceptible = **Type A**; Least = **Type C**. 2. **H**ypoxia (Ischemia): Most susceptible = **Type B**; Least = **Type C**. 3. **L**ocal Anesthetics: Most susceptible = **Type C**; Least = **Type A**. (Small, unmyelinated fibers are blocked first). * **Order of block by Local Anesthetics:** Type C > Type B > Type A. * **Order of loss in Pressure:** Type A > Type B > Type C (This explains why "Saturday Night Palsy" affects motor function and touch before pain sensation).

Practice by Chapter

Resting Membrane Potential

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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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