Nerve and Muscle Physiology — MCQs

Nerve and Muscle Physiology Indian Medical PG Practice Questions and MCQs

Question 181: Group B muscle fibers are:

A. Sympathetic preganglionic (Correct Answer)
B. Sympathetic postganglionic
C. Parasympathetic preganglionic
D. Parasympathetic postganglionic

Explanation: ### Explanation The classification of nerve fibers is a high-yield topic for NEET-PG, primarily based on the **Erlanger-Gasser classification**, which categorizes fibers according to diameter, myelination, and conduction velocity. **1. Why the Correct Answer is Right:** **Group B fibers** are characterized as being **myelinated**, with a small diameter (under 3 μm) and moderate conduction velocity (3–15 m/s). In the autonomic nervous system, **all preganglionic fibers** (both sympathetic and parasympathetic) are Group B fibers. However, in the context of standard medical examinations and the Erlanger-Gasser table, Group B is classically identified with **sympathetic preganglionic fibers** (white rami communicantes). **2. Analysis of Incorrect Options:** * **Option B & D (Postganglionic fibers):** All postganglionic fibers of the autonomic nervous system (both sympathetic and parasympathetic) are **Group C fibers**. These are the smallest, **unmyelinated** fibers with the slowest conduction velocity (0.5–2 m/s). * **Option C (Parasympathetic preganglionic):** While these are technically Group B fibers, standard physiological nomenclature often emphasizes the sympathetic preganglionic fibers as the prototype for Group B in MCQ formats. If both were present and only one choice allowed, sympathetic is the conventional "textbook" answer for this specific classification. **3. High-Yield NEET-PG Pearls:** * **Group A fibers:** Largest and fastest. Subdivided into Alpha (Proprioception, somatic motor), Beta (Touch/Pressure), Gamma (Muscle spindle), and Delta (Fast pain/Temperature). * **Group C fibers:** Smallest and slowest. Responsible for **slow pain**, temperature, and postganglionic autonomic functions. * **Susceptibility Rule:** * **Hypoxia:** Affects Group A first. * **Pressure:** Affects Group A first. * **Local Anesthetics:** Affect **Group C** first (due to small diameter).

Question 182: Which nerve fibres have the largest diameter?

A. Somatic motor A alpha fibres (Correct Answer)
B. Preganglionic autonomic type B fibres
C. Dorsal root C fibres
D. Motor fibres A gamma fibres

Explanation: **Explanation:** The classification of nerve fibers is based on the **Erlanger-Gasser classification**, which categorizes fibers according to their diameter, conduction velocity, and functions. **1. Why A Alpha (Aα) is correct:** Type A alpha fibers are the largest in diameter (**12–20 μm**) and possess the thickest myelin sheath. According to the principles of neurophysiology, conduction velocity is directly proportional to fiber diameter. Therefore, Aα fibers are the fastest (70–120 m/s). They function as **somatic motor fibers** (to extrafusal muscle fibers) and **proprioceptive afferents** (from muscle spindles and Golgi tendon organs). **2. Analysis of Incorrect Options:** * **Type B fibers:** These are preganglionic autonomic fibers. They are myelinated but significantly smaller in diameter (**<3 μm**) than any Type A fiber. * **Type C fibers:** These are the smallest (**0.4–1.2 μm**) and are **unmyelinated**. They have the slowest conduction velocity and carry dull pain, temperature, and postganglionic autonomic signals. * **A gamma (Aγ) fibers:** While these are Type A myelinated fibers, they are smaller (**3–6 μm**) than Aα. They supply the intrafusal fibers of the muscle spindle. **High-Yield Facts for NEET-PG:** * **Order of Diameter/Velocity:** Aα > Aβ > Aγ > Aδ > B > C. * **Sensitivity to Local Anesthetics:** Type C fibers are the most sensitive, while Type A fibers are the least sensitive (Size principle: smaller fibers are blocked first). * **Sensitivity to Pressure:** Type A fibers are the most sensitive to pressure (e.g., a limb "falling asleep"). * **Sensitivity to Hypoxia:** Type B fibers are the most sensitive to oxygen deprivation. * **Fastest Conduction:** Somatic motor (Aα); **Slowest Conduction:** Dorsal root/Pain (C).

Question 183: What forms the myelin sheath around nerve fibers in the central nervous system?

A. Astrocytes
B. Microglia
C. Oligodendrocytes (Correct Answer)
D. Protoplasmic astrocytes

Explanation: **Explanation:** The correct answer is **C. Oligodendrocytes**. **1. Why Oligodendrocytes are correct:** In the Central Nervous System (CNS), myelin is formed by **Oligodendrocytes**. A single oligodendrocyte is unique because it can extend its processes to myelinate segments of **multiple axons** (up to 50). Myelin acts as an electrical insulator, increasing the speed of nerve impulse conduction via saltatory conduction. **2. Why the other options are incorrect:** * **Astrocytes (A & D):** These are the most numerous glial cells. They provide structural support, form the **Blood-Brain Barrier (BBB)**, and regulate the extracellular ionic environment. *Protoplasmic astrocytes* are specifically found in the gray matter, while *fibrous astrocytes* are in the white matter. They do not produce myelin. * **Microglia (B):** These are the resident macrophages of the CNS. Derived from the monocyte-macrophage lineage (mesodermal origin), they act as the primary immune defense and are responsible for phagocytosis. **3. High-Yield Clinical Pearls for NEET-PG:** * **CNS vs. PNS Myelination:** While Oligodendrocytes myelinate the CNS, **Schwann cells** myelinate the Peripheral Nervous System (PNS). Crucially, one Schwann cell myelinates only **one axon** segment. * **Demyelinating Diseases:** * **Multiple Sclerosis (MS):** An autoimmune attack on CNS myelin (Oligodendrocytes). * **Guillain-Barré Syndrome (GBS):** An inflammatory attack on PNS myelin (Schwann cells). * **Origin:** Most glial cells (Astrocytes, Oligodendrocytes) are ectodermal in origin, except **Microglia**, which are **mesodermal**.

Question 184: Increased velocity of conduction in a nerve is favored by which of the following?

A. Increased capacitance (Correct Answer)
B. Decreased capacitance
C. Increased resistance
D. Decreased resistance

Explanation: **Explanation:** The velocity of nerve conduction is determined by the time constant ($\tau$) and the length constant ($\lambda$). The time constant ($\tau = R_m \times C_m$) represents the time taken for a membrane to charge; a **lower** time constant results in **faster** conduction. **Why the Correct Answer is Right:** In the context of this specific question (often based on standard physiological equations), conduction velocity is proportional to the rate at which the membrane can be depolarized to its threshold. According to the cable theory of nerves, velocity is inversely proportional to the product of resistance and capacitance. However, in myelinated fibers, the **decreased capacitance** provided by the myelin sheath is the primary driver for increased velocity (Saltatory conduction). *Note: There is a common point of confusion in some question banks regarding the phrasing of this concept. Physiologically, **Decreased Capacitance** (Option B) is the standard mechanism by which myelin increases speed. If "Increased Capacitance" is marked as correct in your specific source, it is likely referring to the increased "current-carrying capacity" or a specific experimental context, but in standard NEET-PG physiology (Guyton/Ganong), **Decreased Capacitance** is the hallmark of fast conduction.* **Analysis of Options:** * **Decreased Capacitance (B):** This is the physiological hallmark of myelination. Myelin increases the thickness of the membrane, reducing its ability to store charge, allowing the action potential to "jump" faster between nodes. * **Increased Resistance (C):** Increased internal (axial) resistance hinders the flow of ions, thereby **decreasing** conduction velocity. * **Decreased Resistance (D):** Decreased internal resistance (seen in larger diameter axons) **increases** conduction velocity. **High-Yield Facts for NEET-PG:** 1. **Diameter:** Velocity is directly proportional to the diameter of the nerve fiber. 2. **Myelination:** Increases velocity by decreasing membrane capacitance ($C_m$) and increasing membrane resistance ($R_m$). 3. **Temperature:** Increased temperature increases conduction velocity. 4. **Length Constant ($\lambda$):** A larger length constant (increased $R_m$, decreased $R_i$) increases conduction velocity.

Question 185: Conduction along a membrane is dependent on all of the following EXCEPT?

A. Ca2+
B. K+
C. Fe2+ (Correct Answer)
D. Na+

Explanation: **Explanation:** The conduction of electrical impulses along a cell membrane (Action Potential) is primarily governed by the movement of specific ions through voltage-gated channels, a process described by the **Hodgkin-Huxley model**. **Why Fe2+ is the Correct Answer:** Iron (**Fe2+**) is a trace element essential for oxygen transport (hemoglobin) and electron transport (cytochromes), but it plays **no direct role** in the acute generation or propagation of an action potential across a nerve or muscle membrane. It does not have specific voltage-gated channels that contribute to membrane excitability. **Analysis of Other Options:** * **Na+ (Sodium):** Crucial for the **depolarization** phase. The rapid influx of Na+ through voltage-gated sodium channels is responsible for the upstroke of the action potential. * **K+ (Potassium):** Essential for the **repolarization** phase and maintaining the Resting Membrane Potential (RMP). The efflux of K+ restores the negative charge inside the cell. * **Ca2+ (Calcium):** Plays a dual role. In cardiac and smooth muscle, it contributes to the action potential plateau/depolarization. More importantly, extracellular Ca2+ levels stabilize the membrane; **hypocalcemia** increases excitability (tetany) by lowering the threshold for Na+ channel activation. **High-Yield Clinical Pearls for NEET-PG:** * **RMP** is primarily determined by **K+** (due to high resting permeability). * **Depolarization** is primarily determined by **Na+**. * **Hyperkalemia** initially increases excitability but eventually leads to inactivation of Na+ channels, causing paralysis or cardiac arrest. * **Local Anesthetics (e.g., Lidocaine)** work by blocking voltage-gated **Na+ channels**, thereby halting conduction.

Question 186: During skeletal muscle contraction, which of the following occurs?

A. A band shortens
B. Both H and I bands shorten (Correct Answer)
C. Both A and I bands shorten
D. Both A and H bands shorten

Explanation: ### Explanation The fundamental mechanism of skeletal muscle contraction is explained by the **Sliding Filament Theory**. According to this theory, contraction occurs when thin (actin) filaments slide over thick (myosin) filaments toward the center of the sarcomere. **Why Option B is Correct:** * **I Band (Isotropic):** This band contains only thin filaments. As actin filaments slide toward the M-line, the distance between thick filaments of adjacent sarcomeres decreases, causing the I band to **shorten**. * **H Zone (Heller):** This is the central part of the A band containing only thick filaments. As thin filaments slide inward, they overlap more of the thick filaments, causing the H zone to **shorten** or even disappear. * **Sarcomere:** The distance between two Z-lines also decreases, leading to overall shortening of the muscle fiber. **Why Other Options are Incorrect:** * **A Band (Anisotropic):** This band represents the entire length of the thick (myosin) filament. Since the thick filaments themselves do not change length or move, the **A band remains constant** during contraction. * Options A, C, and D are incorrect because they all suggest that the A band shortens. **High-Yield NEET-PG Pearls:** 1. **Mnemonic:** During contraction, **"HI"** disappears (H and I bands shorten), but the **"A"** stays the same (A band is constant). 2. **Z-lines:** Move closer together during contraction. 3. **ATP Role:** ATP binding is required for the **detachment** of the myosin head from actin; hydrolysis of ATP "cocks" the myosin head into a high-energy state. 4. **Calcium:** Binds to **Troponin C**, causing a conformational change in Tropomyosin to uncover the active sites on actin.

Question 187: When a muscle is not contracting, by which mechanism are actin and myosin prevented from interacting?

A. Troponin-tropomyosin complex (Correct Answer)
B. Phosphocreatine
C. Heavy meromyosin
D. Acetylcholinesterase

Explanation: ### Explanation In a resting muscle, the interaction between actin and myosin is prevented by the **Troponin-Tropomyosin complex**, which acts as a physical regulatory barrier. **1. Why Option A is Correct:** The thin filament consists of actin, tropomyosin, and troponin. In the relaxed state, **tropomyosin** lies in the groove of the actin filament, physically covering the active binding sites for myosin heads. **Troponin I** (inhibitory) further stabilizes this position. Contraction only occurs when Calcium binds to **Troponin C**, causing a conformational change that pulls tropomyosin away, exposing the actin-binding sites (the "sliding filament" mechanism). **2. Why the Other Options are Incorrect:** * **B. Phosphocreatine:** This is an energy-storage molecule. It provides a rapid source of high-energy phosphate to regenerate ATP from ADP during the first few seconds of muscle contraction; it has no structural role in blocking actin-myosin binding. * **C. Heavy Meromyosin (HMM):** This is a fragment of the myosin molecule produced by proteolytic digestion (trypsin). It contains the S1 (head) and S2 (neck) regions responsible for ATP hydrolysis and binding to actin, rather than preventing it. * **D. Acetylcholinesterase:** This is an enzyme located in the synaptic cleft of the neuromuscular junction. Its role is to degrade acetylcholine to terminate the signal for contraction; it does not interact with the myofilaments directly. **High-Yield Clinical Pearls for NEET-PG:** * **Troponin T:** Binds the troponin complex to **T**ropomyosin. * **Troponin I:** **I**nhibits the ATPase activity of the actin-myosin interaction. * **Troponin C:** Binds **C**alcium (4 ions per molecule in skeletal muscle). * **Rigor Mortis:** Occurs because ATP is required to *detach* myosin from actin. Without ATP, the cross-bridges remain "locked." * **Cardiac Biomarkers:** Troponin I and T are highly specific markers for myocardial infarction because they are released into the blood when cardiac myocytes are damaged.

Question 188: What is the function of the transverse tubule of the skeletal muscle?

A. Transmit the action potential to the sarcoplasmic reticulum (Correct Answer)
B. Carry ATP from the mitochondria
C. Conduct impulses from the muscle spindle
D. Provide energy for the actin molecule

Explanation: ### Explanation **Correct Option: A (Transmit the action potential to the sarcoplasmic reticulum)** The **Transverse tubules (T-tubules)** are deep invaginations of the sarcolemma (muscle cell membrane) that penetrate the muscle fiber at the junctions of the A and I bands. Their primary function is to ensure that the action potential reaches the interior of the muscle fiber rapidly. When an action potential travels down the T-tubule, it activates voltage-gated **L-type calcium channels (Dihydropyridine receptors - DHPR)**. These receptors are mechanically linked to **Ryanodine receptors (RyR)** on the terminal cisternae of the **Sarcoplasmic Reticulum (SR)**. This mechanical coupling triggers the release of $Ca^{2+}$ from the SR into the sarcoplasm, initiating contraction via the excitation-contraction coupling mechanism. **Analysis of Incorrect Options:** * **Option B:** ATP is transported via simple diffusion within the sarcoplasm or through specific mitochondrial transporters; T-tubules are membrane structures, not transport channels for metabolites. * **Option C:** Muscle spindles are sensory receptors that send impulses to the CNS via afferent nerve fibers (Type Ia and II), not through T-tubules. * **Option D:** Energy for actin-myosin interaction is provided by ATP hydrolysis at the myosin head, not by the T-tubule system. --- ### High-Yield Clinical Pearls for NEET-PG * **The Triad:** In skeletal muscle, a triad consists of one T-tubule and two flanking terminal cisternae of the SR. (Note: In cardiac muscle, it is a **Diad** located at the Z-line). * **DHPR vs. RyR:** Remember that DHPR acts as a voltage sensor in the T-tubule, while RyR is the calcium release channel in the SR. * **Malignant Hyperthermia:** This condition is caused by a mutation in the **Ryanodine receptor (RyR1)**, leading to excessive $Ca^{2+}$ release upon exposure to volatile anesthetics.

Question 189: Which of the following nerve fibers has the highest conduction velocity?

A. Aα (Correct Answer)
B. Aβ
C. Aγ
D. B

Explanation: **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 based on these physical characteristics. **1. Why Aα is Correct:** **Type Aα (Alpha)** fibers are the largest in diameter (12–20 μm) and are heavily myelinated. In physiology, the conduction velocity is directly proportional to the diameter of the fiber (Velocity ≈ 6 × Diameter). Because Aα fibers have the greatest diameter, they possess the highest conduction velocity (approx. 70–120 m/s). They primarily serve somatic motor functions and proprioception. **2. Why the other options are incorrect:** * **Aβ (Beta):** These are slightly smaller than Aα (5–12 μm) and carry sensations like touch and pressure. Their velocity (30–70 m/s) is lower than Aα. * **Aγ (Gamma):** These fibers (3–6 μm) go to muscle spindles (intrafusal fibers). Being smaller than Aβ, they conduct even slower (15–30 m/s). * **B fibers:** These are preganglionic autonomic fibers. While myelinated, they are much smaller in diameter (<3 μm) than any Type A fiber, resulting in significantly slower conduction (3–15 m/s). **High-Yield Facts for NEET-PG:** * **Fastest Fiber:** Aα (Proprioception/Somatic motor). * **Slowest Fiber:** Type C (Pain/Temperature/Post-ganglionic sympathetic). Type C is the only **unmyelinated** fiber. * **Sensitivity to Local Anesthetics:** Type C fibers are the most sensitive, while Type Aα are the least sensitive (Size principle: smaller fibers are blocked more easily). * **Sensitivity to Pressure:** Type A fibers are the most sensitive. * **Sensitivity to Hypoxia:** Type B fibers are the most sensitive.

Question 190: All of the following proteins are involved during muscle contraction, EXCEPT:

A. Myosin
B. Actin
C. Myoglobin (Correct Answer)
D. Troponin

Explanation: The correct answer is **C. Myoglobin**. ### **Explanation** Muscle contraction is a mechanical process governed by the **Sliding Filament Theory**, which involves structural and regulatory proteins organized into sarcomeres. * **Why Myoglobin is the correct answer:** Myoglobin is a cytoplasmic hemeprotein found in muscle cells. Its primary role is the **storage and transport of oxygen** from the cell membrane to the mitochondria. While it supports aerobic metabolism to provide ATP for contraction, it is **not a structural or regulatory component** of the contractile machinery itself. It does not participate in the cross-bridge cycle. ### **Analysis of Other Options** * **A. Myosin:** The primary **thick filament** protein. It possesses ATPase activity and forms cross-bridges with actin to generate force (the "power stroke"). * **B. Actin:** The primary **thin filament** protein. It contains specific binding sites for myosin heads, allowing for filament sliding. * **D. Troponin:** A **regulatory protein** complex (Troponin I, T, and C) located on the thin filament. It acts as a "switch"; when calcium binds to Troponin C, it moves tropomyosin away from actin’s binding sites, initiating contraction. ### **High-Yield NEET-PG Pearls** * **Contractile Proteins:** Actin and Myosin. * **Regulatory Proteins:** Troponin and Tropomyosin. * **Structural Proteins:** Titin (largest protein, connects Z-disk to M-line), Nebulin, and Dystrophin. * **Red vs. White Muscle:** Type I (Slow-twitch) fibers have **high myoglobin** content (red), while Type II (Fast-twitch) fibers have **low myoglobin** (white). * **Clinical Correlation:** Elevated serum **Myoglobin** is an early marker of muscle injury (Rhabdomyolysis) and can lead to acute renal failure due to its nephrotoxic effects.

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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