Nerve and Muscle Physiology Practice Questions

Q: Group B muscle fibers are:

Sympathetic preganglionic. ### 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).

Q: Which nerve fibres have the largest diameter?

Somatic motor A alpha fibres. **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).

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

Oligodendrocytes. **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**.

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

Fe2+. **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.

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

Both H and I bands shorten. ### 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.

Q: What is the function of the transverse tubule of the skeletal muscle?

Transmit the action potential to the sarcoplasmic reticulum. ### 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.

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

Aα. **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.

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

Myoglobin. 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.

Question 1

Group B muscle fibers are:

Accepted Answer

Sympathetic preganglionic

Answer

Sympathetic postganglionic

Answer

Parasympathetic preganglionic

Answer

Parasympathetic postganglionic

Question 2

Which nerve fibres have the largest diameter?

Accepted Answer

Somatic motor A alpha fibres

Answer

Preganglionic autonomic type B fibres

Answer

Dorsal root C fibres

Answer

Motor fibres A gamma fibres

Question 3

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

Accepted Answer

Oligodendrocytes

Answer

Astrocytes

Answer

Microglia

Answer

Protoplasmic astrocytes

Question 4

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

Accepted Answer

Increased capacitance

Answer

Decreased capacitance

Answer

Increased resistance

Answer

Decreased resistance

Question 5

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

Accepted Answer

Fe2+

Answer

Ca2+

Answer

K+

Answer

Na+

Question 6

During skeletal muscle contraction, which of the following occurs?

Accepted Answer

Both H and I bands shorten

Answer

A band shortens

Answer

Both A and I bands shorten

Answer

Both A and H bands shorten

Question 7

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

Accepted Answer

Troponin-tropomyosin complex

Answer

Phosphocreatine

Answer

Heavy meromyosin

Answer

Acetylcholinesterase

Question 8

What is the function of the transverse tubule of the skeletal muscle?

Accepted Answer

Transmit the action potential to the sarcoplasmic reticulum

Answer

Carry ATP from the mitochondria

Answer

Conduct impulses from the muscle spindle

Answer

Provide energy for the actin molecule

Question 9

Which of the following nerve fibers has the highest conduction velocity?

Accepted Answer

Aα

Answer

Aβ

Answer

Aγ

Answer

B

Question 10

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

Accepted Answer

Myoglobin

Answer

Myosin

Answer

Actin

Answer

Troponin

Nerve and Muscle Physiology — MCQs

Nerve and Muscle Physiology — MCQs

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