Nerve and Muscle Physiology Practice Questions

Q: Actin's active site is covered by?

Tropomyosin. ### Explanation **Correct Option: B. Tropomyosin** In a resting muscle fiber, the interaction between actin and myosin is physically prevented to allow for relaxation. The **active sites (myosin-binding sites)** on the filamentous (F) actin strands are covered by **tropomyosin**. Tropomyosin is a long, rod-shaped protein that wraps spirally around the actin filament, specifically masking the sites where myosin heads would otherwise attach to initiate contraction. **Analysis of Options:** * **A. Myosin:** This is the thick filament. It possesses "heads" that seek to bind to actin to form cross-bridges. It does not cover the site; it is the protein being blocked. * **C. Troponin:** This is a complex of three regulatory proteins (I, T, and C) attached to tropomyosin. While Troponin I helps inhibit the binding, it is the **tropomyosin molecule itself** that physically lies over and masks the active site. Troponin acts as the "lock" that moves the tropomyosin "bar" when calcium binds to Troponin C. * **D. Desmin:** This is an intermediate filament found near the Z-line. Its primary role is structural—linking myofibrils together and anchoring them to the sarcolemma—rather than regulating the actin-myosin interface. **High-Yield NEET-PG Pearls:** * **The Troponin Complex:** * **Troponin T:** Binds the complex to **T**ropomyosin. * **Troponin I:** **I**nhibits the actin-myosin interaction. * **Troponin C:** Binds **C**alcium ions (requires 4 $Ca^{2+}$ ions to trigger a conformational change). * **Mechanism of Contraction:** When $Ca^{2+}$ binds to Troponin C, it causes a conformational change that pulls tropomyosin deeper into the actin groove, uncovering the active sites. This is known as the **"Dual-control mechanism."** * **Relaxation:** Occurs when $Ca^{2+}$ is pumped back into the Sarcoplasmic Reticulum (via SERCA), causing tropomyosin to return to its original masking position.

Q: Which of the following muscle fiber types is brown in color?

Type II A. **Explanation:** The classification of skeletal muscle fibers is based on their metabolic profile, contraction speed, and myoglobin content. **Type IIA fibers (Fast-twitch oxidative-glycolytic)** are known as **"Brown fibers"** or "Intermediate fibers." They occupy a middle ground between Type I and Type IIB. They contain a high concentration of myoglobin and mitochondria (similar to Type I) but also possess high glycolytic capacity. The combination of high myoglobin content and specific enzymatic activity gives them a distinct brownish appearance under certain staining conditions. **Analysis of Incorrect Options:** * **Option A: Type I (Slow-twitch):** These are known as **"Red fibers."** They have the highest myoglobin content and mitochondrial density, specialized for endurance and aerobic metabolism. * **Option C: Type IIB (Fast-twitch glycolytic):** These are known as **"White fibers."** They have low myoglobin and mitochondrial content, relying primarily on anaerobic glycolysis for short bursts of power. * **Option D: Type III:** This is a **distractor.** In standard mammalian skeletal muscle classification, fibers are categorized into Types I, IIA, IIB, and sometimes IID/X. Type III is not a recognized skeletal muscle fiber type in this context. **High-Yield NEET-PG Pearls:** * **Mnemonic for Type I:** **"One Slow Red Ox"** (Type **I**, **Slow**-twitch, **Red** color, **Ox**idative metabolism). * **Fatigability:** Type I is fatigue-resistant; Type IIA is relatively resistant; Type IIB fatigues rapidly. * **Myosin ATPase activity:** Low in Type I, High in Type II. * **Order of Recruitment (Henneman’s Size Principle):** Type I fibers are recruited first, followed by Type IIA, and finally Type IIB for maximal force.

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: Which of the following acts as a muscle spring?

Titin. **Explanation:** **1. Why Titin is the Correct Answer:** Titin (also known as connectin) is a giant protein that functions as a molecular spring within the sarcomere. It extends from the Z-disk to the M-line. The segment of titin located in the I-band is highly elastic and foldable. When a muscle is stretched, titin develops passive tension, and when the stretch is released, it acts like a spring to return the sarcomere to its resting length. This provides **passive elasticity** [1] to the muscle and ensures the thick filaments (myosin) remain centered within the sarcomere during contraction and relaxation. **2. Why the Other Options are Incorrect:** * **Alpha-actinin:** This is a structural protein located specifically at the **Z-disk**. Its primary role is to anchor the thin (actin) filaments to the Z-disk, maintaining the structural framework of the sarcomere [1]. It does not possess elastic properties and does not act as a spring. * **Both/None:** Since only titin possesses the unique elastic domain required to function as a spring, these options are incorrect. **3. NEET-PG High-Yield Pearls:** * **Largest Protein:** Titin is the largest known single polypeptide chain in the human body. * **Nebulin:** Often confused with titin, nebulin acts as a "molecular ruler" to regulate the length of actin filaments; it is non-elastic. * **Dystrophin:** Connects the cytoskeleton of a muscle fiber to the surrounding extracellular matrix; its deficiency leads to Duchenne Muscular Dystrophy. * **Desmin:** An intermediate filament that links Z-disks of adjacent myofibrils together.

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: What happens when sodium enters cells?

There is a spike in action potential.. ### Explanation The correct answer is **A. There is a spike in action potential.** **1. Why Option A is correct:** The generation of an action potential is primarily driven by the movement of ions across the cell membrane. When a stimulus reaches the threshold potential, **voltage-gated sodium (Na⁺) channels** open rapidly. Since Na⁺ concentration is much higher extracellularly, it rushes into the cell following its electrochemical gradient. This influx of positive charge causes **depolarization**, making the membrane potential more positive. This rapid upstroke is known as the **"spike"** of the action potential. **2. Why other options are incorrect:** * **B. Plateau in action potential:** This is characteristic of cardiac ventricular muscle cells (Phase 2), caused by the balanced influx of **Calcium (Ca²⁺)** through L-type channels and the efflux of Potassium (K⁺). * **C. Repolarization:** This occurs when the membrane potential returns to its resting state, primarily due to the **efflux of Potassium (K⁺)** out of the cell and the closure of Na⁺ channels. * **D. Hyperpolarization:** This happens when the membrane potential becomes more negative than the resting membrane potential, usually due to an **excessive efflux of K⁺** or an **influx of Chloride (Cl⁻)**. **3. NEET-PG High-Yield Pearls:** * **Tetrodotoxin (from Pufferfish) and Saxitoxin:** Block voltage-gated Na⁺ channels, preventing the spike/depolarization. * **Local Anesthetics (e.g., Lidocaine):** Work by blocking voltage-gated Na⁺ channels from the inside, inhibiting signal conduction. * **Overshoot:** The portion of the action potential where the membrane potential is positive (above 0 mV) is called the overshoot. * **Na⁺-K⁺ ATPase:** This pump does not create the action potential but maintains the ionic gradients necessary for it to occur (3 Na⁺ out, 2 K⁺ in).

Q: The sites where myosin heads bind to actin in skeletal muscles are covered by which of the following?

Tropomyosin. ### Explanation **Correct Answer: A. Tropomyosin** In skeletal muscle, the interaction between actin and myosin is the fundamental step for contraction. In a resting (relaxed) state, the **active sites on the F-actin strand**—where myosin heads must attach to form cross-bridges—are physically blocked. This masking is performed by **Tropomyosin**, a long, rod-like protein that wraps around the actin filament. As long as tropomyosin remains in this position, the myosin heads cannot bind to actin, preventing contraction. **Why other options are incorrect:** * **B. Troponin:** This is a complex of three subunits (I, T, and C). While it is attached to tropomyosin, its role is to act as a "switch." When calcium binds to Troponin C, it undergoes a conformational change that pulls the tropomyosin away from the binding sites. It does not cover the sites itself. * **C. Calcium ions:** Calcium is the trigger for contraction. It binds to Troponin C to *uncover* the binding sites; it does not cover them. * **D. Calmodulin:** This is the calcium-binding protein used in **smooth muscle** contraction (where troponin is absent). It does not play a structural role in masking actin sites in skeletal muscle. **High-Yield NEET-PG Pearls:** * **The Troponin Complex:** * **Troponin T:** Binds to **T**ropomyosin. * **Troponin I:** **I**nhibits the actin-myosin interaction. * **Troponin C:** Binds **C**alcium ions (up to 4 ions per molecule). * **The "Walk-along" Theory:** Contraction occurs when tropomyosin moves into the "groove" of the actin helix, exposing the sites. * **Clinical Correlation:** Cardiac Troponins (I and T) are highly specific biomarkers for myocardial infarction because they are released into the blood when cardiac myocytes are damaged.

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.

Question 1

Active tension in a muscle depends upon which of the following factors?

Accepted Answer

Number of motor units recruited

Answer

Number of muscle fibers

Answer

Aerobic capacity of the muscle

Answer

Length of the muscle fiber

Question 2

Actin's active site is covered by?

Accepted Answer

Tropomyosin

Answer

Myosin

Answer

Troponin

Answer

Desmin

Question 3

Which of the following muscle fiber types is brown in color?

Accepted Answer

Type II A

Answer

Type I

Answer

Type II B

Answer

Type III

Question 4

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 5

Which of the following acts as a muscle spring?

Accepted Answer

Titin

Answer

Alpha actinin

Answer

Both

Answer

None

Question 6

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

Accepted Answer

Fe2+

Answer

Ca2+

Answer

K+

Answer

Na+

Question 7

What happens when sodium enters cells?

Accepted Answer

There is a spike in action potential.

Answer

There is a plateau in action potential.

Answer

There is repolarization.

Answer

There is hyperpolarization.

Question 8

The sites where myosin heads bind to actin in skeletal muscles are covered by which of the following?

Accepted Answer

Tropomyosin

Answer

Troponin

Answer

Calcium ions

Answer

Calmodulin

Question 9

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 10

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

Nerve and Muscle Physiology — MCQs

Nerve and Muscle Physiology — MCQs

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