Nerve and Muscle Physiology — MCQs

Nerve and Muscle Physiology Indian Medical PG Practice Questions and MCQs

Question 151: The force of muscle contraction can be increased by all of the following mechanisms except:

A. Increasing the frequency of activation of motor units
B. Increasing the number of motor units activated
C. Increasing the amplitude of action potentials in the motor neurons (Correct Answer)
D. Recruiting larger motor units

Explanation: ### Explanation The force of skeletal muscle contraction is regulated by the nervous system through spatial and temporal summation, but it is **not** influenced by the amplitude of action potentials. #### Why Option C is Correct (The "Except") Action potentials follow the **All-or-None Law**. Once the threshold is reached, an action potential is generated with a constant amplitude and duration for a given nerve fiber. Increasing the stimulus strength above the threshold does not increase the size of the action potential; therefore, it cannot increase the force of muscle contraction. The signal is coded by **frequency**, not amplitude. #### Why the Other Options are Incorrect * **Option A (Frequency/Temporal Summation):** Increasing the frequency of discharge leads to the overlapping of muscle twitches. If the frequency is high enough, it leads to **tetanization**, significantly increasing the total force. * **Option B (Recruitment/Spatial Summation):** By activating more motor neurons, more muscle fibers are stimulated simultaneously, leading to a stronger collective contraction. * **Option D (Size Principle):** According to **Henneman’s Size Principle**, smaller motor units are recruited first for fine control, followed by larger motor units (which contain more muscle fibers) for tasks requiring greater force. #### NEET-PG High-Yield Pearls * **Henneman’s Size Principle:** Recruitment order is always Small → Medium → Large motor units. * **Quantal Summation:** Another term for spatial summation (recruiting more motor units). * **Treppe (Staircase Effect):** An increase in contraction force when a muscle is stimulated repeatedly after a period of rest, due to increased cytosolic $Ca^{2+}$ and heat. * **Multi-unit vs. Unitary Smooth Muscle:** Remember that multi-unit smooth muscles (like the iris) behave similarly to skeletal muscle recruitment, whereas unitary (visceral) muscles act as a syncytium.

Question 152: Which of the following changes occur in bone growth?

A. Increased acid phosphatase
B. Increased urinary calcium
C. Increased bone nucleotidase
D. Increased osteocalcin (Correct Answer)

Explanation: ### Explanation **Correct Answer: D. Increased osteocalcin** **Why it is correct:** Bone growth and remodeling are driven by the activity of **osteoblasts** (bone-forming cells). **Osteocalcin** (also known as Bone Gamma-Carboxyglutamic Acid Protein) is a non-collagenous protein hormone synthesized exclusively by osteoblasts. It plays a crucial role in bone mineralization and calcium ion binding. Because its synthesis is specific to osteoblasts, serum levels of osteocalcin are considered a highly specific **biochemical marker of bone formation** and turnover. During periods of active bone growth, osteoblast activity increases, leading to elevated osteocalcin levels. **Why the other options are incorrect:** * **A. Increased acid phosphatase:** Tartrate-resistant acid phosphatase (TRAP) is a marker of **osteoclast** activity (bone resorption), not bone growth. * **B. Increased urinary calcium:** High urinary calcium (hypercalciuria) is typically a sign of bone resorption or impaired renal reabsorption, often seen in conditions like hyperparathyroidism or immobilization, rather than healthy bone growth. * **C. Increased bone nucleotidase:** While 5'-nucleotidase is a marker for liver/biliary disease, it is not a specific or standard marker for bone growth or mineralization. **NEET-PG High-Yield Pearls:** * **Markers of Bone Formation:** Osteocalcin, Bone-specific Alkaline Phosphatase (ALP), and Procollagen type 1 N-terminal propeptide (P1NP). * **Markers of Bone Resorption:** Urinary hydroxyproline, Pyridinoline cross-links, and Serum TRAP. * **Vitamin K Dependency:** Osteocalcin requires Vitamin K for carboxylation to become functional; thus, Vitamin K deficiency can impair bone mineralization. * **Metabolic Link:** Osteocalcin also acts as a hormone that increases insulin secretion and sensitivity, linking bone metabolism to energy homeostasis.

Question 153: What is the resting membrane potential of a muscle fiber?

A. -60mV
B. -70mV
C. -80mV
D. -90mV (Correct Answer)

Explanation: The Resting Membrane Potential (RMP) is the electrical potential difference across the cell membrane when the cell is at rest. In skeletal muscle fibers, the RMP is typically **-90 mV**. ### Why -90 mV is Correct The RMP is primarily determined by the equilibrium potential of Potassium ($K^+$). According to the Nernst equation, the equilibrium potential for $K^+$ is approximately -94 mV. Because the resting muscle membrane is highly permeable to $K^+$ (via leak channels) and relatively impermeable to $Na^+$, the RMP sits very close to the $K^+$ equilibrium potential. The slight difference (-90 mV vs -94 mV) is due to a minor influx of $Na^+$ and the electrogenic contribution of the $Na^+-K^+$ ATPase pump. ### Analysis of Incorrect Options * **-70 mV (Option B):** This is the typical RMP for **large neurons**. Neurons have a slightly higher RMP than muscle fibers, making them more excitable. * **-60 mV (Option A):** This is closer to the RMP of **smooth muscle cells** (ranging from -50 to -60 mV) or the threshold potential for firing an action potential in many excitable tissues. * **-80 mV (Option C):** While some cardiac cells or specific fibers may show this value, -90 mV is the standard physiological value cited for skeletal muscle and ventricular myocytes. ### High-Yield NEET-PG Pearls * **Skeletal Muscle vs. Nerve:** Muscle fibers have a more negative RMP (-90 mV) compared to nerves (-70 mV). * **Ionic Basis:** $K^+$ efflux is the most important factor in establishing RMP. * **Maintenance:** The $Na^+-K^+$ ATPase pump maintains the concentration gradients but only contributes about -4 to -5 mV directly to the RMP. * **Clinical Correlation:** Hypokalemia hyperpolarizes the membrane (makes it more negative), making it harder to trigger an action potential, leading to muscle weakness.

Question 154: Muscle weakness may be a feature of all of the following EXCEPT:

A. Progressive degeneration of muscle fibres
B. Magnesium deficiency
C. Myasthenia gravis
D. Physostigmine therapy (Correct Answer)

Explanation: **Explanation:** The core concept here is the regulation of the neuromuscular junction (NMJ). Muscle weakness occurs when there is a failure in signal transmission or a defect in the contractile apparatus. **Why Physostigmine (Option D) is the Correct Answer:** Physostigmine is an **acetylcholinesterase (AChE) inhibitor**. It prevents the breakdown of acetylcholine (ACh) in the synaptic cleft, thereby increasing the concentration and duration of ACh action at the motor endplate. In clinical practice, this **enhances neuromuscular transmission** and is used to *reverse* muscle weakness (e.g., in Myasthenia Gravis or after non-depolarizing neuromuscular blockade). While an extreme overdose can lead to a "cholinergic crisis" (depolarizing block), therapeutic use is intended to improve muscle strength, making it the exception in this list. **Analysis of Other Options:** * **A. Progressive degeneration of muscle fibers:** Seen in conditions like Duchenne Muscular Dystrophy. Loss of structural integrity and contractile proteins directly leads to profound muscle weakness. * **B. Magnesium deficiency:** Magnesium competes with Calcium at the presynaptic terminal. However, severe hypomagnesemia often coexists with hypocalcemia and hypokalemia, leading to impaired ACh release and altered membrane excitability, resulting in muscle weakness and tetany. * **C. Myasthenia gravis:** An autoimmune disorder where antibodies destroy nicotinic ACh receptors (nAChR) at the NMJ, leading to classic fatigable muscle weakness. **Clinical Pearls for NEET-PG:** * **Edrophonium (Tensilon) Test:** A short-acting AChE inhibitor used to diagnose Myasthenia Gravis (improvement in strength = positive test). * **Lambert-Eaton Syndrome:** Weakness caused by antibodies against **presynaptic voltage-gated calcium channels**, reducing ACh release. * **Physostigmine vs. Neostigmine:** Physostigmine crosses the blood-brain barrier (tertiary amine), while Neostigmine does not (quaternary ammonium).

Question 155: All of the following are true about excitation-contraction coupling EXCEPT:

A. Acetylcholine is released at the nerve terminal.
B. Calcium is pumped back into the sarcoplasmic reticulum during relaxation.
C. Calcium is released from the sarcoplasmic reticulum during contraction.
D. Calcium binds to troponin to initiate muscle contraction. (Correct Answer)

Explanation: **Explanation** Excitation-contraction (E-C) coupling is the physiological process where an electrical stimulus (action potential) is converted into a mechanical response (muscle contraction). **Why Option D is the "Except" (Correct Answer):** While it is a common misconception, calcium does not bind to troponin in general; it binds specifically to **Troponin C**. In skeletal muscle, this binding causes a conformational change in the troponin-tropomyosin complex, uncovering the myosin-binding sites on actin. In a NEET-PG context, "Troponin" is often considered too vague when specific subunits are tested, but more importantly, in **smooth muscle**, calcium binds to **Calmodulin**, not troponin. Therefore, stating it binds to "troponin" as a universal rule for all muscle contraction is technically incomplete or incorrect depending on the muscle type. **Analysis of Other Options:** * **Option A:** Correct. The process begins when an action potential reaches the NMJ, triggering the release of **Acetylcholine (ACh)** into the synaptic cleft. * **Option B:** Correct. Relaxation is an active process. The **SERCA pump** (Sarcoplasmic Endoplasmic Reticulum Calcium ATPase) moves calcium back into the SR against its concentration gradient. * **Option C:** Correct. Depolarization of the T-tubules activates **DHPR** (Dihydropyridine receptors), which mechanically opens **RyR** (Ryanodine receptors) on the SR, releasing calcium into the sarcoplasm. **High-Yield NEET-PG Pearls:** * **L-type Ca²⁺ channels** act as voltage sensors in skeletal muscle (DHPR). * **Calsequestrin** is the protein that buffers calcium within the SR. * **Malignant Hyperthermia:** Caused by a mutation in the Ryanodine Receptor (RyR1), leading to excessive calcium release. Treatment is **Dantrolene**. * **Rigor Mortis:** Occurs due to the lack of ATP, which is required to break the actin-myosin cross-bridge and power the SERCA pump.

Question 156: A cross-sectional view of a skeletal muscle fiber through the H-zone would show the presence of what?

A. Actin and myosin
B. Titin and myosin
C. Actin and titin
D. Myosin only (Correct Answer)

Explanation: To understand the cross-sectional anatomy of a sarcomere, one must visualize the arrangement of myofilaments within its specific zones. ### **Explanation of the Correct Answer** The **H-zone** (Hensen’s zone) is the central region of the **A-band**. During muscle relaxation, the thin filaments (actin) do not extend all the way to the center of the sarcomere. Therefore, the H-zone contains **only thick filaments (myosin)**. In a cross-section through this specific area, you would see the hexagonal arrangement of myosin filaments without any overlapping actin. ### **Why Other Options are Incorrect** * **A. Actin and myosin:** This combination is found in the **outer regions of the A-band** (the overlap zone). This is the site where cross-bridges form. * **B. Titin and myosin:** While titin anchors myosin to the Z-discs, it is generally not the primary focus of cross-sectional identification in standard physiological models of the H-zone. * **C. Actin and titin:** This combination (along with other proteins like nebulin) is characteristic of the **I-band**, which contains only thin filaments and no myosin. ### **High-Yield NEET-PG Pearls** * **M-line:** Located in the dead center of the H-zone; it contains proteins (like myomesin) that hold the thick filaments together. * **Sarcomere Dynamics:** During muscle contraction (Sliding Filament Theory), the **H-zone and I-band shorten**, while the **A-band remains constant** in length. * **Pseudo-H Zone:** A narrow region in the center of the H-zone that lacks myosin heads (the "bare zone"). * **Ratio:** In the overlap zone of mammalian skeletal muscle, each thick filament is surrounded by **6 thin filaments**.

Question 157: What is the main component of the thin filament?

A. Actin (Correct Answer)
B. Myosin
C. Tropomyosin
D. Dystrophin

Explanation: **Explanation:** The sarcomere, the functional unit of skeletal muscle, is composed of thick and thin filaments. The **thin filament** is primarily composed of three proteins: **Actin**, Tropomyosin, and the Troponin complex. **1. Why Actin is Correct:** Actin is the backbone of the thin filament. It exists as globular monomers (**G-actin**) that polymerize to form two long, helical strands known as **F-actin** (filamentous actin). Each G-actin molecule possesses a specific binding site for myosin heads, which is essential for cross-bridge formation and muscle contraction. **2. Analysis of Incorrect Options:** * **B. Myosin:** This is the primary component of the **thick filament**. It is a large protein with a "head" (possessing ATPase activity) and a "tail." * **C. Tropomyosin:** While it is a component of the thin filament, it is a regulatory protein, not the "main" structural component. It wraps around the actin helix to cover the myosin-binding sites during rest. * **D. Dystrophin:** This is a cytoskeletal protein that anchors the entire myofibril to the sarcolemma (cell membrane). It is not part of the thin filament itself. **High-Yield NEET-PG Pearls:** * **Troponin Complex:** Consists of **Troponin T** (binds to tropomyosin), **Troponin I** (inhibits actin-myosin binding), and **Troponin C** (binds to Calcium). * **Clinical Correlation:** Mutations in the **Dystrophin** gene lead to Duchenne and Becker Muscular Dystrophies. * **The Sliding Filament Theory:** During contraction, neither the thick nor thin filaments shorten; instead, they slide past each other, shortening the H-zone and I-band, while the **A-band remains constant** in length.

Question 158: In preload, which of the following can be seen?

A. Isotonic contraction without shortening of muscle fibers
B. Isotonic contraction with shortening of muscle fibers (Correct Answer)
C. Isometric contraction without shortening of muscle fibers
D. Isometric contraction with shortening of muscle fibers

Explanation: ### Explanation In muscle physiology, **Preload** refers to the load applied to a muscle *before* it begins to contract. This load stretches the muscle to a certain resting length, determining the initial overlap between actin and myosin filaments. **Why Option B is Correct:** When a muscle is subjected to preload, it is stretched to a specific length. Upon stimulation, if the muscle develops enough tension to overcome this load, it undergoes **isotonic contraction**. In an isotonic contraction, the tension remains constant while the **muscle fibers shorten** to perform work (moving the load). Therefore, preload is fundamentally associated with the length-tension relationship and subsequent shortening during contraction. **Analysis of Incorrect Options:** * **Option A:** Isotonic contraction, by definition, involves a change in muscle length. A contraction "without shortening" cannot be isotonic if the muscle is performing work. * **Option C:** **Isometric contraction** occurs when the muscle develops tension without changing its length (e.g., pushing against a fixed wall). While preload determines the starting point for isometric tension, the hallmark of the *action* following preload in a standard experimental setup is the ability to shorten. * **Option D:** This is a physiological contradiction. Isometric means "same length"; therefore, shortening cannot occur during an isometric contraction. **High-Yield NEET-PG Pearls:** * **Preload vs. Afterload:** Preload sets the **resting length** (related to the Frank-Starling Law in the heart), while Afterload is the resistance the muscle must contract *against*. * **Isotonic vs. Isometric:** Remember: **Iso-metric** = Same Length (No work done, $W=0$); **Iso-tonic** = Same Tension (Work is done). * **Optimal Length ($L_0$):** The length at which the muscle develops maximum active tension. Preload helps the muscle reach this $L_0$.

Question 159: The hyperpolarization phase of the action potential is due to?

A. Opening of voltage-gated Cl- channels
B. Prolonged opening of voltage-gated K+ channels (Correct Answer)
C. Closure of resting Na+ channels
D. Closure of Cl- channels

Explanation: ### Explanation The hyperpolarization phase (also known as the **undershoot**) occurs because voltage-gated potassium ($K^+$) channels are slow to close. **1. Why Option B is Correct:** During the repolarization phase, voltage-gated $K^+$ channels open to allow $K^+$ to exit the cell. Unlike sodium channels, which have a rapid inactivation gate, $K^+$ channels close slowly. This **prolonged conductance** allows $K^+$ efflux to continue even after the membrane potential has reached the resting level (-70 mV). The potential moves closer to the **equilibrium potential of Potassium (-94 mV)**, resulting in a temporary state where the interior of the cell is more negative than at rest. **2. Why the Other Options are Incorrect:** * **Option A & D:** Chloride ($Cl^-$) channels do not play a primary role in the generation of a standard neuronal action potential. While $Cl^-$ influx can cause inhibitory postsynaptic potentials (IPSPs), it is not the mechanism behind the action potential's hyperpolarization phase. * **Option C:** The closure of $Na^+$ channels (specifically the inactivation of the 'h' gate) is responsible for the initiation of **repolarization**, not hyperpolarization. --- ### NEET-PG High-Yield Pearls * **Equilibrium Potentials (Nernst Equation):** $Na^+$ is +60 mV, $K^+$ is -94 mV. The membrane potential always moves toward the equilibrium potential of the ion to which it is most permeable. * **After-hyperpolarization:** This phase is responsible for the **Relative Refractory Period**, as a stronger-than-normal stimulus is required to reach the threshold from a more negative starting point. * **Tetrodotoxin (TTX):** A high-yield toxin (from Pufferfish) that blocks voltage-gated $Na^+$ channels, preventing the depolarization phase. * **Tetraethylammonium (TEA):** Blocks voltage-gated $K^+$ channels, specifically abolishing the hyperpolarization phase.

Question 160: What is the contribution of noncontractile muscle elements to total tension?

A. A
B. E (Correct Answer)
C. C
D. B

Explanation: In skeletal muscle physiology, the **Total Tension** produced by a muscle is the sum of its **Active Tension** (generated by cross-bridge cycling) and **Passive Tension** (generated by noncontractile elastic elements). ### Explanation of the Correct Answer The correct answer is **E**. In a standard length-tension relationship graph: * **Passive Tension (Curve E):** This represents the tension developed by stretching the muscle's noncontractile components (such as the sarcolemma, connective tissue sheaths like endomysium/perimysium, and the protein **titin**) without any electrical stimulation. As the muscle is stretched beyond its resting length, this tension increases exponentially. * **Total Tension:** This is the sum of active and passive tension. At longer muscle lengths, the contribution of noncontractile elements (Curve E) becomes the dominant factor in total tension. ### Explanation of Incorrect Options * **Option A (Active Tension):** This curve typically shows a bell shape, peaking at the optimal resting length ($L_0$) where actin-myosin overlap is maximal, and decreasing as the muscle is overstretched. * **Option B & D:** These usually represent intermediate points or specific components of the active tension curve (like the descending limb) rather than the standalone contribution of elastic elements. * **Option C:** Often represents the "Total Tension" curve itself, which is the resultant of adding the active and passive components. ### High-Yield NEET-PG Pearls * **Titin:** The primary protein responsible for passive tension and for keeping thick filaments centered in the sarcomere. * **Frank-Starling Law:** In cardiac muscle, the passive tension curve is much steeper than in skeletal muscle, preventing overstretch and ensuring efficient pumping. * **Optimal Length ($L_0$):** The length at which active tension is maximal (approx. 2.0–2.2 μm per sarcomere).

Practice by Chapter

Resting Membrane Potential

Practice Questions

Action Potential Generation and Propagation

Practice Questions

Neuromuscular Junction

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Skeletal Muscle Contraction

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Smooth Muscle Physiology

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Cardiac Muscle Properties

Practice Questions

Muscle Metabolism and Fatigue

Practice Questions

Motor Unit Function

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Neurotransmitters and Receptors

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Electrophysiological Measurements

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