Nerve and Muscle Physiology — MCQs

Nerve and Muscle Physiology Indian Medical PG Practice Questions and MCQs

Question 221: What is the main motor supply to intrafusal fibers?

A. Gamma neurons (Correct Answer)
B. Alpha neurons
C. Beta neurons
D. All of the following

Explanation: ### Explanation The muscle spindle is a complex sensory organ that monitors muscle length and the rate of change in length. It consists of specialized muscle fibers called **intrafusal fibers**, which are contained within a connective tissue capsule. **1. Why Gamma (γ) neurons are correct:** Gamma motor neurons are the primary motor supply to the contractile poles of the intrafusal fibers. When gamma neurons fire, they cause the ends of the intrafusal fibers to contract, which stretches the non-contractile central portion. This increases the sensitivity of the muscle spindle, ensuring it can still detect stretch even when the surrounding muscle (extrafusal fibers) is contracted. This process is essential for maintaining **muscle tone** and the **stretch reflex**. **2. Why the other options are incorrect:** * **Alpha (α) neurons:** These are the largest and fastest motor neurons. They supply the **extrafusal fibers**, which are the regular muscle fibers responsible for generating the force of muscle contraction. * **Beta (β) neurons:** These are less common and provide **collateral innervation** to both intrafusal and extrafusal fibers (skeleton-fusimotor fibers). While they do supply intrafusal fibers, they are not the "main" or primary motor supply. * **All of the following:** Incorrect because Alpha neurons specifically avoid intrafusal fibers. **High-Yield Clinical Pearls for NEET-PG:** * **Alpha-Gamma Co-activation:** During voluntary movement, both alpha and gamma neurons are activated simultaneously. This prevents the muscle spindle from going "slack" during contraction, allowing the brain to maintain constant feedback on muscle length. * **Gamma Gain:** The sensitivity of the spindle is regulated by the CNS via gamma neurons. High gamma discharge leads to hyperreflexia (seen in Upper Motor Neuron lesions). * **Sensory Supply:** Remember that **Type Ia** (primary) and **Type II** (secondary) afferent fibers provide the sensory output from the muscle spindle to the spinal cord.

Question 222: What is the term for a decrease in reflex response after repetitive stimulation?

A. Summation
B. Fatigue (Correct Answer)
C. Irradiation
D. Occlusion

Explanation: **Explanation:** The correct answer is **Fatigue**. In the context of reflex activity, repetitive stimulation of the afferent nerve leads to a gradual decrease and eventual disappearance of the reflex response. This occurs primarily due to the **exhaustion of neurotransmitter stores** at the synaptic level within the reflex arc. Unlike nerve fibers, which are relatively indefatigable, the synapse is the most vulnerable site for fatigue in the neural pathway. **Analysis of Options:** * **A. Summation:** This refers to the cumulative effect of multiple stimuli. It can be *temporal* (repeated stimuli from one neuron) or *spatial* (simultaneous stimuli from multiple neurons) that combine to reach the threshold for an action potential. It increases rather than decreases the response. * **C. Irradiation:** This occurs when the strength of a stimulus is increased, causing the impulse to spread to more neurons in the spinal cord, involving more muscle groups (e.g., a strong painful stimulus causing withdrawal of the entire limb instead of just a finger). * **D. Occlusion:** This is a phenomenon where the response to simultaneous stimulation of two afferent nerves is *less* than the sum of their individual responses because they share a common pool of postsynaptic neurons. While it involves a "decrease" in expected output, it is not caused by repetitive stimulation. **High-Yield Facts for NEET-PG:** * **Site of Fatigue:** In a nerve-muscle preparation, the **synapse** (neuromuscular junction) fatigues first, followed by the muscle, while the nerve fiber is the last to fatigue. * **Synaptic Delay:** The time taken for the neurotransmitter to release and act on the postsynaptic membrane (approx. 0.5 ms) is the reason for the delay in reflex action. * **One-way conduction:** Reflexes follow the **Bell-Magendie Law**, stating that impulses pass only from the axon to the dendrite/soma across a synapse.

Question 223: Fast fatigue fibres are recruited during walking?

A. In the beginning
B. In the end (Correct Answer)
C. Throughout the walking process
D. When small neurons are excited

Explanation: This question tests your understanding of the **Size Principle (Henneman’s Principle)** and the classification of skeletal muscle fibers. ### **Explanation of the Correct Answer** According to the **Size Principle**, motor units are recruited in a specific order based on the size of the alpha-motor neuron. Small, fatigue-resistant motor neurons (Type I, Slow Oxidative fibers) have the lowest threshold and are recruited first for low-intensity activities like standing or slow walking. As the intensity or duration of the activity increases and the initial fibers begin to fatigue, the body recruits larger motor neurons. **Fast-fatigable (Type IIb/IIx)** fibers have the highest threshold and the largest cell bodies. These are recruited **last** (at the end) to provide the necessary force when smaller units are no longer sufficient or when a burst of power is needed. ### **Analysis of Incorrect Options** * **A. In the beginning:** Only Slow-Twitch (Type I) fibers are recruited at the start of low-intensity exercise because they are energy-efficient and highly resistant to fatigue. * **C. Throughout the walking process:** Recruitment is hierarchical, not simultaneous. Type IIb fibers are "reserved" for high-force requirements to prevent premature exhaustion. * **D. When small neurons are excited:** Small neurons innervate Type I fibers. Fast-fatigable fibers are innervated by **large** motor neurons. ### **High-Yield Clinical Pearls for NEET-PG** * **Order of Recruitment:** Type I (Slow) → Type IIa (Fast-Resistant) → Type IIb (Fast-Fatigable). * **Type I Fibers:** High myoglobin (Red), many mitochondria, oxidative metabolism (e.g., back muscles for posture). * **Type IIb Fibers:** Low myoglobin (White), high glycogen, glycolytic metabolism (e.g., extraocular muscles or sprinting). * **Size Principle Exception:** During rapid, powerful eccentric contractions (like jumping down), there may be a selective recruitment of fast-twitch units, but for steady activities like walking, the standard hierarchy applies.

Question 224: Which ion has the lowest electrochemical driving force in a typical neuron with a resting membrane potential of -65 millivolts?

A. Chloride (Correct Answer)
B. Potassium
C. Sodium
D. Calcium

Explanation: ### Explanation The **electrochemical driving force** is the net force acting on an ion, determined by the difference between the actual membrane potential ($V_m$) and the ion's equilibrium potential ($E_{ion}$). The formula is: **Driving Force = $V_m - E_{ion}$** **1. Why Chloride (A) is Correct:** In a typical neuron, the resting membrane potential ($V_m$) is approximately **-65 to -70 mV**. The equilibrium potential for Chloride ($E_{Cl}$) is also approximately **-65 to -70 mV**. Because $V_m$ is almost equal to $E_{Cl}$, the net driving force is near zero. Consequently, there is very little net movement of chloride ions across the resting membrane. **2. Why the Other Options are Incorrect:** * **Potassium (B):** $E_K$ is typically around **-90 mV**. The driving force is $|-65 - (-90)| = 25\text{ mV}$. While small, it is significantly higher than that of Chloride. * **Sodium (C):** $E_{Na}$ is approximately **+60 mV**. The driving force is massive: $|-65 - (+60)| = 125\text{ mV}$. This creates a strong inward pressure for $Na^+$ to enter the cell. * **Calcium (D):** $E_{Ca}$ is very positive (approx. **+120 mV**). Combined with a very low intracellular concentration, the driving force is the highest among common ions ($>180\text{ mV}$). **3. High-Yield Facts for NEET-PG:** * **Nernst Equation:** Used to calculate the equilibrium potential for a single ion. * **Goldman-Hodgkin-Katz (GHK) Equation:** Used to calculate the RMP by considering the permeability of all ions. * **RMP Determinant:** The RMP is closest to the equilibrium potential of the ion with the **highest permeability** (which is Potassium at rest). * **Chloride Paradox:** In some neurons, $Cl^-$ is passively distributed, making its $E_{Cl}$ exactly equal to RMP, resulting in zero driving force.

Question 225: In a nerve, the magnitude of the action potential overshoot is normally a function of which of the following?

A. Magnitude of the stimulus
B. Intracellular potassium concentration
C. Extracellular sodium concentration (Correct Answer)
D. Resting membrane potential

Explanation: ### Explanation **1. Why Extracellular Sodium Concentration is Correct:** The **overshoot** of an action potential refers to the phase where the membrane potential becomes positive (above 0 mV). During depolarization, voltage-gated Na⁺ channels open, causing a massive influx of Na⁺ ions into the cell. According to the **Nernst Equation**, the peak of the action potential (and thus the magnitude of the overshoot) is determined by the equilibrium potential for sodium ($E_{Na}$). Since $E_{Na}$ is directly proportional to the ratio of extracellular to intracellular sodium concentrations ($[Na^+]_o / [Na^+]_i$), a change in **extracellular sodium concentration** will directly alter the driving force and the peak voltage reached. **2. Why the Other Options are Incorrect:** * **A. Magnitude of the stimulus:** Nerve fibers follow the **"All-or-None Law."** Once the threshold is reached, the magnitude of the action potential remains constant regardless of the stimulus strength. * **B. Intracellular potassium concentration:** K⁺ concentration primarily determines the **Resting Membrane Potential (RMP)** and the repolarization phase, not the peak of the overshoot. * **C. Resting membrane potential:** While RMP determines the starting point, the peak height (overshoot) is specifically limited by the sodium equilibrium potential. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Tetrodotoxin (Pufferfish) & Saxitoxin:** Block voltage-gated Na⁺ channels, preventing the upstroke of the action potential. * **Hypokalemia:** Increases the concentration gradient for K⁺, leading to hyperpolarization of the RMP (making nerves less excitable). * **Hyperkalemia:** Partially depolarizes the RMP, initially increasing excitability but eventually causing inactivation of Na⁺ channels (leading to paralysis/arrhythmias). * **Local Anesthetics (Lidocaine):** Block voltage-gated Na⁺ channels from the inside, increasing the threshold for excitation.

Question 226: Name the structure that is NOT involved in excitation-contraction coupling in striated muscle?

A. Microtubules (Correct Answer)
B. Sarcolemma
C. Sarcoplasmic Reticulum
D. Motor end plate

Explanation: **Explanation:** **Excitation-Contraction (E-C) Coupling** is the physiological process by which an electrical stimulus (action potential) is converted into a mechanical response (muscle contraction). **Why Microtubules are the correct answer:** Microtubules are components of the cytoskeleton involved in structural integrity, intracellular transport, and cell division. While they provide a framework for the cell, they play **no direct role** in the rapid transmission of electrical impulses or the release of calcium required for E-C coupling. **Analysis of other options:** * **Motor End Plate:** This is the specialized region of the sarcolemma at the neuromuscular junction. It contains nicotinic acetylcholine receptors; its depolarization (End Plate Potential) is the initiating step that triggers the action potential. * **Sarcolemma:** The muscle cell membrane propagates the action potential along its surface and deep into the fiber via **T-tubules**. This ensures that the electrical signal reaches the interior of the muscle fiber. * **Sarcoplasmic Reticulum (SR):** The SR acts as the primary intracellular calcium reservoir. The signal from the T-tubules triggers the release of $Ca^{2+}$ via Ryanodine receptors (RyR), which is the essential "coupling" step that allows actin-myosin interaction. **High-Yield Facts for NEET-PG:** * **The Triad:** In skeletal muscle, a triad consists of one T-tubule and two terminal cisternae of the SR. It is located at the **A-I junction**. * **Voltage Sensor:** The Dihydropyridine (DHP) receptor in the T-tubule acts as the voltage sensor. * **Calcium Release Channel:** The Ryanodine receptor (RyR1 in skeletal muscle) is the channel on the SR. * **Malignant Hyperthermia:** Caused by a mutation in the RyR1 receptor, leading to excessive calcium release upon exposure to volatile anesthetics.

Question 227: Staircase phenomenon (Treppe) is due to?

A. Increased availability of intracellular calcium (Correct Answer)
B. Synthesis of stable troponin C molecules
C. Summation
D. Tetanus

Explanation: **Explanation:** The **Staircase Phenomenon (Treppe)** refers to a gradual increase in the force of contraction when a muscle is stimulated by a series of pulses of the same intensity at low frequencies. **Why Option A is correct:** The primary mechanism behind Treppe is the **increased availability of intracellular calcium ($Ca^{2+}$)**. When a muscle is stimulated repeatedly, the Sarcoplasmic Reticulum (SR) releases $Ca^{2+}$ into the sarcoplasm. If the stimuli occur in rapid enough succession, the $Ca^{2+}$ pumps (SERCA) do not have sufficient time to sequester all the calcium back into the SR before the next stimulus arrives. This leads to a "build-up" of residual calcium in the cytosol, allowing more $Ca^{2+}$ to bind to Troponin C, resulting in more cross-bridge formations and increased contractile force. Additionally, the slight rise in muscle temperature during repeated contractions enhances enzyme efficiency. **Why other options are incorrect:** * **Option B:** Troponin C molecules are structural proteins; they are not "synthesized" or modified into "stable" versions during acute muscle contraction. * **Option C:** **Summation** occurs when a second stimulus is applied *before* the muscle has started to relax (higher frequency than Treppe). Treppe occurs when the muscle relaxes completely between stimuli but still shows increasing tension. * **Option D:** **Tetanus** is a state of sustained maximal contraction caused by high-frequency stimulation where individual twitches fuse. Treppe is a step-like increase, not a continuous fusion. **High-Yield Facts for NEET-PG:** * **Treppe vs. Summation:** In Treppe, the muscle relaxes fully between stimuli; in Summation, it does not. * **Bowditch Effect:** This is the cardiac equivalent of Treppe, where an increase in heart rate leads to increased force of contraction (positive inotropy) due to $Ca^{2+}$ accumulation. * **Warm-up effect:** Treppe is the physiological basis for why athletes "warm up" to achieve maximal contraction strength.

Question 228: What is the function of synaptobrevin?

A. Presynaptic vesicle fusion (Correct Answer)
B. Postsynaptic vesicle fusion
C. Inhibits synaptic transmission
D. Amplify synaptic transmission

Explanation: **Explanation:** The process of neurotransmitter release at the chemical synapse involves a group of proteins known as **SNARE proteins** (Soluble NSF Attachment Protein Receptors). These proteins are essential for the docking and fusion of synaptic vesicles with the presynaptic membrane. **Why Option A is Correct:** **Synaptobrevin** (also called VAMP - Vesicle-Associated Membrane Protein) is a **v-SNARE** (vesicular SNARE). It is located on the membrane of the neurotransmitter vesicle. During synaptic transmission, synaptobrevin interacts with **t-SNAREs** (target SNAREs) on the presynaptic membrane—specifically **Syntaxin** and **SNAP-25**. This interaction forms a "SNARE complex" that pulls the vesicle close to the presynaptic membrane, leading to membrane fusion and exocytosis of the neurotransmitter into the synaptic cleft. **Why Other Options are Incorrect:** * **Option B:** Vesicle fusion occurs at the **presynaptic** terminal to release neurotransmitters. The postsynaptic membrane contains receptors, not fusion machinery for neurotransmitter release. * **Options C & D:** While synaptobrevin is essential for transmission, its primary physiological function is the mechanical act of **fusion**. It does not act as a regulatory inhibitor or an amplifier of the signal itself. **High-Yield Clinical Pearls for NEET-PG:** * **Tetanus Toxin & Botulinum Toxin:** These are proteases that cleave SNARE proteins. * **Tetanus toxin** specifically cleaves **Synaptobrevin** in inhibitory interneurons (Renshaw cells), leading to spastic paralysis. * **Botulinum toxins (B, D, F, G)** also cleave **Synaptobrevin**, while types A and E cleave SNAP-25, leading to flaccid paralysis. * **Synaptotagmin:** This is the calcium sensor on the vesicle that triggers the final fusion step when $Ca^{2+}$ enters the terminal.

Question 229: The cross-bridges of the sarcomere in skeletal muscle are components of what?

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

Explanation: ### Explanation **Correct Option: A (Myosin)** In skeletal muscle, the **thick filaments** are composed of the protein **Myosin**. Each myosin molecule consists of a tail (rod) and two globular heads. These heads, along with the "arm" or hinge region, extend outward toward the thin filaments to form **cross-bridges**. These cross-bridges contain two critical binding sites: 1. **Actin-binding site:** To initiate contraction. 2. **ATP-binding site (Myosin ATPase):** To hydrolyze ATP and provide energy for the "power stroke." **Why Incorrect Options are Wrong:** * **B. Actin:** This is the primary component of the **thin filament**. While actin has a binding site for myosin, it does not form the cross-bridge structure itself; it serves as the "ladder" the cross-bridge climbs. * **C. Troponin:** A regulatory protein complex (TnT, TnI, TnC) located on the thin filament. Its role is to bind Calcium (TnC) and shift tropomyosin to uncover the active sites on actin. * **D. Tropomyosin:** A filamentous protein that wraps around actin. In a relaxed state, it physically blocks the myosin-binding sites on actin, preventing cross-bridge formation. **High-Yield NEET-PG Pearls:** * **The Power Stroke:** Occurs when **ADP and Pi (inorganic phosphate) are released** from the myosin head, causing it to tilt toward the M-line. * **Detachment:** Binding of a **new ATP molecule** is required for the myosin head to detach from actin. * **Rigor Mortis:** Occurs due to the lack of ATP; without ATP, the cross-bridges cannot detach, leaving the muscle in a rigid, contracted state. * **H-Zone:** This region of the sarcomere contains **only thick filaments (myosin)** and lacks cross-bridges in its central "bare zone."

Question 230: Which of the following is NOT an indicator of bone formation?

A. Osteocalcin
B. Alkaline phosphatase
C. Hydroxyproline (Correct Answer)
D. Type 1 procollagen

Explanation: ### Explanation The correct answer is **Hydroxyproline** because it is a marker of **bone resorption** (destruction), not bone formation. **1. Why Hydroxyproline is the correct answer:** Bone matrix consists primarily of Type 1 collagen. During bone resorption, osteoclasts break down this collagen matrix, releasing **Hydroxyproline** and **Pyridinoline cross-links** into the blood and urine. Therefore, elevated levels of urinary hydroxyproline indicate increased bone turnover or destruction (e.g., Paget’s disease, bone metastasis), making it a marker of resorption. **2. Why the other options are markers of bone formation:** * **Osteocalcin (Option A):** This is a non-collagenous protein synthesized by **osteoblasts**. It is the most specific marker for bone formation and reflects osteoblastic activity. * **Alkaline Phosphatase (Option B):** Specifically the **bone-specific isoenzyme (BSAP)**, it is released by osteoblasts during the mineralization process. It is a widely used clinical screening tool for bone formation. * **Type 1 Procollagen (Option D):** Collagen is synthesized as procollagen. During its conversion to mature collagen, N-terminal (PINP) and C-terminal (PICP) propeptides are cleaved and released into the circulation. These propeptides are sensitive indicators of new collagen synthesis by osteoblasts. ### High-Yield Clinical Pearls for NEET-PG: * **Most Specific Marker of Bone Formation:** Osteocalcin. * **Most Sensitive Marker of Bone Resorption:** Serum CTx (C-terminal telopeptide of type 1 collagen). * **Urinary Marker for Resorption:** Pyridinoline and Deoxypyridinoline (more specific than hydroxyproline, as hydroxyproline can also be influenced by dietary intake). * **Enzyme Marker for Resorption:** Tartrate-resistant acid phosphatase (TRAP 5b).

Practice by Chapter

Resting Membrane Potential

Practice Questions

Action Potential Generation and Propagation

Practice Questions

Neuromuscular Junction

Practice Questions

Skeletal Muscle Contraction

Practice Questions

Smooth Muscle Physiology

Practice Questions

Cardiac Muscle Properties

Practice Questions

Muscle Metabolism and Fatigue

Practice Questions

Motor Unit Function

Practice Questions

Neurotransmitters and Receptors

Practice Questions

Electrophysiological Measurements

Practice Questions

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