Nerve and Muscle Physiology Practice Questions

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

Microtubules. **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.

Q: In the Central Nervous System (CNS), myelination is carried out by which type of glial cell?

Oligodendrocytes. **Explanation:** The correct answer is **Oligodendrocytes**. Myelination is the process of forming a lipid-rich insulating layer around axons to increase the speed of nerve impulse conduction (saltatory conduction). In the **Central Nervous System (CNS)**, which includes the brain and spinal cord, this function is performed by oligodendrocytes. A key characteristic of oligodendrocytes is that a single cell can extend multiple processes to myelinate segments of several different axons simultaneously. **Analysis of Incorrect Options:** * **Schwann cells:** These are responsible for myelination in the **Peripheral Nervous System (PNS)**. Unlike oligodendrocytes, one Schwann cell myelinates only a single segment of one axon. * **Astrocytes:** These are the most abundant glial cells in the CNS. They provide structural support, maintain the blood-brain barrier (BBB), regulate the external chemical environment, and form scar tissue (gliosis) after injury. * **Microglia:** These are the resident macrophages of the CNS. They act as the primary immune defense and are derived from the yolk sac (mesodermal origin), unlike other glial cells which are neuroectodermal. **High-Yield Clinical Pearls for NEET-PG:** * **Multiple Sclerosis (MS):** An autoimmune demyelinating disease specifically affecting the **CNS** (oligodendrocytes). * **Guillain-Barré Syndrome (GBS):** An acute inflammatory demyelinating polyneuropathy affecting the **PNS** (Schwann cells). * **Origin:** All glial cells (Astrocytes, Oligodendrocytes, Ependymal cells) originate from the **Neural Tube**, except for Microglia, which originate from **Monocytes/Mesoderm**.

Q: What is the function of synaptobrevin?

Presynaptic vesicle fusion. **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.

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

Myosin. ### 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."

Q: The relaxation of smooth muscle is associated with a reduction in free intracellular calcium ion concentration. What is the effect of this reduction?

Decreased phosphorylation of myosin molecules. ### Explanation The mechanism of smooth muscle contraction and relaxation is fundamentally different from skeletal muscle, primarily because smooth muscle lacks **troponin**. **1. Why Option C is Correct:** In smooth muscle, contraction is initiated when calcium binds to **Calmodulin**. This Ca²⁺-Calmodulin complex activates **Myosin Light Chain Kinase (MLCK)**. MLCK then phosphorylates the myosin light chain, allowing the myosin head to bind to actin and initiate the cross-bridge cycle. When intracellular calcium levels fall, MLCK becomes inactive. Simultaneously, the enzyme **Myosin Light Chain Phosphatase (MLCP)** removes the phosphate group from the myosin. Therefore, a reduction in calcium leads directly to **decreased phosphorylation of myosin**, resulting in relaxation. **2. Why the Other Options are Incorrect:** * **Option A & D:** These describe the mechanism in **skeletal and cardiac muscle**. In those muscles, relaxation occurs when calcium is removed, allowing the troponin-tropomyosin complex to re-establish inhibition by blocking active sites on actin. Smooth muscle does not use this "blocking" mechanism. * **Option B:** Actin molecules do not possess "enzymatic activity" that is deactivated by calcium; the enzymatic (ATPase) activity resides in the myosin head. **High-Yield NEET-PG Pearls:** * **Calcium Source:** Smooth muscle relies on both extracellular calcium (via L-type channels) and intracellular calcium (from the sarcoplasmic reticulum). * **Latch-Bridge Mechanism:** This allows smooth muscle to maintain prolonged tone with minimal ATP consumption, even after dephosphorylation of myosin. * **Caldesmon & Calponin:** These are functional analogues of troponin found in smooth muscle that inhibit actin-myosin binding in the resting state.

Q: Which of the following modalities is conducted fastest along nerve fibers?

Proprioception. The speed of nerve conduction is directly proportional to the **diameter of the nerve fiber** and the **presence of myelin**. According to the **Erlanger-Gasser classification**, nerve fibers are categorized into Types A, B, and C. **Why Proprioception is Correct:** Proprioception (the sense of body position) is carried by **Type A-alpha (Aα)** fibers. These are the thickest and most heavily myelinated fibers in the human body, boasting a diameter of 12–20 μm and a conduction velocity of **70–120 m/s**. Rapid transmission is physiologically essential for maintaining balance and coordinating complex motor movements in real-time. **Analysis of Incorrect Options:** * **Pressure Sensation:** Carried by **Type A-beta (Aβ)** fibers. While myelinated, they are smaller than Aα fibers, with a conduction velocity of approximately 30–70 m/s. * **Temperature (Cold):** Carried by **Type A-delta (Aδ)** fibers. These are thin, lightly myelinated fibers with a much slower velocity of 5–30 m/s. * **Pain Sensation:** Fast pain is carried by Aδ fibers, while slow, chronic pain is carried by **Type C** fibers. Type C fibers are unmyelinated and the smallest in diameter, conducting at a mere 0.5–2 m/s. **High-Yield NEET-PG Pearls:** * **Order of Conduction Velocity:** Proprioception (Aα) > Touch/Pressure (Aβ) > Pain/Temp (Aδ) > Pain/Autonomic (C). * **Susceptibility to Blockade:** * **Local Anesthetics:** Block Type C fibers first (smallest). * **Pressure/Hypoxia:** Block Type A fibers first (largest). * **Rule of Thumb:** Conduction velocity (m/s) ≈ Fiber diameter (μm) × 6.

Q: What is the first change observed in the distal part of a cut nerve?

Axonal degeneration. **Explanation:** The process described in the question refers to **Wallerian Degeneration**, which occurs when a nerve fiber is cut or crushed. **1. Why Axonal Degeneration is correct:** The axon is the most metabolically active part of the nerve but depends entirely on the cell body (soma) for the transport of proteins and organelles via axoplasmic flow. Once the nerve is severed, the distal segment is isolated from its source of nutrients. **Axonal degeneration** is the earliest morphological change, beginning within 24 hours. The neurofilaments and microtubules fragment, and the axoplasm liquefies. **2. Analysis of Incorrect Options:** * **C. Myelin degeneration:** While the myelin sheath does break down (forming ellipsoids), this occurs **secondary** to the collapse of the underlying axon. Myelin degeneration typically becomes prominent after the first 48–72 hours. * **D. Schwann cell proliferation:** This occurs after the axon and myelin have begun to break down. Schwann cells proliferate to clean up debris (along with macrophages) and form the **Bungner bands** to guide regenerating sprouts. * **B. Sprouting:** This is a feature of the **regenerative phase**, not the initial degenerative phase. It occurs from the proximal stump, not the distal part. **High-Yield Clinical Pearls for NEET-PG:** * **Wallerian Degeneration:** Occurs in the distal segment. * **Retrograde Degeneration:** Occurs in the proximal segment (up to the first Node of Ranvier). * **Chromatolysis:** The hallmark change in the **cell body** (soma) following nerve injury, characterized by the disappearance of Nissl bodies and lateral displacement of the nucleus. * **Rate of Regeneration:** Peripheral nerves typically regrow at a rate of **1–3 mm/day**.

Question 1

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

Accepted Answer

Microtubules

Answer

Sarcolemma

Answer

Sarcoplasmic Reticulum

Answer

Motor end plate

Question 2

In the Central Nervous System (CNS), myelination is carried out by which type of glial cell?

Accepted Answer

Oligodendrocytes

Answer

Schwann cells

Answer

Astrocytes

Answer

Microglia

Question 3

Staircase phenomenon (Treppe) is due to?

Accepted Answer

Increased availability of intracellular calcium

Answer

Synthesis of stable troponin C molecules

Answer

Summation

Answer

Tetanus

Question 4

The plateau phase of the myocardial action potential is primarily due to which ion movement?

Accepted Answer

Influx of Ca++

Answer

Efflux of Na+

Answer

Influx of K+

Answer

Closure of voltage-gated K+ channels

Question 5

What is the function of synaptobrevin?

Accepted Answer

Presynaptic vesicle fusion

Answer

Postsynaptic vesicle fusion

Answer

Inhibits synaptic transmission

Answer

Amplify synaptic transmission

Question 6

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

Accepted Answer

Myosin

Answer

Actin

Answer

Troponin

Answer

Tropomyosin

Question 7

The relaxation of smooth muscle is associated with a reduction in free intracellular calcium ion concentration. What is the effect of this reduction?

Accepted Answer

Decreased phosphorylation of myosin molecules

Answer

Reestablishment of inhibition of actin-myosin interaction

Answer

Deactivation of enzymatic activity of the individual actin molecules

Answer

Reduced contractile interaction by blocking the active sites of the myosin molecules

Question 8

Which of the following modalities is conducted fastest along nerve fibers?

Accepted Answer

Proprioception

Answer

Temperature sensation (cold)

Answer

Pain sensation

Answer

Pressure sensation

Question 9

What is the first change observed in the distal part of a cut nerve?

Accepted Answer

Axonal degeneration

Answer

Sprouting

Answer

Myelin degeneration

Answer

Schwann cell proliferation

Question 10

In a demyelinating disease (e.g., multiple sclerosis), symptoms worsen with a hot bath and improve with decreased temperature because:

Accepted Answer

Lower temperatures slow the gating kinetics of Na+ channels

Answer

Action potential duration increases with increased temperature

Answer

Activity of K+ channels is increased at lower temperatures

Answer

Inadequate recharging of the denuded axon at higher temperatures

Nerve and Muscle Physiology — MCQs

Nerve and Muscle Physiology — MCQs

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