Nerve and Muscle Physiology — MCQs

Nerve and Muscle Physiology Indian Medical PG Practice Questions and MCQs

Question 131: What structure facilitates the unidirectional flow of a nerve impulse?

A. Synapse (Correct Answer)
B. Axon
C. Dendrites
D. Node of Ranvier

Explanation: **Explanation:** The correct answer is **Synapse**. This phenomenon is known as the **Bell-Magendie Law** or the principle of **One-Way Conduction**. **Why Synapse is Correct:** In a chemical synapse, neurotransmitters are stored exclusively in the **presynaptic vesicles** and the specific receptors for these neurotransmitters are located on the **postsynaptic membrane**. Therefore, when an action potential reaches the axon terminal, the chemical messenger can only be released from the presynaptic side to act on the postsynaptic side. This structural asymmetry ensures that the nerve impulse travels in only one direction. **Why Other Options are Incorrect:** * **Axon:** While axons conduct impulses away from the cell body, they are theoretically capable of **bidirectional conduction** (orthodromic and antidromic) if stimulated experimentally in the middle of the fiber. * **Dendrites:** These are receptive structures that carry impulses toward the cell body. Like axons, they do not possess the "valve-like" mechanism of the synapse to enforce unidirectionality in a circuit. * **Node of Ranvier:** These are gaps in the myelin sheath that facilitate **Saltatory Conduction** (jumping of the impulse). Their primary function is to increase the velocity of nerve conduction, not to determine its direction. **High-Yield Clinical Pearls for NEET-PG:** * **Synaptic Delay:** The time required for neurotransmitter release and binding (approx. **0.5 msec**). It is the reason why reflex arcs with more synapses are slower. * **Synaptic Fatigue:** Repeated stimulation leads to the exhaustion of neurotransmitter stores in the presynaptic terminal, acting as a protective mechanism against excessive neuronal activity (e.g., terminating a seizure). * **Electrical Synapses:** Unlike chemical synapses, these occur via **gap junctions** and allow for faster, often bidirectional flow (found in cardiac muscle and some brain regions).

Question 132: The major cation directly involved in the interaction of actin and myosin in skeletal muscle is:

A. Ca++ (Correct Answer)
B. Na+
C. K+
D. Mg++

Explanation: **Explanation:** **1. Why Calcium (Ca++) is the Correct Answer:** In skeletal muscle, Calcium is the essential link between excitation and contraction (**Excitation-Contraction Coupling**). In a resting state, the binding sites on actin are covered by the **troponin-tropomyosin complex**. When an action potential reaches the muscle fiber, Ca++ is released from the **Sarcoplasmic Reticulum (SR)**. This Ca++ binds specifically to **Troponin C**, causing a conformational change that pulls tropomyosin away from the myosin-binding sites on the actin filament. This allows the myosin head to bind to actin, forming cross-bridges and initiating the power stroke. **2. Why Other Options are Incorrect:** * **Na+ (Sodium):** Primarily responsible for the **depolarization** phase of the action potential at the neuromuscular junction and along the sarcolemma, but it does not interact with contractile proteins. * **K+ (Potassium):** Responsible for the **repolarization** phase and maintaining the resting membrane potential. * **Mg++ (Magnesium):** Acts as a cofactor for ATPase activity and competes with Ca++ for binding sites. While it is necessary for ATP hydrolysis, it is not the "trigger" cation for actin-myosin interaction; in fact, low magnesium can lead to hyperexcitability (tetany). **3. Clinical Pearls & High-Yield Facts:** * **Calsequestrin:** The protein that buffers and stores Ca++ within the Sarcoplasmic Reticulum. * **Ryanodine Receptors (RyR1):** The calcium release channels in the SR. Mutations in these receptors lead to **Malignant Hyperthermia** (triggered by halothane/succinylcholine). * **SERCA Pump:** Responsible for the reuptake of Ca++ into the SR, which is required for muscle **relaxation**. * **Rigor Mortis:** Occurs due to the lack of ATP, which is needed to break the actin-myosin bond, not due to an excess of Calcium.

Question 133: Small packets of acetylcholine released randomly from the nerve cell membrane at rest produce what?

A. Inhibitory post synaptic potential
B. Miniature end plate potential (Correct Answer)
C. Action potential
D. End plate potential

Explanation: ### Explanation **Correct Answer: B. Miniature end plate potential (MEPP)** **Why it is correct:** At the neuromuscular junction (NMJ), even in the absence of an electrical stimulus, small quantities of acetylcholine (ACh) are released spontaneously from the presynaptic terminal. ACh is stored in vesicles called **quanta**, each containing approximately 5,000–10,000 molecules. The random fusion of a single vesicle with the nerve cell membrane releases one quantum of ACh, which binds to nicotinic receptors on the motor end plate. This results in a small, localized depolarization (typically ~0.5 mV) known as a **Miniature End Plate Potential (MEPP)**. MEPPs are sub-threshold and cannot trigger an action potential on their own. **Why the other options are incorrect:** * **A. Inhibitory Post Synaptic Potential (IPSP):** These are hyperpolarizing potentials (making the cell more negative) usually caused by GABA or Glycine. ACh at the NMJ is always excitatory. * **C. Action Potential:** An action potential is an "all-or-none" electrical impulse triggered only when the membrane potential reaches a specific threshold (approx. -55mV). A single MEPP is far too small to reach this threshold. * **D. End Plate Potential (EPP):** While an MEPP is the result of *one* quantum, an EPP is the result of the *simultaneous* release of many quanta (approx. 100-300) triggered by a nerve impulse (calcium influx). An EPP is a large depolarization that normally exceeds the threshold to trigger an action potential. **High-Yield NEET-PG Pearls:** * **Quantal Theory:** The EPP is the summation of multiple MEPPs. * **Calcium Dependency:** While the EPP is strictly dependent on extracellular $Ca^{2+}$ influx via voltage-gated channels, the spontaneous release of MEPPs is largely independent of nerve stimulation. * **Clinical Correlation:** In **Myasthenia Gravis**, the *amplitude* of the MEPP is reduced because of a decrease in functional ACh receptors, whereas in **Lambert-Eaton Syndrome**, the *number* of quanta released (EPP) is reduced due to antibodies against $Ca^{2+}$ channels.

Question 134: Receptors for the inverse stretch reflex are located in?

A. Desmosomes
B. Hemidesmosomes
C. Gap junctions
D. Golgi tendon organs (Correct Answer)

Explanation: ### Explanation The **Inverse Stretch Reflex** (also known as the autogenic inhibition reflex) is a protective mechanism that prevents muscle damage due to excessive tension. **Why Golgi Tendon Organs (GTO) are the correct answer:** The receptors for this reflex are the **Golgi Tendon Organs**, which are encapsulated sensory receptors located at the **musculo-tendinous junction**. Unlike muscle spindles (which sense changes in muscle *length*), GTOs are arranged in **series** with muscle fibers and are sensitive to changes in **muscle tension**. When a muscle undergoes vigorous contraction, the GTOs fire impulses via **Ib afferent nerve fibers**. these fibers synapse on inhibitory interneurons in the spinal cord, which then inhibit the alpha motor neurons of the same muscle, causing it to relax. **Analysis of Incorrect Options:** * **A & B (Desmosomes and Hemidesmosomes):** These are types of cell-to-cell and cell-to-matrix junctions (anchoring junctions) that provide mechanical stability to tissues like skin and cardiac muscle. They have no sensory or reflex function. * **C (Gap Junctions):** These are communicating junctions that allow the passage of ions and small molecules between cells (e.g., in electrical synapses or cardiac myocytes). They do not function as sensory receptors. **High-Yield Facts for NEET-PG:** * **Stretch Reflex:** Receptor is the **Muscle Spindle** (arranged in parallel); Afferent is **Ia**; Result is contraction. * **Inverse Stretch Reflex:** Receptor is the **GTO** (arranged in series); Afferent is **Ib**; Result is relaxation. * **Clasp-Knife Phenomenon:** This clinical sign seen in upper motor neuron lesions is mediated by the inverse stretch reflex. * **Function:** The primary role of the GTO is to equalize force among muscle fibers and prevent avulsion of the tendon from the bone.

Question 135: What is the characteristic of the skeletal muscle action potential?

A. It is not essential for contraction to occur.
B. It has a prolonged plateau phase.
C. It spreads inward to all parts of the muscle via the T-tubule system. (Correct Answer)
D. It begins with an inward movement of K+ ion.

Explanation: ### Explanation **1. Why Option C is Correct:** In skeletal muscle, the action potential (AP) initiated at the neuromuscular junction travels along the sarcolemma. However, because skeletal muscle fibers have a large diameter, surface depolarization alone cannot reach the deep-seated myofibrils. The **T-tubule system** (transverse tubules) acts as a continuous extension of the cell membrane, conducting the AP deep into the fiber. This triggers the release of $Ca^{2+}$ from the terminal cisternae of the sarcoplasmic reticulum (via DHP and Ryanodine receptors), a process known as **Excitation-Contraction (E-C) Coupling**. **2. Analysis of Incorrect Options:** * **Option A:** The AP is **essential** for contraction. It is the mandatory trigger that leads to the release of calcium required for cross-bridge cycling. * **Option B:** Skeletal muscle APs are "spike-like" and very brief (2-5 ms). A **prolonged plateau phase** is a characteristic of **cardiac ventricular muscle** (due to L-type $Ca^{2+}$ channels), which prevents tetanization. * **Option D:** Depolarization in skeletal muscle begins with an **inward movement of $Na^+$ ions** through voltage-gated sodium channels. $K^+$ movement is outward during the repolarization phase. **3. High-Yield Clinical Pearls for NEET-PG:** * **The Triad:** In skeletal muscle, a triad consists of one T-tubule and two terminal cisternae. It is located at the **A-I junction**. (In cardiac muscle, it is a *diad* located at the Z-line). * **L-type $Ca^{2+}$ Channels:** Also called Dihydropyridine (DHP) receptors; they act as voltage sensors in the T-tubule. * **Ryanodine Receptors (RyR1):** Located on the sarcoplasmic reticulum; they release $Ca^{2+}$ into the sarcoplasm. * **Malignant Hyperthermia:** Caused by a mutation in the RyR1 receptor, leading to excessive $Ca^{2+}$ release upon exposure to certain anesthetics (e.g., Halothane, Succinylcholine).

Question 136: Which of the following events accompanies the rapid voluntary flexion of the arm?

A. An increase in the activity of the Ia afferent fibers from the biceps (the agonist)
B. A decrease in the activity of the Ib afferent fibers from the biceps (the agonist)
C. An increase in the activity of the Ia afferent fibers from the triceps (the antagonist) (Correct Answer)
D. A decrease in the activity of the Ib afferent fibers from the triceps (the antagonist)

Explanation: ### Explanation The correct answer is **C: An increase in the activity of the Ia afferent fibers from the triceps (the antagonist).** #### 1. Why Option C is Correct During rapid voluntary flexion of the arm, the **biceps (agonist)** contracts while the **triceps (antagonist)** is passively stretched. * **Mechanism:** Muscle spindles are stretch receptors located within the muscle belly. When the triceps is stretched during flexion, the **Ia afferent fibers** (which wrap around the nuclear bag and chain fibers of the spindle) are stimulated. * This increase in firing rate is a direct response to the change in muscle length and the rate of stretch in the antagonist muscle. #### 2. Why Other Options are Incorrect * **Option A:** During voluntary contraction, **alpha-gamma co-activation** occurs. While gamma motor neurons prevent the spindle from going completely slack, the shortening of the biceps (agonist) generally leads to a **decrease** or stabilization in Ia firing compared to a stretched state. * **Option B:** **Ib afferents** originate from the **Golgi Tendon Organs (GTO)**, which sense muscle **tension**. During active contraction of the biceps, tension increases; therefore, Ib activity would **increase**, not decrease. * **Option D:** As the triceps is passively stretched, the tension on its tendon increases slightly. This would lead to an **increase** (or no change) in Ib activity, but certainly not a decrease. #### 3. High-Yield NEET-PG Pearls * **Ia Afferents:** Largest and fastest fibers; primary receptors for the **dynamic stretch reflex** (Monosynaptic). * **Ib Afferents:** Mediate the **inverse stretch reflex** (autogenic inhibition) via the Golgi Tendon Organ to prevent muscle damage from excessive tension. * **Alpha-Gamma Co-activation:** Essential during voluntary movement to maintain spindle sensitivity; without it, the spindle would become "unloaded" and silent during contraction. * **Reciprocal Inhibition:** When the agonist (biceps) is stimulated, the antagonist (triceps) is inhibited via inhibitory interneurons in the spinal cord.

Question 137: What is the smooth muscle counterpart of troponin?

A. Nebulin
B. Calsequestrin
C. Calmodulin (Correct Answer)
D. Titin

Explanation: ### Explanation In **skeletal and cardiac muscle**, contraction is regulated by the **troponin-tropomyosin complex**. When calcium enters the sarcoplasm, it binds to Troponin C, causing a conformational change that uncovers the actin-binding sites. However, **smooth muscle lacks troponin**. Instead, the regulatory protein is **Calmodulin**. When intracellular calcium levels rise, calcium binds to Calmodulin to form a **Calcium-Calmodulin complex**. This complex activates an enzyme called **Myosin Light Chain Kinase (MLCK)**, which phosphorylates the myosin head, allowing it to bind to actin and initiate contraction. **Analysis of Incorrect Options:** * **Nebulin:** A large protein in skeletal muscle that acts as a "molecular ruler," regulating the length of the thin (actin) filaments. * **Calsequestrin:** A calcium-binding protein located within the **Sarcoplasmic Reticulum (SR)**. It acts as a buffer, allowing large amounts of calcium to be stored at low free concentrations. * **Titin:** The largest known protein; it acts as a spring, connecting the Z-disk to the M-line in the sarcomere, providing passive elasticity to skeletal muscle. **High-Yield NEET-PG Pearls:** * **Contraction Mechanism:** Smooth muscle uses **thick-filament regulation** (via MLCK), whereas skeletal muscle uses **thin-filament regulation** (via Troponin). * **Relaxation:** In smooth muscle, relaxation requires **Myosin Light Chain Phosphatase (MLCP)** to dephosphorylate the myosin head. * **Latch-bridge Mechanism:** A unique feature of smooth muscle allowing it to maintain prolonged tension with minimal ATP consumption.

Question 138: Which of the following electrodes can be used to detect muscle activity without causing pain?

A. Surface electrode (Correct Answer)
B. Round electrode
C. Hook electrode
D. Needle electrode

Explanation: **Explanation:** The detection of muscle electrical activity (Electromyography or EMG) is achieved through two primary types of electrodes: **Surface** and **Needle**. **1. Why Surface Electrode is correct:** Surface electrodes are **non-invasive** discs (usually made of silver-silver chloride) placed directly on the skin over the muscle belly. They detect the summation of action potentials from many underlying motor units. Because they do not penetrate the skin or the muscle fascia, they are **painless** and ideal for kinesiologic studies or monitoring general muscle activity in pediatric patients. **2. Why the other options are incorrect:** * **Needle electrode:** These are **invasive** electrodes inserted directly into the muscle tissue. While they provide superior data regarding individual motor unit action potentials (MUAPs) and denervation potentials (like fibrillations), the insertion causes significant **pain and discomfort**. * **Hook electrode:** These are a type of fine-wire electrode where the tip is bent into a hook to remain anchored within the muscle. These are invasive and used primarily for deep muscles or during dynamic movement, causing pain upon insertion. * **Round electrode:** This is a generic descriptive term for the shape of an electrode rather than a functional category. While many surface electrodes are round, "Surface electrode" is the specific medical term used to denote the non-invasive, painless method. **Clinical Pearls for NEET-PG:** * **Surface EMG:** Best for assessing global muscle timing and "on/off" patterns; cannot isolate deep muscles or single motor units. * **Needle EMG:** The "Gold Standard" for diagnosing **Lower Motor Neuron (LMN)** lesions, myopathies, and neuropathies. * **Key Finding:** Spontaneous activity (fibrillations and positive sharp waves) on needle EMG is a hallmark of **active denervation**.

Question 139: Where does action potential generate from?

A. Cell body
B. Dendrites
C. Axon
D. Initial segment (Correct Answer)

Explanation: **Explanation:** The generation of an action potential (AP) is determined by the density of **voltage-gated sodium (Na+) channels**. The **Initial Segment** (the region between the axon hillock and the beginning of the myelin sheath) has the highest concentration of these channels. Consequently, this area has the lowest threshold for excitation, making it the site where the "all-or-none" electrical impulse is triggered. **Why the other options are incorrect:** * **Cell Body (Soma):** While the soma contains organelles and receives signals, its membrane has a relatively low density of voltage-gated Na+ channels compared to the initial segment. It primarily conducts graded potentials, not action potentials. * **Dendrites:** These are the primary receiving stations for synaptic input. They generate local, graded potentials (EPSPs and IPSPs) that decay over distance and time. In most neurons, dendrites lack the necessary channel density to initiate a self-propagating AP. * **Axon:** While the axon is responsible for the **propagation** and conduction of the action potential toward the nerve terminal, it is not the site of **origin**. The impulse must first be generated at the initial segment before traveling down the axon. **High-Yield NEET-PG Pearls:** * **Axon Hillock vs. Initial Segment:** While often used interchangeably in casual study, the *Initial Segment* is the precise physiological "trigger zone." The Axon Hillock is the anatomical funnel-shaped region of the soma leading to it. * **Threshold:** The threshold at the initial segment is approximately **-35 to -40 mV**, whereas the soma threshold is much higher (around -10 to -20 mV). * **Nodes of Ranvier:** In myelinated axons, these are the sites of AP regeneration (saltatory conduction) due to high Na+ channel density, but the very first AP still starts at the initial segment.

Question 140: What is the highest equilibrium potential among the following ions?

A. Sodium (Na+)
B. Potassium (K+)
C. Chloride (Cl-)
D. Calcium (Ca++) (Correct Answer)

Explanation: ### Explanation The equilibrium potential ($E_{ion}$) of an ion is the membrane potential at which the electrical gradient exactly balances the chemical concentration gradient, resulting in no net movement of that ion. This is calculated using the **Nernst Equation**. **1. Why Calcium (Ca++) is Correct:** Calcium has the highest concentration gradient across the cell membrane. Extracellular concentration is approximately **1-2 mmol/L**, while intracellular concentration is extremely low (**~0.0001 mmol/L**). Because the gradient is so steep (10,000-fold), a very high positive electrical charge is required inside the cell to repel the influx of $Ca^{++}$. Its equilibrium potential is approximately **+120 to +130 mV**, the highest among all physiological ions. **2. Why the Other Options are Incorrect:** * **Sodium (Na+):** Sodium is higher extracellularly (142 mEq/L) than intracellularly (14 mEq/L). Its equilibrium potential is approximately **+60 to +65 mV**. While positive, it is significantly lower than Calcium. * **Potassium (K+):** Potassium is the primary intracellular cation. Its equilibrium potential is negative (approximately **-90 to -94 mV**) because a negative internal charge is needed to keep $K^+$ from leaking out. * **Chloride (Cl-):** Chloride's equilibrium potential is usually close to the Resting Membrane Potential (RMP), approximately **-70 to -85 mV**. **3. NEET-PG High-Yield Pearls:** * **Resting Membrane Potential (RMP):** In neurons, it is -70 mV; in skeletal muscle, it is -90 mV. * **Determinant of RMP:** The RMP is closest to the equilibrium potential of the ion with the highest permeability (Potassium). * **Nernst Equation Simplified:** $E = 61 \times \log([Ion]_{out} / [Ion]_{in})$ for monovalent cations at body temperature. * **Goldman-Hodgkin-Katz Equation:** Used to calculate the membrane potential when multiple ions are permeable.

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

Practice Questions

Neurotransmitters and Receptors

Practice Questions

Electrophysiological Measurements

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