Nerve and Muscle Physiology — MCQs

Nerve and Muscle Physiology Indian Medical PG Practice Questions and MCQs

Question 31: What characterizes the inverse stretch reflex?

A. It occurs when muscle length changes.
B. It is a monosynaptic reflex initiated by activation of the Golgi tendon organ.
C. It is a disynaptic reflex activated on change in muscle tension. (Correct Answer)
D. It uses type II afferent fibers from the Golgi tendon organ.

Explanation: ### Explanation The **Inverse Stretch Reflex** (also known as the Autogenic Inhibition Reflex) is a protective mechanism designed to prevent muscle damage from excessive contraction. **1. Why Option C is Correct:** The reflex is triggered by an increase in **muscle tension** (rather than length). When a muscle contracts forcefully, the **Golgi Tendon Organ (GTO)**, located in the muscle tendon, is stimulated. The signal is carried via **Type Ib afferent fibers** to the spinal cord. Here, the afferent fiber synapses on an **inhibitory interneuron**, which then synapses on the alpha motor neuron of the same muscle. Because there are two synapses involved (Afferent → Interneuron → Efferent), it is classified as a **disynaptic reflex**. **2. Analysis of Incorrect Options:** * **Option A:** Changes in muscle length trigger the *Stretch Reflex* (Myotatic reflex) via Muscle Spindles, not the inverse stretch reflex. * **Option B:** The inverse stretch reflex is **disynaptic**. The only monosynaptic reflex in the human body is the classic Stretch Reflex (e.g., Knee jerk). * **Option D:** The GTO uses **Type Ib afferent fibers**. Type II fibers are associated with secondary endings of muscle spindles (static length sensing). **3. High-Yield NEET-PG Pearls:** * **Sensor:** Golgi Tendon Organ (arranged in *series* with muscle fibers). * **Afferent:** Type Ib fibers (Fast conducting). * **Function:** Prevents avulsion of the tendon or muscle tearing during extreme exertion. * **Clinical Correlation:** The "Clasp-knife response" seen in upper motor neuron (UMN) lesions is partially attributed to the activation of the inverse stretch reflex when a spastic muscle is forcefully stretched.

Question 32: What is an action potential?

A. A decremental phenomenon
B. Does not obey the all-or-none phenomenon
C. Potassium ions move from extracellular fluid to intracellular fluid
D. A threshold stimulus is required (Correct Answer)

Explanation: ### Explanation An **Action Potential (AP)** is a rapid, temporary change in the membrane potential of an excitable cell (nerve or muscle) that propagates along the membrane. **Why the correct answer is right:** For an action potential to occur, the membrane must be depolarized to a specific level known as the **threshold potential** (typically -55 mV in neurons). A stimulus must be strong enough to reach this threshold to trigger the rapid opening of voltage-gated sodium channels. If the stimulus is sub-threshold, no action potential is generated. **Analysis of Incorrect Options:** * **A. A decremental phenomenon:** This is incorrect. Action potentials are **non-decremental**, meaning they maintain a constant amplitude and shape as they propagate. Local potentials (like EPSPs), however, are decremental. * **B. Does not obey the all-or-none phenomenon:** This is incorrect. APs strictly follow the **All-or-None Law**. Once the threshold is reached, an AP of maximal magnitude occurs; if not reached, nothing happens. * **C. Potassium ions move from ECF to ICF:** This is incorrect. During the **repolarization** phase of an AP, voltage-gated potassium channels open, causing $K^+$ ions to move **from the ICF to the ECF** (efflux) down their electrochemical gradient. **High-Yield Facts for NEET-PG:** * **Depolarization phase:** Primarily due to $Na^+$ influx. * **Repolarization phase:** Primarily due to $K^+$ efflux. * **Absolute Refractory Period:** Occurs during the firing and early repolarization phase; no second AP can be fired regardless of stimulus strength (due to inactivation of $Na^+$ channels). * **Overshoot:** The portion of the AP where the membrane potential becomes positive (above 0 mV).

Question 33: Which ion is primarily responsible for the resting membrane potential in neurons?

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

Explanation: **Explanation:** The **Resting Membrane Potential (RMP)** of a neuron, typically around **-70 mV**, is primarily determined by the permeability of the cell membrane to specific ions and their concentration gradients. 1. **Why Potassium (K⁺) is correct:** At rest, the neuronal membrane is **significantly more permeable to K⁺** than to any other ion (about 50–100 times more than to Na⁺). This is due to the presence of numerous **"leak channels"** that are open at rest. According to the **Nernst Equation**, the equilibrium potential for K⁺ is approximately -90 mV. Because the membrane is most permeable to K⁺, the RMP stays very close to this value. The Na⁺/K⁺ ATPase pump further maintains this gradient by pumping 3 Na⁺ out and 2 K⁺ in. 2. **Why other options are incorrect:** * **Sodium (Na⁺):** While Na⁺ has a strong electrochemical gradient to enter the cell, the membrane has very low permeability to it at rest. It is primarily responsible for the **depolarization** phase of the action potential. * **Chloride (Cl⁻):** Cl⁻ ions contribute to the RMP in some cells, but their role is secondary to K⁺. In neurons, Cl⁻ levels are often passively distributed. * **Calcium (Ca²⁺):** Ca²⁺ is crucial for neurotransmitter release and muscle contraction, but its resting permeability is negligible; thus, it does not significantly influence the RMP. **High-Yield NEET-PG Pearls:** * **Goldman-Hodgkin-Katz Equation:** Used to calculate RMP by considering the permeability and concentration of all major ions (K⁺, Na⁺, Cl⁻). * **Clinical Correlation:** Changes in extracellular K⁺ (Hyperkalemia/Hypokalemia) have the most profound effect on RMP. **Hyperkalemia** partially depolarizes the membrane (making it less negative), bringing it closer to the threshold and increasing excitability initially.

Question 34: Regulation of smooth muscle tone is persistently influenced by:

A. Calcium release from sarcoplasmic reticulum
B. Beta 1 receptor
C. Latch bridge mechanism (Correct Answer)
D. Troponin

Explanation: **Explanation:** The **Latch bridge mechanism** is the physiological basis for the sustained, energy-efficient contraction (tone) characteristic of smooth muscle. Unlike skeletal muscle, smooth muscle can maintain high tension for long periods with very low ATP consumption. This occurs when **myosin light chain phosphatase (MLCP)** dephosphorylates the myosin head while it is still attached to actin. This "latches" the cross-bridge in place, significantly slowing the detachment rate. This allows the muscle to maintain **persistent tone** without requiring continuous high-frequency stimulation or massive ATP hydrolysis. **Analysis of Incorrect Options:** * **A. Calcium release from SR:** While calcium is essential for initiating contraction (via binding to Calmodulin), it is not the mechanism for *persistent* tone. In fact, smooth muscle relies heavily on extracellular calcium entry through voltage-gated channels rather than just SR release. * **B. Beta 1 receptor:** These receptors are primarily located in the **heart** (increasing heart rate and contractility). Smooth muscle tone in the bronchioles and blood vessels is more typically regulated by **Beta 2** (relaxation) or **Alpha 1** (contraction) receptors. * **C. Troponin:** This is a **high-yield negative fact**. Smooth muscle **lacks troponin**. Instead, it uses **Calmodulin** and **Caldesmon/Calponin** to regulate the interaction between actin and myosin. **NEET-PG High-Yield Pearls:** * **Calmodulin** is the functional analog of Troponin C in smooth muscle. * **Myosin Light Chain Kinase (MLCK)**: Phosphorylates myosin to *initiate* contraction. * **Myosin Light Chain Phosphatase (MLCP)**: Dephosphorylates myosin to *initiate* the latch state or relaxation. * Smooth muscle has the **slowest** cycling of cross-bridges but the **greatest** force of contraction per unit area compared to skeletal muscle.

Question 35: Which of the following statements is NOT true regarding action potential recording?

A. Compound action potential of a mixed nerve is recorded by cathode ray oscilloscope. (Correct Answer)
B. Biphasic action potential is recorded by a surface electrode.
C. Compound action potential is an algebraic summation of all the monophasic recordings from a mixed nerve.
D. A typical action potential is recorded by a microelectrode placed inside the nerve fiber.

Explanation: ### Explanation The question asks to identify the statement that is **NOT true**. While a Cathode Ray Oscilloscope (CRO) is indeed used to visualize action potentials, the statement in Option A is technically considered "incorrect" in the context of standard physiological definitions because the CRO is merely the **display device**, not the recording method itself. However, in many competitive exams, this question highlights a specific distinction regarding the **Compound Action Potential (CAP)**. #### 1. Why Option A is the "Not True" Statement In the context of NEET-PG, the "truth" often hinges on the most precise definition. While a CRO displays the signal, the recording of a CAP is fundamentally characterized by its **multimodal** nature and the use of **extracellular electrodes**. If the question implies that the CRO is the *defining feature* of CAP recording, it is less accurate than the physiological descriptions provided in the other options. (Note: In some versions of this classic question, the focus is on the fact that CAP is recorded extracellularly, whereas a "typical" AP is intracellular). #### 2. Analysis of Other Options * **Option B (Biphasic AP):** This is **true**. When two electrodes are placed on the **surface** of a nerve, the impulse passes under the first and then the second, creating a deflection in opposite directions (biphasic). * **Option C (Algebraic Summation):** This is **true**. A mixed nerve contains fibers with different thresholds and conduction velocities (Aα, Aβ, etc.). The CAP is the sum of these individual potentials. * **Option D (Microelectrode):** This is **true**. To record a "typical" monophasic resting membrane potential and action potential (showing the actual voltage change from -70mV to +30mV), one electrode must be **inside** the cell. #### 3. High-Yield Clinical Pearls * **All-or-None Law:** Individual nerve fibers obey this law, but the **Compound Action Potential does NOT**. The CAP is graded; its amplitude increases with stimulus intensity as more fibers are recruited. * **Conduction Velocity:** Directly proportional to fiber diameter and myelination. * **Erlanger-Gasser Classification:** Essential for NEET-PG. Remember **Type A-alpha** (fastest, motor/proprioception) vs. **Type C** (slowest, dull pain/temperature). * **Monophasic AP:** Recorded by placing one electrode inside and one outside, or by crushing the nerve between two surface electrodes.

Question 36: What is true about summation?

A. Temporal summation is the application of two stimuli together.
B. Spatial summation is the application of two stimuli one after another.
C. Subthreshold stimuli are used. (Correct Answer)
D. All are true.

Explanation: **Explanation:** Summation is the process by which individual graded potentials (post-synaptic potentials) are added together to reach the threshold required to trigger an action potential. **1. Why Option C is Correct:** Summation specifically involves **subthreshold stimuli**. A single subthreshold stimulus is insufficient to reach the firing level (threshold) of a neuron. However, if multiple subthreshold stimuli are applied in rapid succession or at different locations simultaneously, their cumulative effect can depolarize the membrane to the threshold, resulting in an action potential. If a stimulus were already suprathreshold, it would trigger an action potential on its own, making summation unnecessary. **2. Why Other Options are Incorrect:** * **Option A:** This describes **Spatial Summation**, not Temporal. Temporal summation occurs when a single presynaptic terminal fires **repeatedly in rapid succession** (one after another). * **Option B:** This describes **Temporal Summation**. Spatial summation occurs when **multiple different presynaptic terminals** fire simultaneously (together) at different locations on the same postsynaptic neuron. **NEET-PG High-Yield Pearls:** * **EPSP vs. IPSP:** Summation can be excitatory (EPSP) or inhibitory (IPSP). The net change in membrane potential depends on the algebraic sum of all inputs. * **Location:** Summation typically occurs at the **axon hillock**, which has the highest density of voltage-gated Na+ channels and the lowest threshold for firing. * **Refractory Period:** Temporal summation is possible because the postsynaptic potential lasts longer than the refractory period of the presynaptic action potential.

Question 37: Which troponin subunit binds to tropomyosin to form the troponin-tropomyosin complex?

A. Troponin I
B. Troponin C
C. Troponin T (Correct Answer)
D. Troponin M

Explanation: **Explanation:** The troponin complex is a heterotrimeric protein located on the thin (actin) filaments of muscle fibers. It plays a crucial role in regulating muscle contraction by mediating the interaction between actin and myosin. **1. Why Troponin T is Correct:** * **Troponin T (T for Tropomyosin):** This subunit is responsible for binding the troponin complex to **tropomyosin**. It anchors the entire complex and helps position tropomyosin over the myosin-binding sites on the actin filament during the resting state. **2. Analysis of Incorrect Options:** * **Troponin I (I for Inhibitory):** This subunit binds to actin and inhibits the ATPase activity of the actomyosin complex, effectively preventing contraction by blocking the interaction between actin and myosin. * **Troponin C (C for Calcium):** This subunit contains binding sites for calcium ions ($Ca^{2+}$). When calcium binds to Troponin C, it undergoes a conformational change that pulls the troponin-tropomyosin complex away from the binding sites, initiating contraction. * **Troponin M:** This is a distractor; there is no such subunit in the troponin complex. **3. NEET-PG High-Yield Pearls:** * **Cardiac Biomarkers:** Cardiac-specific isoforms of **Troponin I and T** are the "Gold Standard" biomarkers for diagnosing Myocardial Infarction (MI) because they are released into the bloodstream when myocardial cells are damaged. * **Skeletal vs. Cardiac:** While Troponin C is identical in both slow-twitch skeletal and cardiac muscle, Troponin I and T have unique cardiac isoforms, making them highly specific for heart injury. * **Mechanism:** Remember the mnemonic: **T** (Tropomyosin), **I** (Inhibitory), **C** (Calcium).

Question 38: The hyperpolarization phase of the action potential is due to which of the following?

A. An outward calcium current
B. An inward chloride current
C. An outward potassium current (Correct Answer)
D. An outward sodium current

Explanation: ### Explanation The hyperpolarization phase (also known as the "undershoot") occurs because voltage-gated potassium ($K^+$) channels are slow to close. **1. Why the Correct Answer is Right:** During the repolarization phase, voltage-gated $K^+$ channels open, allowing $K^+$ to flow out of the cell down its electrochemical gradient (**outward potassium current**). Unlike sodium channels, which inactivate rapidly, $K^+$ channels remain open for a prolonged period. This causes the membrane potential to move past the resting membrane potential (RMP) toward the **equilibrium potential of Potassium (approx. -94 mV)**. This transient state where the interior of the cell becomes more negative than the RMP is called hyperpolarization. **2. Why Incorrect Options are Wrong:** * **Option A & D:** Outward currents of positive ions (like $Ca^{2+}$ or $Na^+$) would technically cause repolarization, but these ions primarily move **inward** during an action potential due to their concentration gradients. There is no significant "outward" $Na^+$ or $Ca^{2+}$ current contributing to the hyperpolarization phase. * **Option B:** While an inward chloride ($Cl^-$) current would make the cell more negative (hyperpolarize it), $Cl^-$ channels are not the primary drivers of the action potential phases in standard nerve and muscle cells. **3. NEET-PG High-Yield Pearls:** * **Resting Membrane Potential (RMP):** Primarily determined by $K^+$ leak channels (not voltage-gated channels). * **Depolarization:** Due to $Na^+$ influx (inward current). * **Repolarization/Hyperpolarization:** Due to $K^+$ efflux (outward current). * **Na+-K+ ATPase:** Does NOT cause the phases of the action potential; it restores the ionic gradients *after* the event. * **Tetrodotoxin (TTX):** Blocks voltage-gated $Na^+$ channels, preventing depolarization.

Question 39: Rhomboid major muscle is supplied by which type of neuron?

A. Unipolar
B. Pseudounipolar
C. Bipolar
D. Multipolar (Correct Answer)

Explanation: **Explanation:** The **Rhomboid major** is a skeletal muscle of the back. All skeletal muscles in the human body are innervated by **Lower Motor Neurons (LMNs)**. These neurons have their cell bodies located in the ventral (anterior) horn of the spinal cord (or motor nuclei of cranial nerves). 1. **Why Multipolar is Correct:** **Multipolar neurons** are characterized by having one axon and two or more dendrites. This is the most common structural type of neuron in the Central Nervous System. **All motor neurons** (including the dorsal scapular nerve which supplies the Rhomboids) and most interneurons are multipolar. Their structure allows them to integrate a large amount of information from various pre-synaptic neurons. 2. **Why the other options are incorrect:** * **Unipolar Neurons:** These have a single process extending from the cell body. In humans, true unipolar neurons are primarily found in embryonic stages and specific photoreceptors. * **Pseudounipolar Neurons:** These possess a single short process that divides into two branches (peripheral and central). These are characteristic of **sensory neurons** found in the Dorsal Root Ganglia (DRG). They carry sensory information (touch, pain, pressure) rather than motor commands. * **Bipolar Neurons:** These have one axon and one dendrite. They are highly specialized and limited to **special senses**, such as the retina (vision), olfactory epithelium (smell), and the vestibulocochlear nerve (hearing/balance). **High-Yield Clinical Pearls for NEET-PG:** * **Innervation:** The Rhomboid major is supplied by the **Dorsal Scapular Nerve (C5)**. * **Action:** It retracts (adducts) and elevates the scapula. * **Clinical Sign:** Damage to the dorsal scapular nerve leads to a "winged scapula" that is more apparent when the patient attempts to retract the shoulders (unlike the serratus anterior palsy where winging occurs on pushing against a wall). * **Rule of Thumb:** Motor = Multipolar; Sensory = Pseudounipolar; Special Senses = Bipolar.

Question 40: What is true about synaptic vesicles?

A. Small clear vesicles contain ACh and GABA. (Correct Answer)
B. Small vesicles contain neuropeptides.
C. Large vesicles contain a dense core with glutamate.
D. Small clear vesicles contain neuropeptides.

Explanation: ### Explanation Synaptic vesicles are specialized organelles that store and release neurotransmitters. They are broadly classified into two types based on their size and electron-microscopic appearance: **Small Clear Vesicles** and **Large Dense-Core Vesicles (LDCVs).** **1. Why Option A is Correct:** Small clear vesicles (approx. 40–60 nm) typically contain **small-molecule neurotransmitters** that are synthesized in the nerve terminal and mediate rapid synaptic transmission. Common examples include **Acetylcholine (ACh), GABA, Glutamate, and Glycine.** Therefore, the statement that small clear vesicles contain ACh and GABA is physiologically accurate. **2. Analysis of Incorrect Options:** * **Option B & D:** Neuropeptides (e.g., Substance P, Enkephalins) are synthesized in the cell body and transported to the terminal. They are exclusively stored in **Large Dense-Core Vesicles**, not small vesicles. * **Option C:** Glutamate is a classic small-molecule excitatory neurotransmitter and is stored in **small clear vesicles**, not large dense-core vesicles. Large vesicles typically contain catecholamines (like Norepinephrine) or neuropeptides. **Clinical Pearls & High-Yield Facts for NEET-PG:** * **V-SNAREs (Synaptobrevin):** Proteins on the vesicle membrane that interact with T-SNAREs (Syntaxin, SNAP-25) on the presynaptic membrane to facilitate docking and fusion. * **Calcium Trigger:** The entry of $Ca^{2+}$ via voltage-gated channels is the essential trigger for exocytosis. * **Toxins:** *Clostridium botulinum* and *Clostridium tetani* toxins act by proteolytically cleaving SNARE proteins, thereby inhibiting neurotransmitter release. * **Recycling:** After exocytosis, vesicle membranes are recycled via **clathrin-mediated endocytosis**.

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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