Nerve and Muscle Physiology — MCQs

Nerve and Muscle Physiology Indian Medical PG Practice Questions and MCQs

Question 41: Twitch of a single motor unit is called:

A. Myoclonic jerk
B. Fasciculation (Correct Answer)
C. Tremor
D. Chorea

Explanation: ### Explanation **Correct Answer: B. Fasciculation** **Understanding the Concept:** A **fasciculation** is defined as the spontaneous, involuntary contraction of a **single motor unit** (one lower motor neuron and all the muscle fibers it innervates). Because these contractions involve an entire motor unit, they are often visible under the skin as small flickers or ripples but are usually insufficient to move a joint. In clinical practice, fasciculations are a hallmark sign of **Lower Motor Neuron (LMN) lesions**, such as Amyotrophic Lateral Sclerosis (ALS) or poliomyelitis, though they can occur benignly (e.g., due to caffeine or stress). **Analysis of Incorrect Options:** * **A. Myoclonic jerk:** This is a sudden, brief, shock-like contraction of a **muscle or group of muscles**. It originates in the central nervous system (cortex, brainstem, or spinal cord) rather than a single peripheral motor unit. * **C. Tremor:** This is a **rhythmic, oscillatory movement** produced by alternating or synchronous contractions of antagonistic muscles. It is not a single twitch but a continuous purposeless movement. * **D. Chorea:** This refers to **brief, semi-purposeful, irregular, and "dance-like" involuntary movements**. It involves multiple muscle groups and is typically associated with Basal Ganglia disorders (e.g., Huntington’s disease). **High-Yield Clinical Pearls for NEET-PG:** * **Fibrillation vs. Fasciculation:** While a fasciculation is the contraction of a *motor unit* (visible), a **fibrillation** is the spontaneous contraction of a *single muscle fiber* (not visible to the naked eye; detected only on EMG). * **LMN Lesion Signs:** Fasciculations, fibrillations, hypotonia, hyporeflexia, and significant muscle atrophy. * **Benign Fasciculations:** Most commonly seen in the orbicularis oculi (eyelid twitching).

Question 42: NN Nicotinic acetylcholine receptors are present on which of the following structures?

A. Myocardium
B. Motor endplates
C. Presynaptic glutamate-secreting axon terminals (Correct Answer)
D. Neuromuscular junction

Explanation: **Explanation:** Nicotinic acetylcholine receptors (nAChRs) are ligand-gated ion channels categorized into two main subtypes: **$N_M$ (Muscle-type)** and **$N_N$ (Neuronal-type)**. **Why Option C is Correct:** $N_N$ receptors are primarily found in the central nervous system (CNS) and autonomic ganglia. In the CNS, they are frequently located on **presynaptic terminals** of non-cholinergic neurons (such as glutamate-secreting axons). When acetylcholine binds to these presynaptic $N_N$ receptors, it triggers an influx of $Ca^{2+}$, which facilitates the release of other neurotransmitters like glutamate, dopamine, or norepinephrine. This is a key mechanism of neuromodulation in the brain. **Analysis of Incorrect Options:** * **Options B & D (Motor endplates / Neuromuscular junction):** These contain **$N_M$ receptors**, not $N_N$. While both are nicotinic, $N_M$ receptors have a different subunit composition ($\alpha_1, \beta_1, \delta, \epsilon/\gamma$) and are specifically targeted by muscle relaxants like succinylcholine or vecuronium. * **Option A (Myocardium):** The heart contains **Muscarinic ($M_2$) receptors**, which are G-protein coupled receptors (GPCRs), not nicotinic ion channels. Activation of $M_2$ receptors leads to a decrease in heart rate (negative chronotropy). **High-Yield Facts for NEET-PG:** * **$N_N$ Location:** Autonomic ganglia (both sympathetic and parasympathetic), adrenal medulla, and CNS. * **$N_N$ Blockers:** Hexamethonium and Trimethaphan (Ganglionic blockers). * **$N_M$ Blockers:** d-Tubocurarine (Competitive) and Succinylcholine (Depolarizing). * **Key Difference:** $N_N$ receptors are primarily involved in ganglionic transmission and neurotransmitter modulation, whereas $N_M$ receptors mediate skeletal muscle contraction.

Question 43: Which of the following is a true statement about smooth muscle?

A. Smooth muscle cells have multiple nuclei.
B. Smooth muscle cells have striations.
C. Sarcomeres are absent in smooth muscle. (Correct Answer)
D. Cross bridges are formed in smooth muscle during contraction.

Explanation: ### Explanation **Correct Answer: C. Sarcomeres are absent in smooth muscle.** Smooth muscle is characterized by its lack of a highly organized internal structure compared to skeletal or cardiac muscle. While it contains actin (thin) and myosin (thick) filaments, these are not arranged in the repetitive, linear units known as **sarcomeres**. Instead, actin filaments are attached to **dense bodies** (analogous to Z-discs), which are scattered throughout the cytoplasm and attached to the cell membrane. This non-linear arrangement allows smooth muscle to contract in multiple directions and maintain tension over a wide range of lengths (the plasticity effect). **Analysis of Incorrect Options:** * **A. Smooth muscle cells have multiple nuclei:** Incorrect. Smooth muscle cells are **uninucleated** (single, centrally located nucleus). Skeletal muscle is multinucleated. * **B. Smooth muscle cells have striations:** Incorrect. Striations are the visual result of organized sarcomeres. Since smooth muscle lacks sarcomeres, it appears "smooth" or non-striated under a microscope. * **D. Cross bridges are formed in smooth muscle during contraction:** This is actually a **true physiological statement**; however, in the context of standard medical examinations, Option C is the defining structural hallmark of smooth muscle. *Note: If this were a "Multiple True" type question, D would be correct as smooth muscle uses the sliding filament mechanism via myosin cross-bridges.* **High-Yield NEET-PG Pearls:** * **Calmodulin:** Smooth muscle lacks Troponin. Calcium binds to Calmodulin to activate Myosin Light Chain Kinase (MLCK). * **Latch-bridge mechanism:** Allows smooth muscle to maintain prolonged contraction with very little ATP consumption (essential for vascular tone). * **Caveolae:** These are small invaginations of the sarcolemma that act like a rudimentary T-tubule system. * **Unitary vs. Multi-unit:** Unitary (visceral) smooth muscle has **gap junctions** and acts as a syncytium (e.g., GI tract, uterus). Multi-unit acts independently (e.g., Iris, ciliary body).

Question 44: If the refractory period of a nerve is 1/2500th of a second, what is the maximum frequency of excitation of the nerve?

A. 100 times/sec
B. 250 times/sec
C. 2000 times/sec
D. 2500 times/sec (Correct Answer)

Explanation: ### Explanation **1. Understanding the Core Concept** The **Refractory Period** is the time interval following an action potential during which a nerve fiber is either unable to respond to a second stimulus (Absolute Refractory Period) or requires a stronger-than-normal stimulus (Relative Refractory Period). The **maximum frequency of excitation** (the number of impulses a nerve can transmit per second) is mathematically determined by the duration of the refractory period. Since a nerve cannot fire another action potential until the refractory period is over, the formula is: > **Maximum Frequency = 1 / Refractory Period (in seconds)** In this question: * Refractory Period = 1/2500 seconds * Frequency = 1 / (1/2500) = **2500 times/sec (or 2500 Hz)** **2. Analysis of Incorrect Options** * **Option A (100 times/sec) & B (250 times/sec):** These values are far below the physiological limit dictated by a 1/2500s refractory period. Such frequencies would be seen in nerves with much longer refractory periods (e.g., 10ms or 4ms). * **Option C (2000 times/sec):** This is a mathematical distractor. While high, it does not utilize the full capacity of the nerve's recovery time as defined by the provided value. **3. Clinical Pearls & High-Yield Facts for NEET-PG** * **Absolute Refractory Period (ARP):** Corresponds to the period from the firing level until approximately one-third of repolarization is complete. It is due to the **inactivation of Voltage-Gated Na+ channels**. * **Relative Refractory Period (RRP):** Corresponds to the period from the end of ARP to the start of after-hyperpolarization. It is due to **continued outward K+ flow**. * **Cardiac Muscle vs. Nerve:** The refractory period of skeletal muscle is short (~5ms), but in cardiac muscle, it is very long (~250-300ms). This long refractory period in the heart prevents **tetanization**, ensuring the heart relaxes to fill with blood. * **Accommodation:** If a nerve is subjected to a slowly increasing constant current, the threshold for firing rises; this is known as accommodation.

Question 45: In myasthenia gravis, antibodies are formed against which of the following?

A. Acetylcholine receptor (Correct Answer)
B. Calcium channel
C. Potassium channel
D. Histamine H2 receptor

Explanation: **Explanation:** **Myasthenia Gravis (MG)** is an autoimmune neuromuscular junction (NMJ) disorder characterized by muscle weakness and fatigability. The primary pathology involves the production of autoantibodies against the **post-synaptic Nicotinic Acetylcholine Receptors (nAChR)** at the motor endplate. These antibodies lead to receptor degradation, complement-mediated damage, and competitive inhibition, resulting in reduced end-plate potentials and failure of muscle fiber depolarization. **Analysis of Options:** * **A. Acetylcholine receptor (Correct):** As stated, IgG antibodies against nAChR are found in ~85% of cases. (Note: Antibodies against MuSK or LRP4 may be present in seronegative cases). * **B. Calcium channel:** Antibodies against **P/Q-type voltage-gated calcium channels (VGCC)** are characteristic of **Lambert-Eaton Myasthenic Syndrome (LEMS)**, a pre-synaptic disorder often associated with small cell lung cancer. * **C. Potassium channel:** Antibodies against voltage-gated potassium channels (VGKC) are associated with **Neuromyotonia (Isaac’s Syndrome)** or limbic encephalitis, not MG. * **D. Histamine H2 receptor:** These receptors are primarily involved in gastric acid secretion and have no role in the pathogenesis of neuromuscular disorders. **High-Yield Clinical Pearls for NEET-PG:** * **Clinical Presentation:** Ptosis and diplopia (most common initial symptoms), "Cogan’s lid twitch," and weakness that worsens with activity (fatigability). * **Associated Pathology:** 75% of MG patients have **thymic abnormalities** (65% hyperplasia, 10% thymoma). * **Diagnosis:** Ice pack test (improves ptosis), Edrophonium (Tensilon) test, and repetitive nerve stimulation (shows a **decremental response**). * **Treatment:** Pyridostigmine (AChE inhibitor) is the first-line symptomatic treatment.

Question 46: Which of the following statements is NOT true regarding Type 1 muscle fibers?

A. High glycolytic capacity (Correct Answer)
B. High capillary density
C. High number of mitochondria
D. High myoglobin content

Explanation: **Explanation:** Type 1 muscle fibers, also known as **Slow-Twitch (Red) fibers**, are specialized for endurance and aerobic metabolism. **1. Why Option A is the Correct Answer (NOT true):** Type 1 fibers have a **low glycolytic capacity**. They rely primarily on oxidative phosphorylation (aerobic metabolism) rather than glycolysis (anaerobic metabolism) to generate ATP. High glycolytic capacity is a hallmark of **Type 2 (Fast-Twitch/White) fibers**, which are designed for short bursts of powerful activity and fatigue quickly. **2. Why the other options are incorrect (True for Type 1):** * **High capillary density (Option B):** To support continuous aerobic metabolism, Type 1 fibers require a rich blood supply for the steady delivery of oxygen and nutrients. * **High number of mitochondria (Option C):** Mitochondria are the sites of the Krebs cycle and the electron transport chain; their high density in Type 1 fibers facilitates efficient ATP production over long periods. * **High myoglobin content (Option D):** Myoglobin is an iron-binding protein that stores oxygen within the muscle. Its high concentration gives these fibers their characteristic red color and ensures oxygen availability during sustained contraction. **High-Yield NEET-PG Pearls:** * **Mnemonic:** **"One Slow Red Ox"** (Type **1**, **Slow**-twitch, **Red** color, **Ox**idative metabolism). * **Fatigability:** Type 1 fibers are highly resistant to fatigue (e.g., soleus, back muscles for posture), whereas Type 2B fibers fatigue rapidly. * **ATPase Activity:** Type 1 fibers have low myosin ATPase activity, leading to a slower velocity of contraction compared to Type 2. * **Lipid Content:** Type 1 fibers have higher triglyceride stores, while Type 2 fibers have higher glycogen stores.

Question 47: Which of the following events happens when a nerve impulse arrives at the Neuromuscular Junction?

A. Release of Calcium from the Sarcoplasmic Reticulum
B. Acetylcholine binds to receptors on the postsynaptic sarcolemmal membrane (Correct Answer)
C. Acetylcholine binds to receptors on the postsynaptic sarcolemmal membrane
D. Efflux of calcium through synaptic vesicles into the synaptic cleft

Explanation: ### Explanation The neuromuscular junction (NMJ) is a specialized chemical synapse where a motor neuron communicates with a muscle fiber. The sequence of events is highly specific and a frequent target for NEET-PG questions. **Why Option B/C is Correct:** When an action potential reaches the presynaptic terminal, it triggers the opening of voltage-gated calcium channels. **Calcium influx** into the neuron causes synaptic vesicles to fuse with the membrane and release **Acetylcholine (ACh)** into the synaptic cleft via exocytosis. ACh then diffuses across the cleft and **binds to Nicotinic Acetylcholine Receptors (nAChR)** on the motor endplate (postsynaptic sarcolemma). This binding opens ligand-gated cation channels, leading to sodium influx and the generation of an End Plate Potential (EPP). **Analysis of Incorrect Options:** * **Option A:** Release of calcium from the Sarcoplasmic Reticulum (SR) occurs *after* the muscle membrane has been depolarized and the action potential has traveled down the T-tubules to reach the DHP receptors. This is part of **Excitation-Contraction Coupling**, not the initial arrival of the nerve impulse. * **Option D:** This is physiologically incorrect. Calcium does not "efflux" through vesicles into the cleft; rather, **ACh** is released from vesicles, and calcium **influxes into the neuron** from the extracellular fluid to trigger that release. **High-Yield Clinical Pearls for NEET-PG:** * **Lambert-Eaton Syndrome:** Antibodies against presynaptic voltage-gated calcium channels (impairs ACh release). * **Myasthenia Gravis:** Antibodies against postsynaptic nAChR (reduces EPP magnitude). * **Botulinum Toxin:** Inhibits ACh release by degrading SNARE proteins. * **Safety Factor:** The EPP is normally much larger than required to reach the threshold, ensuring every nerve impulse results in a muscle contraction.

Question 48: Each myosin molecule of a sarcomere is composed of how many monomers?

A. Two monomers
B. Four monomers
C. Six monomers (Correct Answer)
D. Eight monomers

Explanation: **Explanation:** The correct answer is **Six monomers (Option C)**. Myosin II, the type found in skeletal muscle, is a large hexameric protein (molecular weight ~480,000 Da). It is composed of **six polypeptide chains**: 1. **Two Heavy Chains:** These wrap around each other in a double helix to form the "tail" (rod) of the molecule. At one end, they fold outward to form two globular "heads." 2. **Four Light Chains:** Two light chains are associated with each myosin head. These are categorized as **Essential Light Chains** (provide structural stability) and **Regulatory Light Chains** (regulate ATPase activity). **Analysis of Incorrect Options:** * **Option A (Two):** This refers only to the number of heavy chains. * **Option B (Four):** This refers only to the number of light chains. * **Option D (Eight):** This is incorrect as the standard Myosin II molecule does not contain eight subunits. **High-Yield Facts for NEET-PG:** * **The Myosin Head:** Contains two critical sites: the **Actin-binding site** and the **Catalytic site (ATP-binding site)** which has ATPase activity. * **HMM vs. LMM:** When treated with the enzyme trypsin, myosin breaks into **Heavy Meromyosin (HMM)**, which contains the heads and short neck (cross-bridges), and **Light Meromyosin (LMM)**, which forms the tail. * **Power Stroke:** The actual "rowing" motion occurs when ADP is released from the myosin head, causing it to tilt toward the arm. * **Clinical Correlation:** Mutations in the genes encoding cardiac myosin heavy chains are a primary cause of **Familial Hypertrophic Cardiomyopathy (HCM)**.

Question 49: Which of the following statements is NOT true for smooth muscle?

A. No Z-discs are present.
B. The actin to myosin ratio is approximately 2:1. (Correct Answer)
C. Contraction is primarily initiated by the thick filament.
D. A 'latch bridge' phenomenon is observed.

Explanation: ### Explanation **Why Option B is the Correct Answer (The False Statement):** In smooth muscle, the ratio of actin (thin filaments) to myosin (thick filaments) is significantly higher than in skeletal muscle. While skeletal muscle has a ratio of approximately 2:1 or 3:1, **smooth muscle has a ratio of about 10:1 to 15:1**. This high density of actin allows smooth muscle to generate significant force despite having less myosin. **Analysis of Other Options:** * **A. No Z-discs are present:** This is **true**. Smooth muscle lacks the organized sarcomere structure of striated muscle. Instead of Z-discs, thin filaments are anchored to **Dense Bodies** (composed of $\alpha$-actinin), which are distributed throughout the sarcoplasm and attached to the cell membrane. * **C. Contraction is primarily initiated by the thick filament:** This is **true**. Unlike skeletal muscle (thin-filament regulated via Troponin), smooth muscle is **thick-filament regulated**. Contraction begins when Calcium-Calmodulin activates **Myosin Light Chain Kinase (MLCK)**, which phosphorylates the myosin head. * **D. A 'latch bridge' phenomenon is observed:** This is **true**. This unique mechanism allows smooth muscle to maintain high tension for long periods with very low ATP consumption. It occurs when myosin is dephosphorylated while still attached to actin, slowing the detachment rate. **High-Yield NEET-PG Pearls:** * **Calmodulin** is the functional counterpart of Troponin C in smooth muscle. * **Troponin is absent** in smooth muscle; instead, **Caldesmon** and **Calponin** inhibit the actin-myosin interaction. * Smooth muscle can shorten to a much greater degree than skeletal muscle (up to 80% of its length). * **Caveolae** serve as rudimentary analogs to the T-tubule system.

Question 50: Which group of nerve fibers is least susceptible to hypoxia?

A. A alpha
B. A beta
C. B
D. C (Correct Answer)

Explanation: The susceptibility of nerve fibers to various insults depends on their metabolic requirements and anatomical characteristics. This is a high-yield topic often tested via the **Erlanger-Gasser classification**. ### **Explanation of the Correct Answer** The correct answer is **D (C fibers)**. Susceptibility to **hypoxia** follows the order: **B > A > C**. * **Type B fibers** (preganglionic autonomic) are the most sensitive to oxygen deprivation. * **Type C fibers** (small, unmyelinated, slow-conducting) are the **least susceptible** to hypoxia. Because they are unmyelinated and have a smaller surface area, they have lower metabolic demands and can maintain function longer in anaerobic conditions compared to larger, myelinated fibers. ### **Analysis of Incorrect Options** * **Options A & B (A alpha and A beta):** These are large, heavily myelinated fibers. While they are the most sensitive to **pressure** (Order: A > B > C), they are moderately sensitive to hypoxia—more so than C fibers but less than B fibers. * **Option C (B fibers):** As mentioned, these are the **most sensitive** to hypoxia. ### **High-Yield Clinical Pearls for NEET-PG** To master this topic, remember the "ABC" of fiber sensitivity: 1. **Hypoxia:** **B > A > C** (B is most sensitive; C is least). 2. **Pressure:** **A > B > C** (A is most sensitive; C is least). This explains why your foot "falls asleep" (loss of touch/motor) before you lose pain sensation when a nerve is compressed. 3. **Local Anesthetics:** **C > B > A** (C is most sensitive; A is least). This is why pain is the first sensation lost during local anesthesia. **Summary Table for Quick Revision:** | Insult | Most Sensitive | Least Sensitive | | :--- | :--- | :--- | | **Hypoxia** | Type B | **Type C** | | **Pressure** | Type A | Type C | | **Local Anesthesia** | Type C | Type A |

Practice by Chapter

Resting Membrane Potential

Practice Questions

Action Potential Generation and Propagation

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Neuromuscular Junction

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Skeletal Muscle Contraction

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Smooth Muscle Physiology

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Cardiac Muscle Properties

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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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