Nerve and Muscle Physiology Practice Questions

Q: Twitch of a single motor unit is called:

Fasciculation. ### 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).

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

Presynaptic glutamate-secreting axon terminals. **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.

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

Acetylcholine receptor. **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.

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

High glycolytic capacity. **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.

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

Six monomers. **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)**.

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

The actin to myosin ratio is approximately 2:1.. ### 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.

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

C. 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 |

Question 1

Twitch of a single motor unit is called:

Accepted Answer

Fasciculation

Answer

Myoclonic jerk

Answer

Tremor

Answer

Chorea

Question 2

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

Accepted Answer

Presynaptic glutamate-secreting axon terminals

Answer

Myocardium

Answer

Motor endplates

Answer

Neuromuscular junction

Question 3

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

Accepted Answer

Sarcomeres are absent in smooth muscle.

Answer

Smooth muscle cells have multiple nuclei.

Answer

Smooth muscle cells have striations.

Answer

Cross bridges are formed in smooth muscle during contraction.

Question 4

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

Accepted Answer

2500 times/sec

Answer

100 times/sec

Answer

250 times/sec

Answer

2000 times/sec

Question 5

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

Accepted Answer

Acetylcholine receptor

Answer

Calcium channel

Answer

Potassium channel

Answer

Histamine H2 receptor

Question 6

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

Accepted Answer

High glycolytic capacity

Answer

High capillary density

Answer

High number of mitochondria

Answer

High myoglobin content

Question 7

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

Accepted Answer

Acetylcholine binds to receptors on the postsynaptic sarcolemmal membrane

Answer

Release of Calcium from the Sarcoplasmic Reticulum

Answer

Efflux of calcium through synaptic vesicles into the synaptic cleft

Question 8

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

Accepted Answer

Six monomers

Answer

Two monomers

Answer

Four monomers

Answer

Eight monomers

Question 9

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

Accepted Answer

The actin to myosin ratio is approximately 2:1.

Answer

No Z-discs are present.

Answer

Contraction is primarily initiated by the thick filament.

Answer

A 'latch bridge' phenomenon is observed.

Question 10

Which group of nerve fibers is least susceptible to hypoxia?

Accepted Answer

C

Answer

A alpha

Answer

A beta

Answer

B

Nerve and Muscle Physiology — MCQs

Nerve and Muscle Physiology — MCQs

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