Nerve and Muscle Physiology — MCQs

Nerve and Muscle Physiology Indian Medical PG Practice Questions and MCQs

Question 281: What are the main types of muscle cells?

A. Skeletal and cardiac
B. Smooth and cardiac
C. Smooth and skeletal
D. All of the above (Correct Answer)

Explanation: **Explanation:** The human body contains three distinct types of muscle tissue, categorized based on their structure (striated vs. non-striated) and control mechanism (voluntary vs. involuntary). 1. **Skeletal Muscle:** These are striated, multinucleated fibers under **voluntary** control (somatic nervous system). They are primarily attached to bones and responsible for locomotion. 2. **Cardiac Muscle:** Found exclusively in the heart wall (myocardium), these are striated, branched cells with intercalated discs. They are **involuntary** and possess inherent rhythmicity (autorhythmicity). 3. **Smooth Muscle:** These are non-striated (spindle-shaped) cells found in the walls of hollow organs (e.g., GI tract, blood vessels). They are **involuntary** and regulated by the autonomic nervous system. **Why Option D is Correct:** Since skeletal, cardiac, and smooth muscles are the three fundamental types of muscle cells in the human body, options A, B, and C are only partially correct. "All of the above" is the most comprehensive and accurate choice. **Analysis of Other Options:** Options A, B, and C are incorrect because they are **incomplete**. Selecting any one of them would exclude the third essential muscle type, leading to an inaccurate classification of human muscular histology. **High-Yield NEET-PG Pearls:** * **Troponin:** Present in skeletal and cardiac muscle, but **absent** in smooth muscle (which uses **Calmodulin** instead). * **Gap Junctions:** Present in cardiac and single-unit smooth muscle; absent in skeletal muscle. * **Regeneration:** Skeletal muscle has limited regeneration via **satellite cells**; cardiac muscle has virtually no regenerative capacity; smooth muscle can undergo hypertrophy and hyperplasia. * **T-tubules:** Largest in cardiac muscle (located at Z-lines), smaller in skeletal muscle (located at A-I junctions), and absent in smooth muscle (replaced by **caveolae**).

Question 282: Which of the following is an example of an electrical synapse?

A. Tight junction
B. Gap junctions (Correct Answer)
C. Anchoring junction
D. Neuromuscular junction

Explanation: **Explanation:** **1. Why Gap Junctions are the Correct Answer:** Electrical synapses are specialized connections between neurons where transmission occurs via the direct flow of ions through low-resistance channels. These channels are formed by **Gap Junctions**. A gap junction consists of two hemichannels called **connexons** (each made of six **connexin** proteins) that bridge the pre- and post-synaptic membranes. Unlike chemical synapses, electrical synapses are characterized by **minimal synaptic delay**, **bidirectional flow**, and the ability to synchronize the activity of a group of neurons. **2. Analysis of Incorrect Options:** * **A. Tight Junctions (Zonula Occludens):** These function as barriers that seal the space between epithelial cells to prevent the paracellular leakage of molecules. They do not facilitate electrical communication. * **C. Anchoring Junctions (e.g., Desmosomes):** These provide mechanical stability by tethering the cytoskeletons of adjacent cells together. They are essential for structural integrity (especially in skin and heart) but do not transmit electrical signals. * **D. Neuromuscular Junction (NMJ):** This is a classic example of a **chemical synapse**. It involves the release of a neurotransmitter (Acetylcholine) into a synaptic cleft, resulting in a significant synaptic delay (approx. 0.5 ms), which is absent in electrical synapses. **3. High-Yield Facts for NEET-PG:** * **Location:** In humans, electrical synapses are found in the **retina**, **cerebral cortex**, and **olfactory bulb**. * **Cardiac Muscle:** Gap junctions are a key component of **intercalated discs**, allowing the heart to function as a functional syncytium. * **Comparison:** Chemical synapses are more common, unidirectional, and allow for signal amplification/modulation, whereas electrical synapses are faster and primarily used for rapid, synchronized firing.

Question 283: Which muscle is the chief mover of the mandible towards the left?

A. Left medial pterygoid
B. Left lateral pterygoid
C. Right medial pterygoid
D. Right lateral pterygoid (Correct Answer)

Explanation: The movement of the mandible is a high-yield topic in head and neck anatomy and physiology. To understand side-to-side (lateral) movement, one must grasp the specific action of the **Lateral Pterygoid** muscle. ### 1. Why the Right Lateral Pterygoid is Correct The lateral pterygoid is the only muscle of mastication that assists in opening the mouth by depressing the mandible. However, its most unique feature is its role in **lateral deviation**. * **Mechanism:** When the right lateral pterygoid contracts, it pulls the condyle of the mandible and the articular disc forward (protrusion) and medially. * **Result:** Because the right side moves forward while the left side remains relatively fixed, the chin is pushed toward the **opposite (contralateral) side**. Therefore, to move the mandible to the **left**, the **right** lateral pterygoid must contract. ### 2. Analysis of Incorrect Options * **Left Medial/Lateral Pterygoid (Options A & B):** Contraction of these muscles on the left side would result in the mandible moving toward the **right**. * **Right Medial Pterygoid (Option C):** While the medial pterygoid does assist in lateral deviation to the opposite side, it is primarily an elevator of the mandible (closing the mouth). The lateral pterygoid is considered the **chief mover** or primary initiator of this horizontal displacement. ### 3. NEET-PG High-Yield Pearls * **Unilateral Contraction:** Moves the jaw to the opposite side. * **Bilateral Contraction:** Results in protrusion (protrusion) and depression of the mandible. * **Nerve Supply:** All muscles of mastication are supplied by the **Mandibular Nerve (V3)**. * **Clinical Correlation:** In cases of **Trigeminal Nerve palsy**, when the patient is asked to protrude the jaw, it deviates **toward the side of the lesion** because the healthy contralateral lateral pterygoid acts unopposed.

Question 284: Which one of the following is a regulatory protein of the muscle?

A. Troponin (Correct Answer)
B. Myosin
C. Actin
D. Protein-C

Explanation: **Explanation:** The contractile machinery of skeletal muscle is composed of three functional categories of proteins: contractile, regulatory, and structural. **1. Why Troponin is Correct:** **Troponin** and **Tropomyosin** are the primary **regulatory proteins**. In a resting state, tropomyosin covers the myosin-binding sites on the actin filament. Troponin consists of three subunits: * **Troponin C:** Binds to Calcium ions. * **Troponin I:** Inhibits the actin-myosin interaction. * **Troponin T:** Tethers the troponin complex to tropomyosin. When $Ca^{2+}$ binds to Troponin C, a conformational change occurs that moves tropomyosin away, allowing the "power stroke" to occur. **2. Why the Other Options are Incorrect:** * **Myosin (Option B):** This is a **contractile protein** (thick filament). It possesses ATPase activity and forms cross-bridges with actin. * **Actin (Option C):** This is a **contractile protein** (thin filament). It provides the binding sites for myosin heads. * **Protein-C (Option D):** This is a physiological **anticoagulant** synthesized in the liver (Vitamin K dependent). It is not a structural or regulatory component of the muscle sarcomere. **High-Yield NEET-PG Pearls:** * **Sarcomere:** The functional unit of contraction, defined as the segment between two Z-lines. * **Dystrophin:** A vital structural protein; its deficiency leads to Duchenne Muscular Dystrophy. * **Titin:** The largest protein in the body; it acts as a molecular spring providing passive elasticity to the muscle. * **Clinical Marker:** Cardiac Troponin I and T are highly specific gold-standard biomarkers for diagnosing Myocardial Infarction (MI).

Question 285: Which type of nerve fibers are the fastest?

A. A fibers (Correct Answer)
B. B fibers
C. C fibers
D. All nerve fibers

Explanation: **Explanation:** The speed of nerve impulse conduction is determined by two primary factors: **myelination** and **fiber diameter**. According to the **Erlanger-Gasser classification**, nerve fibers are categorized into types A, B, and C based on these characteristics. **Why A fibers are correct:** Type A fibers are the **fastest** because they are both **heavily myelinated** and have the **largest diameter** (up to 20 μm). Myelin allows for saltatory conduction, where the action potential "jumps" between Nodes of Ranvier, significantly increasing velocity. Within this group, **Type A-alpha (Aα)** fibers are the fastest of all, conducting at speeds of 70–120 m/s (primarily serving motor neurons and proprioception). **Why other options are incorrect:** * **B fibers:** These are preganglionic autonomic fibers. While they are myelinated, their diameter is much smaller than Type A fibers, resulting in intermediate conduction velocities (3–15 m/s). * **C fibers:** These are the **slowest** nerve fibers. They are **unmyelinated** and have the smallest diameter (0.4–1.2 μm), conducting at speeds of only 0.5–2 m/s. They carry slow pain, temperature, and postganglionic autonomic signals. * **All nerve fibers:** This is incorrect as conduction velocity varies significantly across different fiber types to suit their physiological functions. **High-Yield Clinical Pearls for NEET-PG:** * **Order of Susceptibility to Local Anesthetics:** B > C > A (Small myelinated > Small unmyelinated > Large myelinated). * **Order of Susceptibility to Pressure:** A > B > C (Large fibers are affected first). * **Order of Susceptibility to Hypoxia:** B > A > C. * **Fastest Fiber:** A-alpha (Proprioception/Somatic motor). * **Slowest Fiber:** C fibers (Dull/Slow pain, Olfaction).

Question 286: Increase in cytosolic calcium from intracellular storage, during smooth muscle contraction is/are due to?

A. CAMP
B. CGMP
C. CCMP
D. IP3-DAG (Correct Answer)

Explanation: **Explanation:** In smooth muscle, contraction is primarily initiated by an increase in cytosolic calcium. Unlike skeletal muscle, which relies heavily on voltage-gated triggers, smooth muscle contraction often involves **Pharmacomechanical Coupling** via G-protein coupled receptors (GPCRs). **1. Why IP3-DAG is correct:** When a ligand (like Acetylcholine or Norepinephrine) binds to a **Gq-protein-coupled receptor**, it activates the enzyme **Phospholipase C (PLC)**. PLC cleaves membrane phospholipids into two second messengers: * **Inositol triphosphate (IP3):** This is the primary trigger for calcium release. It binds to IP3-gated calcium channels on the **Sarcoplasmic Reticulum (SR)**, releasing stored calcium into the cytosol. * **Diacylglycerol (DAG):** This activates Protein Kinase C (PKC), which phosphorylates proteins that enhance contractile sensitivity. The released calcium then binds to **Calmodulin**, activating Myosin Light Chain Kinase (MLCK) to initiate contraction. **2. Why other options are incorrect:** * **cAMP (Option A):** In smooth muscle, increased cAMP (via Gs proteins) usually leads to **relaxation** (e.g., Bronchodilation by Beta-2 agonists) by inhibiting MLCK and promoting calcium sequestration. * **cGMP (Option B):** This acts as a second messenger for Nitric Oxide (NO) and ANP. It activates Protein Kinase G, which causes **vasodilation/relaxation** by dephosphorylating myosin light chains. * **cCMP (Option C):** Cyclic Cytidine Monophosphate is not a standard second messenger involved in the regulation of smooth muscle contraction. **High-Yield Clinical Pearls for NEET-PG:** * **Calcium Source:** Smooth muscle uses both extracellular calcium (via L-type channels) and intracellular calcium (via IP3-receptors). * **Calmodulin:** Smooth muscle lacks Troponin; Calmodulin is its functional equivalent. * **Latching Mechanism:** Smooth muscle can maintain prolonged tension with low ATP consumption, a state known as the "Latch state."

Question 287: Initiation of nerve impulse occurs at the axon hillock because:

A. It has a lower threshold than the rest of the axon (Correct Answer)
B. It is unmyelinated
C. Neurotransmitter release occurs here
D. None of the above

Explanation: The **axon hillock** (specifically the initial segment) is known as the **"Trigger Zone"** of the neuron. ### Why Option A is Correct The initiation of an action potential depends on the density of **voltage-gated sodium (Na+) channels**. The axon hillock has the highest concentration of these channels per unit area compared to the cell body or the rest of the axon. Because there are more channels available to open in response to depolarization, the **threshold for excitation is significantly lower** (approx. -45 mV) compared to the soma (approx. -30 mV). This makes it the site with the highest excitability, where graded potentials are summed to trigger an all-or-nothing impulse. ### Why Other Options are Incorrect * **Option B:** While the axon hillock is unmyelinated, this is not the functional reason for impulse *initiation*. Many parts of a neuron (like the dendrites and soma) are unmyelinated but cannot initiate an action potential as easily because they lack the necessary density of Na+ channels. * **Option C:** Neurotransmitter release occurs at the **axon terminals** (presynaptic boutons), not the hillock. The hillock is responsible for signal *generation*, while the terminal is responsible for signal *transmission*. ### High-Yield Facts for NEET-PG * **Initial Segment:** The actual site of impulse generation is the unmyelinated segment between the axon hillock and the first myelin sheath. * **Refractory Period:** This period is determined by the inactivation gate (h-gate) of the voltage-gated Na+ channels. * **Accommodation:** If a nerve is subjected to a slowly rising subthreshold stimulus, the threshold for firing increases; this is called accommodation. * **Safety Factor:** The ratio of action potential strength to the excitability threshold. In normal nerve fibers, this is usually greater than 1.

Question 288: Which type of nerve injury has the best prognosis?

A. Neuropraxia (Correct Answer)
B. Axonotemesis
C. Neurotemesis
D. Complete transaction

Explanation: **Explanation:** Nerve injuries are classified by the **Seddon Classification** based on the severity of damage to the nerve components. **1. Why Neuropraxia is the correct answer:** Neuropraxia is the mildest form of nerve injury. It involves a **temporary physiological conduction block** (usually due to focal demyelination or ischemia) without any physical disruption of the axon or the connective tissue sheath (endoneurium, perineurium, or epineurium). Since the axon remains intact, there is **no Wallerian degeneration**. Recovery is spontaneous, complete, and rapid (usually within days to weeks) once the inciting cause (like pressure) is removed. **2. Why the other options are incorrect:** * **Axonotmesis:** This involves physical disruption of the **axon**, leading to Wallerian degeneration distal to the injury. However, the supporting connective tissue framework (endoneurium) remains intact. Recovery is possible but slow (1mm/day) and depends on axonal regeneration. * **Neurotmesis:** This is the most severe grade, involving **complete transection** of both the axon and the entire connective tissue sheath. Spontaneous recovery is impossible; surgical intervention is required, and the prognosis is poor. * **Complete Transection:** This is synonymous with Neurotmesis (Grade V in Sunderland’s classification). It has the worst prognosis due to the loss of the guiding channel for regenerating axons. **High-Yield Clinical Pearls for NEET-PG:** * **Sunderland Classification:** Expands Seddon’s into 5 grades. Grade I is Neuropraxia; Grade V is Neurotmesis. * **Wallerian Degeneration:** Occurs in Axonotmesis and Neurotmesis, but **NEVER** in Neuropraxia. * **Tinel’s Sign:** Distal tingling on percussion. It is **absent** in Neuropraxia (as there is no axonal regeneration) but **present** in Axonotmesis as the nerve heals. * **Common Example:** "Saturday Night Palsy" (Radial nerve compression) is a classic clinical example of Neuropraxia.

Question 289: The thin filament of muscle consists of all the following except:

A. Actin
B. Troponin
C. Myosin (Correct Answer)
D. Tropomyosin

Explanation: **Explanation:** The sarcomere, the functional unit of skeletal muscle, is composed of two primary types of myofilaments: **thick filaments** and **thin filaments**. **1. Why Myosin is the Correct Answer:** **Myosin** is the primary constituent of the **thick filament**, not the thin filament. Each thick filament consists of approximately 300 myosin molecules. A myosin molecule is a hexamer composed of two heavy chains (forming the tail and globular heads) and four light chains. The myosin heads contain binding sites for ATP and actin, facilitating the "power stroke" during muscle contraction. **2. Analysis of Incorrect Options (Components of the Thin Filament):** * **Actin (Option A):** This is the backbone of the thin filament. It exists as globular G-actin, which polymerizes into filamentous F-actin. It contains the specific binding sites for myosin heads. * **Troponin (Option B):** A complex of three regulatory proteins: **Troponin T** (binds to tropomyosin), **Troponin I** (inhibits actin-myosin binding), and **Troponin C** (binds calcium). * **Tropomyosin (Option D):** A regulatory protein that wraps around the actin helix. In a resting state, it physically covers the myosin-binding sites on actin, preventing contraction. **High-Yield NEET-PG Pearls:** * **Regulatory Proteins:** Troponin and Tropomyosin are termed "regulatory proteins," while Actin and Myosin are "contractile proteins." * **The "I" and "A" Bands:** The **I-band** (Isotropic) contains only thin filaments, while the **A-band** (Anisotropic) contains the entire length of the thick filaments (with some thin filament overlap). * **Clinical Correlation:** **Troponin I and T** are gold-standard biomarkers for diagnosing Myocardial Infarction (MI) because they are released into the blood when cardiac muscle is damaged. * **Dystrophin:** A vital structural protein that anchors the cytoskeleton of the muscle fiber to the surrounding extracellular matrix; its deficiency leads to Duchenne Muscular Dystrophy.

Question 290: As a neuron’s diameter increases, what effect manifests?

A. Conduction velocity decreases
B. Membrane resistance increases
C. Action potential amplitude increases
D. Conduction velocity increases (Correct Answer)

Explanation: ***Conduction velocity increases*** - Increasing the diameter of an axon decreases the **internal (axial) resistance** ($R_i$) to passive current flow longitudinal to the axon. - Reduced internal resistance allows local current loops to spread further and faster, significantly increasing the **length constant**, thereby increasing conduction velocity. *Conduction velocity decreases* - This is incorrect, as larger diameter decreases internal resistance, leading directly to a **faster electrotonic spread** of depolarization and a higher conduction speed. - Decreased conduction velocity is typically observed in **small-diameter** or **demyelinated** axons where internal resistance is higher or membrane capacitance is altered. *Membrane resistance increases* - Membrane resistance ($R_m$) is determined by the density and activity of **leak ion channels** within the cell membrane, which is independent of the overall axon diameter. - While the total membrane area increases, the **specific membrane resistance** (resistance per unit area) does not change with diameter. *Action potential amplitude increases* - Action potential (AP) conduction is an **all-or-none** phenomenon, meaning the amplitude is fixed and determined by the electrochemical gradient of **voltage-gated sodium channels**. - Changes in axon diameter influence the **speed** of propagation (conduction velocity), but they do not alter the required peak voltage (amplitude) of the action potential.

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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