Nerve and Muscle Physiology — MCQs
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When the tension becomes great enough, contraction suddenly ceases and the muscle relaxes, this phenomenon is known as:
What is the physiological condition in which the ratio of potassium permeability to sodium permeability (PK/PNa) is maximized?
Which of the following components is primarily found in thick filaments?
Which of the following statements about defensive attitude is false?
What initiates the grasp palmar reflex?
Which of the following pathways is the major energy-providing pathway for fast-twitch muscle?
Unidirectional flow of a nerve impulse is characterized by
Sarcomere is the area between which two Z lines?
What protein anchors actin filaments to the Z-line in muscle fibers?
Which of the following proteins prevents contraction by covering binding sites on actin, thereby preventing its interaction with myosin?
Nerve and Muscle Physiology Indian Medical PG Practice Questions and MCQs
Question 461: When the tension becomes great enough, contraction suddenly ceases and the muscle relaxes, this phenomenon is known as:
- A. Autocrine innervation
- B. Autogenic inhibition (Correct Answer)
- C. Converse stretch reflex
- D. Reciprocal innervation
Explanation: ***Autogenic inhibition*** - This reflex is mediated by **Golgi tendon organs (GTOs)**, which are proprioceptors located within the muscle tendons. - When muscle tension becomes excessively high, GTOs are activated and send inhibitory signals to the alpha motor neurons supplying that same muscle, causing it to relax and preventing injury. *Autocrine innervation* - **Autocrine signaling** refers to a form of cell signaling in which a cell secretes a hormone or chemical messenger that binds to receptors on the same cell, leading to changes in the cell. - This term does not describe a reflex mechanism involving muscle tension and relaxation. *Converse stretch reflex* - There is no recognized physiological reflex termed the "converse stretch reflex." - The **stretch reflex** (or myotatic reflex) is a muscle contraction in response to stretching within the muscle, acting to maintain a constant muscle length, which is the opposite of muscle relaxation due to high tension. *Reciprocal innervation* - **Reciprocal innervation** (or reciprocal inhibition) is a reflex where the contraction of one muscle is accompanied by the simultaneous relaxation of its antagonist muscle. - While it involves coordinated muscle activity, it does not explain the sudden cessation of contraction and relaxation of a single muscle due to high tension.
Question 462: What is the physiological condition in which the ratio of potassium permeability to sodium permeability (PK/PNa) is maximized?
- A. Depolarization
- B. Action Potential
- C. Resting Membrane Potential
- D. Hyperpolarization (Correct Answer)
Explanation: ***Hyperpolarization*** - During **hyperpolarization**, the membrane potential becomes more negative than the **resting membrane potential**, primarily due to the outflow of **potassium (K+)** ions or influx of **chloride (Cl-)** ions. - This increased K+ efflux or Cl- influx signifies a state where potassium permeability is maximal relative to sodium permeability, making the membrane less excitable. *Action Potential* - An **action potential** involves a rapid **depolarization** phase due to a massive influx of **sodium (Na+)** ions, causing the PNa/PK ratio to be high, followed by repolarization where K+ efflux restores the resting potential. - Therefore, during an action potential, the ratio of PK/PNa is at its lowest during the rising phase when sodium channels are open. *Depolarization* - **Depolarization** is characterized by a decrease in the absolute value of the membrane potential, making it less negative or even positive, primarily due to the influx of **sodium (Na+)** ions. - During depolarization, the permeability to sodium is significantly higher than to potassium, thus the PK/PNa ratio is low. *Resting Membrane Potential* - At **resting membrane potential**, potassium permeability is already much higher than sodium permeability due to **leak potassium channels**, but it is not maximized to the extent seen during hyperpolarization. - The resting potential is established by a balance of ion movements, primarily K+ efflux and limited Na+ influx, maintained by the **Na+/K+-ATPase pump**.
Question 463: Which of the following components is primarily found in thick filaments?
- A. Dystrophin, Titin, and Actin
- B. Titin, Actin, and Nebulin
- C. Heavy chain of Myosin and Light chain of Myosin (Correct Answer)
- D. Nebulin and Actin
Explanation: ***Heavy chain of Myosin and Light chain of Myosin*** - **Myosin** is the primary component of **thick filaments** in muscle tissue, consisting of heavy chains that form the rod-like tail and heads, and light chains that regulate the myosin head function. - The **myosin heads** bind to **actin** during muscle contraction, driven by ATP hydrolysis. *Dystrophin, Titin, and Actin* - **Actin** is the main component of **thin filaments**, not thick filaments, and interacts with myosin during contraction. - **Dystrophin** is a structural protein linking actin to the sarcolemma, and **titin** is involved in muscle elasticity, but neither are primary components of the thick filament itself, although titin is associated with it. *Titin, Actin, and Nebulin* - **Actin** and **nebulin** are components of **thin filaments**, with nebulin regulating the length of actin filaments. - **Titin** is a large protein associated with thick filaments, providing elasticity and maintaining their position, but not forming the filament's structural core. *Nebulin and Actin* - Both **nebulin** and **actin** are integral components of **thin filaments**. - **Nebulin** helps regulate the length of **actin filaments**, while actin forms the backbone of the thin filament where myosin heads bind.
Question 464: Which of the following statements about defensive attitude is false?
- A. Characterized by flexion of upper extremities
- B. Also called as fencing attitude
- C. Occurs only in severe burns (Correct Answer)
- D. Occurs due to coagulation of proteins
Explanation: ***Occurs only in severe burns*** - The **defensive attitude (pugilistic attitude)** can occur with varying degrees of thermal injury, not exclusively in severe burns. - The underlying mechanism of **protein coagulation** begins at temperatures around 60-70°C and can happen with different burn severities. - This statement is **false** as it represents an oversimplification. *Characterized by flexion of upper extremities* - The defensive attitude is indeed characterized by **flexion of the elbows, wrists, and fingers** due to thermal contraction of flexor muscles. - This creates a characteristic posture resembling a boxer's stance or fencing position. *Also called as fencing attitude* - The defensive attitude is also known as the **pugilistic attitude** or **fencing attitude**. - These terms describe the characteristic flexed posture adopted due to thermal injury causing muscle contraction. *Occurs due to coagulation of proteins* - The contracted posture results from **thermal coagulation and denaturation of muscle proteins**. - Heat causes protein shortening in muscles, with **flexor muscles** (being stronger and bulkier) contracting more than extensors, creating the characteristic flexed position.
Question 465: What initiates the grasp palmar reflex?
- A. A sudden loud noise.
- B. A gentle touch. (Correct Answer)
- C. A sudden movement of the neck.
- D. A sudden bright light.
Explanation: ***A gentle touch.*** - The **palmar grasp reflex** is initiated by a gentle touch or stroking of the infant's palm. - This sensory input triggers an involuntary response where the infant's fingers flex and grasp the stimulating object. *A sudden movement of the neck.* - A sudden movement of the neck, especially when the head turns to one side, typically initiates the **asymmetrical tonic neck reflex (ATNR)**. - The ATNR causes the infant to extend the arm and leg on the side the head is turned, and flex the limbs on the opposite side, rather than a grasping action. *A sudden loud noise.* - A sudden loud noise or sudden repositioning of the head and body usually triggers the **Moro reflex**. - The Moro reflex involves symmetric extension of the arms and legs, followed by adduction and flexion, often accompanied by crying, and is distinct from the palmar grasp. *A sudden bright light.* - A sudden bright light would typically elicit a **blink reflex** or cause the infant to turn away from the light. - It does not directly initiate the palmar grasp reflex, which is a tactile rather than visual response.
Question 466: Which of the following pathways is the major energy-providing pathway for fast-twitch muscle?
- A. Glycolysis (Correct Answer)
- B. Utilisation of ketone bodies
- C. Amino acid breakdown
- D. β oxidation of fatty acids
Explanation: ***Glycolysis*** - **Fast-twitch muscle fibers** (Type II) are designed for rapid, powerful contractions over short periods and rely primarily on **anaerobic metabolism**. - **Glycolysis** is the major energy-providing pathway under these conditions, quickly converting **glucose** into ATP without the need for oxygen, leading to lactate production. *β oxidation of fatty acids* - This pathway is the primary energy source for **slow-twitch muscle fibers** (Type I) which are adapted for sustained activity and rely on **aerobic respiration**. - **Fatty acid oxidation** is slower and requires oxygen, making it less suitable for the rapid ATP demands of fast-twitch muscles. *Utilisation of ketone bodies* - **Ketone bodies** are typically used as an alternative fuel source by the **brain** and **muscle** during prolonged fasting or starvation, when glucose availability is low. - While muscles can utilize ketone bodies, they are not the primary or major energy source for fast-twitch muscle activity, especially during immediate, intense exertion. *Amino acid breakdown* - **Amino acid breakdown** (protein catabolism) is primarily used for energy during conditions of severe calorie restriction or prolonged exercise when other fuel sources are depleted, or for glucose synthesis via **gluconeogenesis**. - It is not a major or rapidly accessible energy source for the immediate, high-demand ATP requirements of fast-twitch muscle.
Question 467: Unidirectional flow of a nerve impulse is characterized by
- A. Synaptic junction (Correct Answer)
- B. Nerve fiber
- C. Neuronal branches
- D. Myelin sheath
Explanation: ***Synaptic junction*** - The **synapse** ensures that neurotransmitters are released from the presynaptic neuron and bind to receptors on the postsynaptic neuron, enforcing a **unidirectional flow** of information. - This structural and functional arrangement prevents the impulse from traveling backward to the transmitting neuron. *Nerve fiber* - While a nerve fiber (axon) conducts impulses, it does not inherently prevent backward propagation; the **synapse** is crucial for establishing directionality between neurons. - An isolated nerve fiber can conduct in both directions if artificially stimulated mid-axon, but the synaptic structure prevents this in vivo. *Neuronal branches* - **Neuronal branches** (dendrites and axon terminals) increase the surface area for receiving and transmitting signals but do not inherently establish or maintain the **unidirectional flow** of the impulse itself. - They allow for complex integration and divergence of signals but rely on the synapse for directionality. *Myelin sheath* - The **myelin sheath** insulates the axon and increases the speed of impulse conduction through **saltatory conduction**. - It does not, however, determine the direction of the impulse; that function is primarily regulated by the **synapse**.
Question 468: Sarcomere is the area between which two Z lines?
- A. Two adjacent Z lines (Correct Answer)
- B. A band and I band
- C. Two H zones
- D. Two consecutive I bands
Explanation: ***Two adjacent Z lines*** - A **sarcomere** is defined as the fundamental contractile unit of muscle, extending from one **Z line** to the next. - The **Z lines** anchor the **actin (thin) filaments**, and their proximity helps delineate the functional sarcomere unit. *Two consecutive I bands* - The **I band** contains only **thin (actin) filaments** and is bisected by a Z line. It is not a boundary that defines a sarcomere. - A sarcomere encompasses parts of two I bands, plus an A band in the middle. *A band and I band* - The **A band** contains **thick (myosin) filaments** and overlapping thin filaments, while the **I band** contains only thin filaments. - These bands are components within a sarcomere, not boundaries that define its extent. *Two H zones* - The **H zone** is a region within the A band that contains only **thick (myosin) filaments** and is visible in relaxed muscle. - It is located in the center of the A band and does not serve as a boundary for the entire sarcomere.
Question 469: What protein anchors actin filaments to the Z-line in muscle fibers?
- A. Titin
- B. Alpha actinin (Correct Answer)
- C. Nebulin
- D. Dystrophin
Explanation: ***Alpha actinin*** - **Alpha-actinin** is a protein that cross-links **actin filaments** and anchors them to the **Z-disc** in skeletal muscle. - It plays a crucial role in maintaining the **structural integrity** of the sarcomere. *Titin* - **Titin** is a giant protein that extends from the **Z-disc to the M-line** and acts as a molecular spring. - It maintains the **passive elasticity** of muscle and helps in sarcomere assembly. *Nebulin* - **Nebulin** is a large protein that co-extends with **actin filaments** from the Z-disc. - It acts as a **molecular ruler** and regulates the length of actin filaments. *Dystrophin* - **Dystrophin** connects the **intracellular cytoskeleton to the extracellular matrix** via the dystrophin-glycoprotein complex. - It is located at the **sarcolemma**, not at the Z-line, and its deficiency causes **Duchenne muscular dystrophy**.
Question 470: Which of the following proteins prevents contraction by covering binding sites on actin, thereby preventing its interaction with myosin?
- A. Thymosin
- B. Troponin
- C. Calmodulin
- D. Tropomyosin (Correct Answer)
Explanation: ***Tropomyosin*** - **Tropomyosin** is a regulatory protein that wraps around the **actin filament**, covering the **myosin-binding sites** in a resting muscle. This physically blocks the myosin heads from attaching to actin. - When **calcium ions** bind to troponin, it causes a conformational change that shifts tropomyosin away, exposing the binding sites and allowing muscle contraction. *Thymosin* - **Thymosin** is a protein involved in the regulation of **actin polymerization**, primarily by sequestering **G-actin monomers**. - It plays a role in the formation and regulation of the **actin cytoskeleton** but is not directly involved in blocking myosin binding sites on actin filaments during muscle contraction. *Troponin* - **Troponin** is a complex of three proteins (troponin C, I, and T) that is attached to **tropomyosin**. - Its primary role is to **bind calcium ions**, which then causes a conformational change that moves tropomyosin, *exposing* the myosin-binding sites on actin, rather than *covering* them. *Calmodulin* - **Calmodulin** is a ubiquitous calcium-binding messenger protein that plays a key role in various cellular processes, including smooth muscle contraction. - In smooth muscle, **calmodulin binds calcium** and activates myosin light chain kinase, but it does not directly cover myosin-binding sites on actin in skeletal muscle.
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
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Motor Unit Function
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Neurotransmitters and Receptors
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Electrophysiological Measurements
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