Nerve and Muscle Physiology — MCQs

Nerve and Muscle Physiology Indian Medical PG Practice Questions and MCQs

Question 411: When the tension in a muscle fibre is maximum, its length is called?

A. None of the options
B. Initial length
C. Equilibrium length
D. Optimum length (Correct Answer)

Explanation: ***Optimum length*** - This is the muscle length at which the **maximum number of cross-bridges** can form between actin and myosin filaments. - At this length, the sarcomere allows for the **greatest overlap** of thick and thin filaments without excessive stretching or compression, leading to peak tension generation. *Equilibrium length* - This term usually refers to the **resting length** of a muscle fiber when no external forces are acting upon it. - At equilibrium, the tension generated by the muscle may not necessarily be at its maximum. *Initial length* - This is a general term that refers to the **starting length** of a muscle fiber before it contracts or is stretched. - It does not specifically denote the length at which maximum tension is achieved. *None of the options* - This option is incorrect because **optimum length** accurately describes the muscle length yielding maximum tension.

Question 412: A man slept with his head over his forearm. The next morning, he complains of tingling and numbness over the forearm. If this were primarily due to hypoxia affecting nerve fibers, which of the following statements about nerve fiber sensitivity to hypoxia is correct?

A. B fibers are most sensitive to hypoxia, followed by A fibers, and C fibers are least sensitive. (Correct Answer)
B. All nerve fibers are equally sensitive to hypoxia.
C. C fibers are most sensitive to hypoxia, followed by B fibers, and A fibers are least sensitive.
D. A fibers are most sensitive to hypoxia, followed by B fibers, and C fibers are least sensitive.

Explanation: ***B fibers are most sensitive to hypoxia, followed by A fibers, and C fibers are least sensitive.*** - **B fibers** (preganglionic autonomic) have intermediate myelination with high metabolic demands for **axoplasmic transport** and **neurotransmitter synthesis**, making them most vulnerable to **hypoxia**. - According to **Ganong's differential sensitivity table**, **B fibers** show the greatest susceptibility to oxygen deprivation due to their unique metabolic requirements for autonomic function. *All nerve fibers are equally sensitive to hypoxia.* - Nerve fibers have vastly different **metabolic demands** and **oxygen requirements** based on their structure and function, creating distinct sensitivity patterns. - **Ganong's classification** clearly demonstrates differential responses where **B fibers > A fibers > C fibers** in terms of hypoxic sensitivity. *C fibers are most sensitive to hypoxia, followed by B fibers, and A fibers are least sensitive.* - **C fibers** are unmyelinated with the lowest metabolic demands and can function with minimal **ATP** requirements, making them most resistant to hypoxia. - This completely reverses the established sensitivity pattern; **C fibers** carrying **pain and temperature** are actually the most resistant to oxygen deprivation. *A fibers are most sensitive to hypoxia, followed by B fibers, and C fibers are least sensitive.* - **A fibers** are most sensitive to **mechanical compression** (pressure), not hypoxia - this is a common confusion between different types of nerve injury. - While **A fibers** have high energy needs for **saltatory conduction**, their hypoxic sensitivity is intermediate, not the highest among all fiber types.

Question 413: What is the primary factor that determines the resting membrane potential in a nerve fiber?

A. Is equal to the resting potential of cardiac muscle fibers.
B. Can be accurately measured using intracellular electrodes.
C. Increases with elevated extracellular potassium concentration.
D. Is primarily determined by the equilibrium potential of potassium ions. (Correct Answer)

Explanation: ***Is primarily determined by the equilibrium potential of potassium ions*** - The **resting membrane potential** of a nerve fiber is predominantly set by the efflux of **potassium ions** through leak channels, bringing the membrane potential close to potassium's equilibrium potential. - The high permeability of the nerve membrane to **potassium** at rest means that K+ movement is the most significant factor influencing the potential. *Is equal to the resting potential of cardiac muscle fibers* - **Cardiac muscle fibers** have a distinct resting potential (around -80 to -90 mV) influenced by different ion channels and regulatory mechanisms compared to nerve fibers (around -70 mV). - While both involve potassium currents, their specific conductances and the contribution of other ions differ significantly. *Can be accurately measured using intracellular electrodes* - While **intracellular electrodes** are indeed used to measure the resting membrane potential, this statement describes a measurement method, not the *primary factor* that determines the potential itself. - The method of measurement does not explain the underlying biophysical mechanisms that establish the potential. *Increases with elevated extracellular potassium concentration* - An **elevated extracellular potassium concentration** would make the resting membrane potential *less negative* (depolarize) rather than "increase" it in the typical sense of a more positive value. - This is because a higher external K+ reduces the concentration gradient for potassium efflux, bringing the membrane potential closer to zero.

Question 414: What is a key difference between smooth muscle and skeletal muscle physiology?

A. Calcium is required for contraction.
B. Troponin is absent in smooth muscle. (Correct Answer)
C. Myosin is essential for contraction.
D. Potassium is required for contraction.

Explanation: ***Troponin is absent in smooth muscle.*** * Smooth muscle contraction is regulated by **calcium-calmodulin complex** and subsequent activation of **myosin light chain kinase (MLCK)**, in contrast to skeletal muscle's reliance on the troponin-tropomyosin system. * **Troponin** is a calcium-binding protein found in skeletal and cardiac muscle, which plays a critical role in regulating muscle contraction by initiating the movement of tropomyosin, thereby exposing myosin-binding sites on actin. *Calcium is required for contraction.* * While calcium is indeed required for contraction in both smooth and skeletal muscle, the **mechanism of its action** differs, making this statement insufficiently discriminative as a *key difference*. * In both muscle types, an increase in intracellular **calcium** initiates the contractile process, but the downstream signaling pathways diverge significantly. *Myosin is essential for contraction.* * **Myosin** is a fundamental motor protein essential for contraction in *all* muscle types, including skeletal, cardiac, and smooth muscle. * This statement highlights a similarity, not a key difference, as **actin-myosin cross-bridge cycling** is the basis of force generation in all muscle tissues. *Potassium is required for contraction.* * **Potassium ions** are crucial for maintaining the resting membrane potential and for repolarization following an action potential, which is necessary for muscle excitability, but they do not directly trigger muscle contraction. * The influx of calcium (or release from intracellular stores) is the direct trigger for contraction, not potassium.

Question 415: Which of the following is NOT a location where multi-unit smooth muscle is present?

A. Blood vessels
B. Iris
C. Gut (Correct Answer)
D. Ciliary muscle

Explanation: ***Gut*** - The gut primarily contains **unitary (single-unit) smooth muscle**, characterized by cells connected by **gap junctions** that allow for synchronized contractions (e.g., peristalsis). - This type of smooth muscle exhibits **spontaneous rhythmic contractions** due to pacemaker cells, and its activity is modulated by neural and hormonal inputs rather than requiring individual innervation of each cell. - Multi-unit smooth muscle is **NOT present** in the gut. *Blood vessels* - Many larger blood vessels (e.g., large arteries) contain **multi-unit smooth muscle**, which allows for **fine, graded control** over vascular tone and blood flow. - Each muscle cell is typically **innervated individually**, enabling precise regulation of contraction strength. *Iris* - The iris contains **multi-unit smooth muscle** (e.g., sphincter pupillae and dilator pupillae muscles) which control pupil size. - These muscles require **individual innervation** to allow for very fine and precise movements in response to light intensity changes. *Ciliary muscle* - The ciliary muscle of the eye contains **multi-unit smooth muscle**, which controls the shape of the lens for accommodation (focusing). - These muscle fibers are **individually innervated** to allow precise control of lens curvature for near and far vision.

Question 416: Which of the following is NOT a characteristic of a biphasic action potential of a mixed nerve?

A. Refractory period
B. All or none phenomenon
C. Recorded on surface
D. Two or more positive peaks (Correct Answer)

Explanation: ***Two or more positive peaks*** - A **biphasic action potential** of a mixed nerve, when recorded extracellularly, typically consists of two phases: an initial **negative deflection** followed by a **positive deflection**. It does not exhibit multiple positive peaks for a single action potential. - The shape is determined by the propagation of the action potential past two recording electrodes, illustrating the **depolarization and repolarization** of the nerve. *All or none phenomenon* - This is a fundamental characteristic of **individual nerve fibers** and thus applies to the action potentials propagating within a mixed nerve. - If a stimulus reaches a threshold, a full-sized action potential is generated; otherwise, none is generated, regardless of stimulus strength. *Refractory period* - The **refractory period** is a crucial characteristic of nerve excitability, ensuring unidirectional propagation and limiting the frequency of action potentials. - This period, comprising absolute and relative phases, applies to the individual fibers within the mixed nerve. *Recorded on surface* - **Compound action potentials (CAPs)** of mixed nerves are typically recorded extracellularly (on the surface) using electrodes, often seen in nerve conduction studies. - This contrasts with intracellular recordings which measure the potential across the cell membrane directly.

Question 417: What is the fixed length of a myosin filament?

A. 0.16 nm
B. 1.6 micrometers (Correct Answer)
C. 16 nm
D. 1.6 mm

Explanation: ***1.6 micrometers*** - Myosin filaments, also known as **thick filaments**, are integral components of muscle contraction and have a characteristic fixed length. This length is precisely **1.6 micrometers** in mammalian skeletal muscle. - This consistent length is crucial for the **sliding filament model** of muscle contraction, ensuring proper overlap with actin filaments and efficient force generation. *0.16 nm* - This value is significantly too small; **nanometers (nm)** are typically used for atomic or molecular distances, not for entire protein filaments like myosin. - A myosin filament is composed of hundreds of myosin molecules, making its overall length much larger than a fraction of a nanometer. *16 nm* - While nanometers are used for molecular structures, 16 nm is still too small for a myosin filament. The entire filament is roughly **100 times larger** than this value. - This dimension might be more appropriate for the diameter of a single myosin molecule's head region, but not the entire filament's length. *1.6 mm* - This value is significantly too large; **millimeters (mm)** are visible to the naked eye and represent macroscopic objects. - Muscle filaments are microscopic structures, and a length of 1.6 mm would imply they are many times longer than an entire muscle cell.

Question 418: What is the consequence of tibial nerve injury/palsy?

A. Loss of plantar flexion (Correct Answer)
B. Dorsiflexion of foot at ankle joint
C. Loss of sensation of dorsum of foot
D. Paralysis of muscles of anterior compartment of leg

Explanation: **Loss of plantar flexion** - The **tibial nerve** innervates the muscles of the **posterior compartment of the leg**, which are primarily responsible for **plantar flexion** of the foot. - Injury to this nerve directly impairs the function of muscles like the gastrocnemius, soleus, and tibialis posterior, leading to a significant loss of the ability to point the foot downwards. *Dorsiflexion of foot at ankle joint* - **Dorsiflexion** is primarily mediated by muscles in the **anterior compartment of the leg**, such as the tibialis anterior, which are innervated by the **deep fibular nerve**. - Tibial nerve injury would not directly affect these muscles or their function; rather, it leads to issues with the opposing action. *Loss of sensation of dorsum of foot* - Sensation to the **dorsum of the foot** is primarily supplied by the **superficial fibular nerve** (for most of the dorsum) and the **deep fibular nerve** (for the first web space). - While the tibial nerve provides sensation to the sole of the foot, it does not typically innervate the dorsum. *Paralysis of muscles of anterior compartment of leg* - The muscles of the **anterior compartment of the leg** (e.g., tibialis anterior, extensor digitorum longus, extensor hallucis longus) are innervated by the **deep fibular nerve**. - A tibial nerve injury would paralyze muscles in the posterior compartment, not the anterior compartment.

Question 419: Tetanic contraction is due to accumulation of?

A. Na+
B. K+
C. Ca2+ (Correct Answer)
D. Cl-

Explanation: ***Ca2+*** - **Tetanic contraction** results from a rapid succession of muscle stimulations, leading to the sustained elevation of **intracellular calcium (Ca2+)** levels. - This persistent high Ca2+ concentration in the sarcoplasm allows for continuous binding to **troponin**, maintaining the activation of cross-bridge cycling. *Na+* - **Sodium (Na+)** influx is primarily responsible for the **depolarization** of the muscle cell membrane, leading to an **action potential**. - While essential for initiating the contraction, Na+ accumulation itself does not directly cause the sustained high Ca2+ levels characteristic of tetany. *K+* - **Potassium (K+)** efflux is crucial for the **repolarization** of the muscle cell membrane after an action potential. - Accumulation of K+ in the extracellular space can contribute to muscle fatigue and reduce excitability, but it does not directly lead to tetanic contraction. *Cl-* - **Chloride (Cl-)** ions play a significant role in stabilizing the resting membrane potential and contributing to muscle **repolarization**, particularly in skeletal muscle. - While important for muscle function, changes in Cl- concentration do not directly cause the sustained Ca2+ release required for tetanic contraction.

Question 420: The most distinguishing feature between skeletal and smooth muscle is the absence of ------ in smooth muscle.

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

Explanation: ***Troponin*** - **Smooth muscle** lacks the **troponin complex** (troponin I, C, and T) that is essential for initiating contraction in skeletal and cardiac muscle. - Instead of troponin, smooth muscle uses **calmodulin** to bind calcium, which then activates **myosin light chain kinase** to regulate contraction. *Tropomyosin* - **Tropomyosin** is present in both **skeletal** and **smooth muscle**, though it plays a different regulatory role in smooth muscle. - In smooth muscle, tropomyosin does not block myosin binding sites, but rather modulates the interaction between actin and myosin. *Myosin* - **Myosin** is a fundamental motor protein found in all types of muscle, including both **skeletal** and **smooth muscle**. - It forms thick filaments and interacts with actin to generate force and muscle contraction. *Actin* - **Actin** is a primary component of thin filaments and is universally present in all muscle types, including **skeletal** and **smooth muscle**. - It provides the framework along which myosin heads slide to produce muscle shortening.

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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