Nerve and Muscle Physiology — MCQs

Nerve and Muscle Physiology Indian Medical PG Practice Questions and MCQs

Question 401: Which of the following fiber types is classically categorized as Group B nerve fibers?

A. Sympathetic postganglionic
B. Parasympathetic preganglionic
C. Parasympathetic post ganglionic
D. Sympathetic preganglionic (Correct Answer)

Explanation: ***Sympathetic preganglionic*** - **Group B nerve fibers** are **myelinated preganglionic autonomic fibers** with intermediate diameter (3-15 μm) and moderate conduction velocity (3-15 m/s) - Both **sympathetic and parasympathetic preganglionic fibers** are classified as Group B fibers - **Sympathetic preganglionic** neurons are the classical example, originating from T1-L2 spinal segments and synapsing in paravertebral or prevertebral ganglia *Sympathetic postganglionic* - These are **unmyelinated Group C fibers** with slow conduction velocity (0.5-2 m/s) - They extend from ganglia to target organs *Parasympathetic preganglionic* - These are also **Group B fibers** (myelinated preganglionic) - However, **sympathetic preganglionic** is the more commonly cited classical example in standard classifications - They originate from cranial nerves (III, VII, IX, X) and sacral segments (S2-S4) *Parasympathetic postganglionic* - These are **unmyelinated Group C fibers** with the slowest conduction velocities - Short fibers extending from ganglia near or within target organs to effector cells

Question 402: Which of the following statements is true about red muscle fibers?

A. Contain fewer mitochondria than white muscle fibers
B. Have less myoglobin than white muscle fibers
C. Exhibit more oxidative capacity (Correct Answer)
D. Utilize glycolytic metabolism

Explanation: ***Exhibit more oxidative capacity*** - **Red muscle fibers**, also known as **slow-twitch fibers**, are rich in **mitochondria** and enzymes for aerobic respiration, allowing for sustained contractions and high oxidative capacity. - Their high oxidative capacity is crucial for activities requiring **endurance**, such as long-distance running or maintaining posture through efficient **ATP production** via the **electron transport chain**. *Contain fewer mitochondria than white muscle fibers* - **Red muscle fibers** contain **more mitochondria** than white muscle fibers to support their greater reliance on **aerobic metabolism** for sustained energy production. - **Mitochondria** are the primary sites of **oxidative phosphorylation**, which is essential for the continuous ATP supply needed by these endurance specialized fibers. *Utilize glycolytic metabolism* - While red fibers can perform some glycolysis, their primary metabolic pathway is **oxidative phosphorylation**, utilizing **fatty acids** and **glucose** aerobically. - **Glycolytic metabolism** is more characteristic of **white muscle fibers (fast-twitch)**, which rely on anaerobic pathways for rapid, high-intensity contractions. *Have less myoglobin than white muscle fibers* - **Red muscle fibers** are characterized by a **high content of myoglobin**, which gives them their characteristic red color and high oxygen storage capacity. - **Myoglobin** is crucial for oxygen delivery to the mitochondria, supporting the sustained aerobic metabolism of these fibers, in contrast to white fibers which have less myoglobin.

Question 403: Which type of muscle fibers has fewer mitochondria?

A. Type I fibers (Red fibers)
B. Type IIb fibers (Fast-twitch fibers) (Correct Answer)
C. Type IIa fibers
D. Type IIx fibers (Intermediate fibers)

Explanation: ***Type IIb fibers (Fast-twitch fibers)*** - These fibers rely primarily on **anaerobic glycolysis** for ATP production, which is a less efficient process than aerobic respiration and therefore requires fewer mitochondria. - Their primary function is rapid, powerful contractions over short durations, leading to quick fatigue. *Type IIa fibers* - These fibers are **fast-twitch oxidative-glycolytic** fibers, meaning they have a moderate number of mitochondria to support both aerobic and anaerobic metabolism. - They are capable of generating strong contractions and are more fatigue-resistant than Type IIb fibers but less so than Type I fibers. *Type I fibers (Red fibers)* - Known as **slow-twitch oxidative fibers**, they have a high density of mitochondria to support continuous **aerobic respiration** for sustained, low-intensity contractions. - Their rich blood supply and high myoglobin content give them their characteristic red color and make them highly fatigue-resistant. *Type IIx fibers (Intermediate fibers)* - These fibers are very similar to Type IIb fibers in their metabolic profile, often considered an intermediate or even functionally equivalent type depending on the species. - They also primarily utilize **anaerobic glycolysis** and have a relatively low mitochondrial content, making them prone to fatigue.

Question 404: What is the primary action observed in the withdrawal reflex?

A. Extension
B. Flexion (Correct Answer)
C. Flexion followed by extension
D. Not applicable

Explanation: ***Flexion*** - The **withdrawal reflex** is a protective reflex that causes the affected limb to **flex** and withdraw from a painful stimulus. - This **flexion** is mediated by the contraction of flexor muscles and relaxation of extensor muscles, moving the limb away from danger. *Extension* - **Extension** is the opposite of flexion and would move the limb closer to or maintain its position relative to the painful stimulus. - This action is typically observed in the **crossed extensor reflex**, where the contralateral limb extends to support the body, not in the direct withdrawal of the stimulated limb. *Flexion followed by extension* - While **flexion** is the primary action, it is not typically followed immediately by extension within the same limb in a simple withdrawal reflex. - If a coordinated movement were to occur, such as shifting weight, the **crossed extensor reflex** would involve extension in the opposite limb. *Not applicable* - The withdrawal reflex involves a clear and defined muscle action which is **flexion**, making "not applicable" incorrect. - This reflex is a fundamental component of the nervous system's response to noxious stimuli.

Question 405: Which type of nerve fibers are responsible for carrying joint position and vibration sense?

A. A-beta fibers (Correct Answer)
B. A-alpha fibers
C. C fibers
D. A-delta fibers

Explanation: ***A-beta fibers*** - **A-beta fibers** are **large-diameter, myelinated** sensory fibers that conduct impulses rapidly. - They are primarily responsible for transmitting **light touch, pressure, joint position (proprioception), and vibration sense**. - **This is the correct answer** because A-beta fibers carry **BOTH joint position AND vibration sense together**. *A-alpha fibers* - **A-alpha fibers** are the **largest and fastest** conducting nerve fibers. - They include **motor nerves to skeletal muscles** and **sensory fibers from muscle spindles (Ia) and Golgi tendon organs (Ib)**. - While Ia fibers (A-alpha) do carry **proprioception from muscle spindles**, they do **NOT carry vibration sense**. - The question asks for fibers carrying **BOTH** joint position **AND** vibration sense, making A-beta the correct answer. *C fibers* - **C fibers** are **small-diameter, unmyelinated** nerve fibers with **slow conduction velocities**. - They are responsible for transmitting **dull, aching pain, temperature, and crude touch**. *A-delta fibers* - **A-delta fibers** are **small-diameter, thinly myelinated** nerve fibers with an intermediate conduction velocity. - They primarily transmit **sharp, fast pain** and **cold temperature sensations**.

Question 406: What is the primary function of the 'patch-clamp' technique in electrophysiology?

A. To record facilitated diffusion
B. To record osmotic pressure around semipermeable membrane
C. To record RMP
D. To record ionic currents through single or multiple ion channels (Correct Answer)

Explanation: ***To record ionic currents through single or multiple ion channels*** - The **patch-clamp technique** uses a microscopic glass pipette to form a tight seal with a cell membrane, allowing direct measurement of electrical currents flowing through individual or multiple **ion channels**. - This method is crucial for understanding the biophysical properties of **ion channels**, including their opening and closing kinetics, conductance, and sensitivity to various stimuli. *To record facilitated diffusion* - **Facilitated diffusion** is a passive transport process involving carrier proteins, which does not generate measurable electrical currents directly recorded by patch clamp. - While ion channels can facilitate diffusion, the patch-clamp technique specifically measures the **ionic current** generated by their activity, not the overall diffusive movement itself. *To record osmotic pressure around semipermeable membrane* - **Osmotic pressure** refers to the pressure that needs to be applied to a solution to prevent the inward flow of water across a semipermeable membrane. - The patch-clamp technique is designed to measure electrical phenomena, not **osmotic pressure** or water movement across membranes. *To record RMP* - While the patch-clamp technique can be used in a **whole-cell configuration** to measure the **resting membrane potential (RMP)**, its primary and most distinctive function is to resolve **single ion channel activity**. - Other, simpler electrophysiological methods can also measure RMP, but patch-clamp excels at the high-resolution study of **ionic currents** through specific channels.

Question 407: Most common type of calcium channels of skeletal muscles is?

A. N-type
B. T-type
C. R-type
D. L-type (Correct Answer)

Explanation: ***L type*** - **L-type calcium channels**, also known as **dihydropyridine receptors (DHPRs)**, are the predominant type of calcium channel found in skeletal muscle. - In skeletal muscle, they serve as voltage sensors that **mechanically link** to ryanodine receptors (RyRs) on the sarcoplasmic reticulum to trigger calcium release without significant calcium influx from the extracellular space. *N-type* - **N-type calcium channels** are primarily found in **neurons** and play a crucial role in neurotransmitter release at synapses. - They are not the primary calcium channels involved in skeletal muscle excitation-contraction coupling. *T-type* - **T-type calcium channels** are low-voltage activated channels found in various excitable cells, including cardiac muscle and neurons, where they contribute to **pacemaker activity** and repetitive firing. - They are not the main calcium channels responsible for excitation-contraction coupling in skeletal muscle. *R-type* - **R-type calcium channels** are found in various neural cells and are involved in diverse functions, including **synaptic transmission**, but their precise physiological role is less clearly defined compared to other types. - These channels are not the primary calcium entry pathways in skeletal muscle and do not play a significant role in its contraction.

Question 408: ATPase activity is present in

A. Myosin (Correct Answer)
B. Actin
C. Actin during interaction with myosin
D. None of the options

Explanation: ***Myosin*** - Myosin heads possess intrinsic **ATPase activity**, meaning they can hydrolyze ATP into ADP and inorganic phosphate. - This **ATP hydrolysis** provides the energy required for the **power stroke** during muscle contraction, detaching the myosin head from actin. *Actin* - Actin filaments themselves do not have ATPase activity. - Actin's primary role is to form the **thin filaments** and bind to myosin heads during contraction. *Actin during interaction with myosin* - While actin interacts with myosin, it does not acquire ATPase activity. - The **myosin head**, not actin, is responsible for ATP hydrolysis during this interaction. *None of the options* - This option is incorrect because **myosin** clearly possesses ATPase activity, which is crucial for muscle function.

Question 409: Which of the following does not have sympathetic noradrenergic fibers?

A. Heart
B. Eye
C. Sweat gland (Correct Answer)
D. Blood vessels

Explanation: ***Sweat gland*** - While sweat glands are innervated by the **sympathetic nervous system**, their postganglionic fibers are **cholinergic**, releasing **acetylcholine** rather than noradrenaline. - This is an important exception where sympathetic stimulation leads to acetylcholine release, causing sweating. *Blood vessels* - Most blood vessels, particularly resistance vessels such as **arterioles**, receive substantial **sympathetic noradrenergic innervation** that causes vasoconstriction. - This sympathetic tone is crucial for regulating **blood pressure** and distributing blood flow. *Heart* - The heart is richly innervated by **sympathetic noradrenergic fibers** that increase **heart rate**, **contractility**, and **conduction velocity** via beta-1 adrenergic receptors. - This makes noradrenaline a key neurotransmitter in the sympathetic regulation of cardiac function. *Eye* - The eye receives sympathetic noradrenergic innervation primarily to the **dilator pupillae muscle**, causing **mydriasis** (pupil dilation) upon activation. - These fibers also contribute to the sympathetic control of the **tarsal muscle** (Müller's muscle) in the eyelid.

Question 410: Duration of maximum contraction depends upon?

A. Both
B. Absolute refractory period (Correct Answer)
C. None of the two
D. Relative refractory period

Explanation: ***Absolute refractory period*** - The duration of **maximum (sustained) contraction** in skeletal muscle depends primarily on the **absolute refractory period** - The absolute refractory period (1-2 ms in skeletal muscle) is much **shorter than the contraction duration** (20-200 ms), allowing for **temporal summation** - When stimuli arrive after the refractory period but before complete relaxation, contractions **summate** to produce **tetanus** (sustained maximum contraction) - A shorter refractory period allows **higher frequency stimulation** → more complete summation → stronger and longer sustained contraction - This is why skeletal muscle can achieve **complete tetanus** at stimulation frequencies of 50-100 Hz *Relative refractory period* - While the relative refractory period affects excitability, it is the **absolute refractory period** that sets the fundamental limit on maximum stimulation frequency - The relative refractory period is less critical for determining the duration of maximum contraction *None of the two* - This is incorrect because the refractory period directly determines the **maximum frequency** at which muscle can be stimulated - Higher stimulation frequency (limited by refractory period) → better temporal summation → sustained maximum contraction (tetanus) - The refractory period is the key factor enabling or limiting the duration of maximum contraction *Both* - While both refractory periods influence excitability, the **absolute refractory period** is the primary determinant - It sets the absolute limit on stimulation frequency and thus the ability to achieve and maintain tetanic contraction

Practice by Chapter

Resting Membrane Potential

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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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