Nerve and Muscle Physiology — MCQs

Nerve and Muscle Physiology Indian Medical PG Practice Questions and MCQs

Question 371: In a muscle fiber at rest, the length of the I band is 1 mm and A band is 1.5 mm. What is the length of the sarcomere?

A. 0.5 mm
B. 2.5 mm (Correct Answer)
C. 3.5 mm
D. 5 mm

Explanation: ***2.5 mm*** - A **sarcomere** is defined as the region between two successive **Z-discs** and is the functional contractile unit of muscle. - Each sarcomere contains: one complete **A band** (1.5 mm) in the center, and **half of each adjacent I band** on either side. - The given **I band length of 1 mm** represents the full I band, so each sarcomere contains two halves of I bands: 0.5 mm + 0.5 mm = 1 mm total. - Therefore, sarcomere length = **A band + two half I-bands** = 1.5 mm + 0.5 mm + 0.5 mm = **2.5 mm**. *0.5 mm* - This value represents only half of the I band length, which is the portion of I band within one sarcomere from one side. - This does not account for the **A band** or the other half of the **I band**, so it grossly underestimates sarcomere length. *3.5 mm* - This incorrect value would result from adding the **A band** to two complete **I bands** (1.5 mm + 1 mm + 1 mm = 3.5 mm). - This is wrong because each sarcomere contains only **two halves** of I bands (totaling one I band), not two complete I bands. *5 mm* - This value has no anatomical basis in sarcomere structure given the measurements provided. - It represents a significant overestimation and misunderstanding of how the **A band** and **I band** components sum to form the sarcomere length.

Question 372: Which type of pain is characterized by unknown etiology?

A. Nociceptive pain
B. Neuropathic pain
C. Idiopathic pain (Correct Answer)
D. Inflammatory pain

Explanation: ***Idiopathic pain*** - This term refers to pain where the **underlying cause** or pathology cannot be identified, despite thorough investigation. - It signifies that the **etiology is unknown**, fitting the description in the question directly. *Nociceptive pain* - This type of pain arises from the activation of **nociceptors** due to actual or threatened tissue damage. - Its etiology is typically clear, involving an injury, inflammation, or mechanical stress. *Neuropathic pain* - This pain results from damage or disease affecting the **somatosensory nervous system**. - The etiology is known to be nerve damage or dysfunction, not an unknown origin. *Inflammatory pain* - This pain is driven by the inflammatory process, involving the release of **pro-inflammatory mediators** at the site of tissue injury or infection. - The cause is directly linked to inflammation, making its etiology known.

Question 373: Excitability of cells is maximally affected by a change in the concentration of which ion?

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

Explanation: ***Correct Option: Ca2+*** - Extracellular **calcium concentration** has the **maximal effect** on cellular excitability by directly altering the **threshold potential** for action potential generation. - **Hypocalcemia** (↓ Ca2+) → **Decreases threshold** → Brings it closer to resting potential → **Increases excitability** → Clinical manifestations include tetany, hyperreflexia, paresthesias, and seizures (Chvostek's and Trousseau's signs). - **Hypercalcemia** (↑ Ca2+) → **Increases threshold** → Moves it away from resting potential → **Decreases excitability** → Clinical manifestations include muscle weakness, hyporeflexia, and lethargy. - Ca2+ ions bind to the **external surface of voltage-gated Na+ channels** and affect their voltage sensitivity, making this the most critical ion for regulating neuronal and muscular excitability. *Incorrect Option: Na+* - While Na+ influx is responsible for the **rapid depolarization phase** of the action potential, changes in **extracellular Na+ concentration** have minimal effect on cellular excitability. - The body tightly regulates serum Na+ concentration (135-145 mEq/L), and clinical conditions of hyponatremia/hypernatremia primarily affect **osmolality and cell volume** rather than membrane excitability directly. - Na+ is the mediator of excitability, but its concentration changes don't maximally affect the threshold. *Incorrect Option: K+* - K+ is crucial for establishing the **resting membrane potential** through the Nernst equilibrium. - **Hyperkalemia** → Depolarizes resting potential → Initially may increase excitability, but sustained depolarization inactivates Na+ channels → **Decreases excitability**. - **Hypokalemia** → Hyperpolarizes resting potential → Decreases excitability. - While K+ changes significantly affect excitability, the effect is not as maximal as Ca2+. *Incorrect Option: Cl-* - Chloride ions contribute to **inhibitory neurotransmission** (GABA and glycine receptors) and help stabilize the resting membrane potential. - Changes in Cl- concentration have relatively minor effects on cellular excitability compared to Ca2+, K+, or Na+. - Cl- primarily modulates rather than determines excitability.

Question 374: Afferents for stretch reflexes are carried by which fibers?

A. Ia (Group Ia) (Correct Answer)
B. Gamma (γ) efferents
C. Type B
D. Type C

Explanation: ***Ia (Group Ia)*** - **Type Ia (Group Ia) afferent fibers** from muscle spindles are the primary sensory receptors that carry information about **muscle stretch and velocity of stretch**. - Type Ia fibers are the **largest diameter** and **fastest conducting sensory fibers** in the peripheral nervous system, ensuring rapid transmission for the monosynaptic stretch reflex. - They synapse directly on alpha motor neurons in the spinal cord, forming the basis of the **stretch reflex (myotatic reflex)**. - **Type II (Group II) afferents** also contribute to stretch reflexes by detecting static muscle length. *Gamma (γ) efferents* - **Gamma motor neurons** are **efferent (motor) fibers**, not afferent (sensory) fibers. - They innervate the **intrafusal muscle fibers** within the muscle spindle to regulate spindle sensitivity. - Their role is motor control of spindle tension, not carrying sensory information from the stretch reflex. *Type B* - **Type B fibers** are **preganglionic autonomic fibers** (sympathetic and parasympathetic). - They are **myelinated** but smaller and slower than Type Ia fibers, and are not involved in somatic stretch reflexes. *Type C* - **Type C fibers** are **unmyelinated**, small-diameter, and the **slowest conducting** nerve fibers. - They primarily transmit **pain (nociception)**, **temperature**, and some **autonomic** signals, not proprioceptive information for stretch reflexes.

Question 375: In cardiac muscles, T-tubules are present at?

A. Z lines (Correct Answer)
B. A lines
C. I lines
D. A-I junction

Explanation: ***Z lines*** - In **cardiac muscle**, the T-tubules are located at the level of the **Z lines**, which delineate the sarcomere boundaries. - This placement allows for efficient excitation-contraction coupling by bringing the **action potential** into close proximity with the sarcoplasmic reticulum. *A lines* - The **A line** represents the length of the **myosin (thick) filaments** within the sarcomere. - T-tubules are not typically found at the A line; their primary function requires them to be positioned at the ends of the sarcomere. *I lines* - The **I line** (or I band) contains only **actin (thin) filaments** and spans from the edge of the A band to the Z disk. - While I lines are adjacent to Z lines, the precise location for T-tubules in cardiac muscle is at the Z line itself, not the I line. *A-I junction* - The **A-I junction** is the region where the A band (myosin) meets the I band (actin). - In **skeletal muscle**, T-tubules are located at the A-I junction, but in **cardiac muscle**, they are specifically at the Z line.

Question 376: Which of the following statements about myosin is true?

A. Myosin is a thin filament.
B. Myosin has ATPase activity. (Correct Answer)
C. Myosin is a calcium-binding protein.
D. Myosin covers the active site of actin.

Explanation: ***Myosin has ATPase activity.*** - The **myosin head** contains a binding site for **ATP** and possesses **ATPase activity**, which is crucial for muscle contraction. - This **ATPase activity** hydrolyzes ATP into ADP and inorganic phosphate, providing the energy for the **power stroke** during muscle contraction. *Myosin is a thin filament.* - Myosin is the primary component of **thick filaments** in muscle cells. - **Thin filaments** are primarily composed of **actin**, along with regulatory proteins like **tropomyosin** and **troponin**. *Myosin is a calcium-binding protein.* - The primary **calcium-binding protein** in muscle contraction is **troponin C**, which is part of the thin filament complex. - While calcium plays a crucial role in initiating myosin-actin interaction, myosin itself does not directly bind calcium to trigger this interaction. *Myosin covers the active site of actin.* - **Tropomyosin** is the protein that covers the **active sites** on actin filaments, preventing myosin from binding in a resting muscle. - Upon calcium binding to **troponin**, tropomyosin moves away, exposing the active sites for myosin to bind.

Question 377: Spinal cord has how many synapses in golgi tendon reflex?

A. 1 synapse
B. 3 synapses
C. 4 synapses
D. 2 synapses (Correct Answer)

Explanation: ***2 synapses*** - The **Golgi tendon reflex** is a **disynaptic** reflex. - It involves an afferent neuron communicating with an interneuron, which then synapses with an efferent neuron. *1 synapse* - A **monosynaptic reflex** involves only one synapse between the afferent and efferent neuron, such as the **stretch reflex**. - The Golgi tendon reflex is more complex in its neuronal pathway. *3 synapses* - While polysynaptic reflexes can involve multiple interneurons, the direct pathway for the Golgi tendon reflex specifically involves two synapses in the spinal cord. - Three synapses would imply an additional interneuronal connection beyond the standard inhibitory interneuron. *4 synapses* - This number of synapses would be present in highly complex or polysynaptic reflexes involving extensive spinal cord processing. - The Golgi tendon reflex is simpler than a polysynaptic reflex that would demand four synapses.

Question 378: What is the primary function of muscle spindles?

A. All of the options
B. Muscle tone regulation
C. Detection of muscle stretch (Correct Answer)
D. Facilitating voluntary muscle contraction

Explanation: ***Detection of muscle stretch*** - Muscle spindles are **sensory receptors** located within the belly of a muscle that primarily detect changes in the **length of the muscle** and the rate of change of length. - This information is crucial for the **stretch reflex**, which helps prevent overstretching and maintains muscle tone. *Muscle tone regulation* - While muscle spindles contribute to muscle tone, this is an **effect** or outcome of their primary function, not the primary function itself. - Muscle tone results from the continuous, **low-level contractions** of muscles, often mediated by the stretch reflex that muscle spindles initiate. *Facilitating voluntary muscle contraction* - Voluntary muscle contraction is initiated by signals from the **motor cortex** and upper motor neurons, not directly by muscle spindles. - Muscle spindles provide feedback during contraction, but they do not initiate or directly facilitate voluntary movement. *All of the options* - This option is incorrect because while muscle spindles contribute to muscle tone regulation, their **primary and most direct function** is the detection of muscle stretch. - The other options describe consequences or related actions rather than the fundamental role of the muscle spindle.

Question 379: Which of the following statements accurately describes Type C nerve fibers?

A. They are involved in motor functions.
B. They can transmit both sensory and motor signals.
C. They are primarily responsible for transmitting pain and temperature sensations. (Correct Answer)
D. They are involved in various types of nerve signal transmission.

Explanation: ***They are primarily responsible for transmitting pain and temperature sensations.*** - **Type C nerve fibers** are **unmyelinated** and have a small diameter, leading to slow conduction velocities. - Their primary role in the sensory system is to transmit **dull, slow pain**, **temperature sensation**, and **crude touch**. *They are involved in motor functions.* - While some autonomic C fibers can modulate organ function, **somatic motor functions** (muscle contraction) are primarily carried by **large myelinated A-alpha nerve fibers**. - C fibers have **minimal to no direct role in voluntary motor control**. *They can transmit both sensory and motor signals.* - **Type C fibers are predominantly sensory in function**, although some autonomic C fibers exist. - They are not typically involved in the diverse range of both somatic sensory and motor signaling seen in larger, mixed nerves. *They are involved in various types of nerve signal transmission.* - While C fibers do transmit signals, their role is quite specific and limited to **certain types of slower sensory inputs** (pain, temperature) and **autonomic functions**. - The statement is too broad; other fiber types are responsible for the "various types" of rapid and precise signal transmission.

Question 380: A patient with external hemorrhoids develops pain while passing stools. Which of the following nerve mediates this pain?

A. Pudendal nerve (Correct Answer)
B. Hypogastric nerve
C. Sympathetic plexus
D. Splanchnic visceral nerve

Explanation: ***Pudendal nerve*** - The **pudendal nerve** provides **somatic innervation** to the entire perineum, including the **anal canal** and external hemorrhoids, making it responsible for perception of sharp pain. - External hemorrhoids are located **distal to the dentate line**, in a richly innervated area of the anus by somatic nerves, primarily derived from the pudendal nerve. *Hypogastric nerve* - The **hypogastric nerves** are part of the **autonomic nervous system** and primarily carry **visceral afferent** and efferent fibers to pelvic organs. - They are involved in sensations like stretching and fullness but do not mediate sharp, localized pain from external structures like hemorrhoids. *Sympathetic plexus* - The **sympathetic plexus** (e.g., inferior mesenteric plexus, superior hypogastric plexus) primarily carries **visceral afferent fibers** from internal organs. - These fibers contribute to diffuse, poorly localized visceral pain and not the sharp somatic pain associated with external hemorrhoids. *Splanchnic visceral nerve* - **Splanchnic nerves** are part of the **autonomic nervous system** and mainly transmit **visceral sensation** from abdominal and pelvic organs. - They are responsible for sensations like cramping and dull pain in internal organs but do not innervate the skin and somatic structures like external hemorrhoids.

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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