Nerve and Muscle Physiology — MCQs

Nerve and Muscle Physiology Indian Medical PG Practice Questions and MCQs

Question 431: What is the role of tropomyosin in muscle contraction?

A. Helps in the fusion of actin and myosin
B. Slides over myosin
C. Causes Ca2+ release
D. Covers the binding sites on actin to regulate the interaction with myosin (Correct Answer)

Explanation: **Covers the binding sites on actin to regulate the interaction with myosin.** - **Tropomyosin** is a regulatory protein that wraps around the actin helix, physically blocking the **myosin-binding sites** on actin in a relaxed muscle. - Its role is to prevent the formation of **cross-bridges** between actin and myosin until calcium is present, thereby regulating muscle contraction. *Helps in the fusion of actin and myosin* - Tropomyosin does not facilitate the fusion of actin and myosin; rather, it **regulates their interaction** by blocking or unblocking binding sites. - The actual binding (cross-bridge formation) occurs when **myosin heads** attach to actin binding sites, not a fusion process. *Slides over myosin* - Tropomyosin does not slide over myosin; instead, it **slides along the actin filament** in response to calcium binding to troponin. - The sliding filament model describes how **actin and myosin filaments slide past each other**, but tropomyosin's movement is confined to the actin filament. *Causes Ca2+ release* - **Tropomyosin does not cause Ca2+ release**; its position is influenced by calcium. - **Ca2+ is released from the sarcoplasmic reticulum** and binds to troponin, which then causes tropomyosin to shift.

Question 432: What mediates the response observed in Lewis's triple response?

A. Axon reflex (Correct Answer)
B. Arteriolar dilation
C. Histamine release
D. Local hormones

Explanation: ***Axon reflex*** - The **axon reflex** is the characteristic **neural mechanism** that mediates the spreading **flare** (the distinctive third component) of Lewis's triple response. - This antidromic reflex occurs when sensory nerve endings are stimulated, causing impulses to travel along branches of the same axon and release **substance P and CGRP** (calcitonin gene-related peptide), producing vasodilation in surrounding areas. - The axon reflex is what makes Lewis's triple response a **neurovascular** phenomenon, distinguishing it from simple inflammatory responses. - While histamine initiates the cascade, the **axon reflex** is considered the key mediating mechanism for the complete triple response pattern. *Arteriolar dilation* - Arteriolar dilation is the **effect** or result seen in the flare, not the mediating mechanism itself. - This is what happens as a consequence of axon reflex activation, not what mediates the response. *Histamine release* - **Histamine** from mast cells is the initial **chemical trigger** that causes the red line (direct capillary dilation) and wheal (increased permeability). - However, histamine alone cannot explain the **spreading flare** beyond the site of injury, which requires the neurogenic mechanism of the axon reflex. - The question asks for the **mediating mechanism** of the observed response pattern, where the axon reflex is the distinguishing feature. *Local hormones* - This term is too vague and non-specific to describe the precise mechanism. - While bradykinin and other mediators may contribute, the **axon reflex** is the specific neural mechanism that characterizes Lewis's triple response.

Question 433: Function of phospholamban is

A. Regulates sodium and potassium levels
B. Transports calcium out of the cell
C. Binds to actin and myosin
D. Regulates calcium uptake by inhibiting SERCA (Correct Answer)

Explanation: ***Regulates calcium uptake by inhibiting SERCA*** - **Phospholamban (PLN)** is a **regulatory protein** that, in its unphosphorylated state, **inhibits the sarco/endoplasmic reticulum Ca2+-ATPase (SERCA)** pump. - This inhibition reduces the rate of **calcium reuptake** into the sarcoplasmic reticulum, thus influencing myocardial relaxation and contractility. - When phosphorylated by **protein kinase A (PKA)** or **CaMKII**, phospholamban's inhibition is relieved, allowing increased calcium uptake. *Regulates sodium and potassium levels* - The regulation of **sodium and potassium levels** is primarily mediated by the **Na+/K+-ATPase pump**, which is distinct from phospholamban's function. - Phospholamban's role is specifically in **calcium handling** within the sarcoplasmic reticulum, particularly in cardiomyocytes. *Transports calcium out of the cell* - Transport of **calcium out of the cell** is primarily performed by the **plasma membrane Ca2+-ATPase (PMCA)** and the **Na+/Ca2+ exchanger (NCX)**. - **SERCA**, which phospholamban regulates, pumps calcium **into the sarcoplasmic reticulum**, not out of the cell. - Phospholamban modulates SERCA activity but does not directly transport calcium itself. *Binds to actin and myosin* - **Actin and myosin** are the primary contractile proteins involved in muscle contraction. - Proteins like **troponin and tropomyosin** bind to these contractile proteins to regulate contraction, whereas phospholamban is a **regulatory protein** affecting calcium handling in the sarcoplasmic reticulum.

Question 434: Upstroke of the action potential would lead to:

A. Cell interior becomes less negative
B. Net current in an inward direction
C. Cell interior becomes more positive
D. All of the options (Correct Answer)

Explanation: ***All of the options*** - The **upstroke** or **depolarization phase** of an action potential is characterized by a rapid influx of **positive ions** (primarily sodium) into the cell. - This influx causes the **cell's membrane potential** to become less negative and then positive relative to the outside, representing an inward net current and making the cell interior more positive. *Net current in an inward direction* - The **upstroke** is driven primarily by the rapid opening of **voltage-gated sodium channels**, allowing **Na+ ions** to rush into the cell. - This movement of positive charge into the cell constitutes an **inward current**, making the inside of the cell more positive. *Cell interior becomes more positive* - As **positively charged sodium ions** flow into the cell during the upstroke, the accumulation of these charges within the cell leads to a change in membrane potential. - This change shifts the membrane potential from a **negative resting state** towards a **positive value**. *Cell interior becomes less negative* - The initial phase of the action potential upstroke involves the **depolarization** of the membrane, meaning the potential moves from its resting negative value (e.g., -70mV) towards zero. - Before becoming positive, the cell interior first becomes **less negative** as it approaches the threshold potential and then rapidly depolarizes.

Question 435: Which of the following functions is primarily associated with neuroactive substances such as substance P, 5-hydroxytryptamine, vasoactive intestinal peptide, somatostatin, and prostaglandins?

A. None of the above.
B. Both modify excitability of nerve endings and affect vascular tone. (Correct Answer)
C. Affect vascular tone only.
D. Modify excitability of nerve endings only.

Explanation: ***Both modify excitability of nerve endings and affect vascular tone.*** - Neurotransmitters like **substance P**, **5-hydroxytryptamine**, **vasoactive intestinal peptide**, **somatostatin**, and **prostaglandins** play diverse roles in neural and vascular physiology. - They are known to modulate the **excitability of nerve endings**, influencing pain transmission and sensory perception, and also have significant effects on **vascular tone**, leading to vasoconstriction or vasodilation. *Affect vascular tone only.* - While these neurotransmitters do influence **vascular tone**, stating this as their *only* function is incomplete and inaccurate. - Their roles extend beyond merely affecting blood vessel diameter. *None of the above.* - This option is incorrect because the aforementioned neurotransmitters clearly have established roles in both nerve excitability and vascular tone. - There is a correct combination of functions among the other choices. *Modify excitability of nerve endings only.* - Similar to the vascular tone only option, limiting their function to just **modifying nerve ending excitability** is an oversimplification. - These substances frequently exert pleiotropic effects on various physiological systems.

Question 436: Which of the following conditions does not involve neuronal degeneration?

A. Aging process of neurons
B. Temporary conduction block (Neuropraxia) (Correct Answer)
C. Growth and differentiation in the fetal nervous system
D. Crush injury to a nerve

Explanation: ***Temporary conduction block (Neuropraxia)*** - **Neuropraxia** is the mildest form of nerve injury, involving a **temporary conduction block** without structural damage to the axon. - It causes a transient loss of function but does not involve **neuronal degeneration** or Wallerian degeneration. *Growth and differentiation in the fetal nervous system* - During fetal development, a significant amount of **neuronal apoptosis** (programmed cell death) occurs. - This process is crucial for sculpting the developing nervous system by eliminating redundant or improperly connected neurons. *Aging process of neurons* - The aging process is associated with some degree of **neuronal loss** and **degeneration**, particularly in specific brain regions. - This can lead to cognitive decline and other neurological changes common in older adults. *Crush injury to a nerve* - A **crush injury** to a nerve typically results in significant **axonal disruption** and **Wallerian degeneration**. - This involves the breakdown of the axon distal to the injury site, which is a form of neuronal degeneration.

Question 437: Decrease in reflex response after repetitive stimulation

A. Fatigue (Correct Answer)
B. Occlusion
C. Spatial summation
D. Temporal summation

Explanation: ***Fatigue*** - **Fatigue** in the context of reflex arcs refers to a temporary decline in the responsiveness of a reflex following repeated or prolonged stimulation. - This decrease is often attributed to the depletion of **neurotransmitter** stores at the presynaptic terminals or changes in the postsynaptic membrane excitability. *Spatial summation* - **Spatial summation** is the process where multiple **presynaptic neurons** simultaneously release neurotransmitters onto a **postsynaptic neuron**, and their combined effects reach the threshold for an action potential. - It involves the integration of signals from different locations at the same time, not a decrease in response due to repetitive stimulation. *Occlusion* - **Occlusion** occurs when the combined effect of stimulating two separate afferent nerves simultaneously is **less than the sum** of their individual effects. - This happens because some interneurons or motoneurons are common to both pathways, and once activated by one, they cannot be activated further by the other, leading to a smaller than expected overall response. *Temporal summation* - **Temporal summation** refers to the rapid firing of a **single presynaptic neuron** that causes successive graded potentials in the postsynaptic neuron to add up, eventually reaching the threshold for an action potential. - This mechanism involves repeated stimulation from a single source over time to *increase* the likelihood of an action potential, which is the opposite of a decrease in reflex response.

Question 438: How is the rate of skeletal muscle relaxation related to calcium ion reuptake by the sarcoplasmic reticulum?

A. When phosphocreatine is metabolized.
B. During ATP hydrolysis.
C. During acetylcholine resynthesis.
D. When free Ca++ is removed from the sarcoplasm. (Correct Answer)

Explanation: ***When free Ca++ is removed from the sarcoplasm*** - Skeletal muscle relaxation occurs as **calcium ions (Ca++)** are actively pumped back into the **sarcoplasmic reticulum (SR)** by **SERCA pumps**. - This removal of Ca++ from the **sarcoplasm** reduces Ca++ concentration around the myofibrils, causing **troponin to return to its original conformation**, which **blocks myosin-binding sites** on actin, leading to muscle relaxation. - The **rate of muscle relaxation is directly proportional to the rate of Ca++ reuptake** by the SR. *When phosphocreatine is metabolized* - **Phosphocreatine** is primarily involved in quickly regenerating **ATP** for muscle contraction, especially during the initial phases of intense activity. - Its metabolism does not directly regulate the rate of calcium reuptake or muscle relaxation. *During ATP hydrolysis* - **ATP hydrolysis** by **myosin heads** is essential for muscle contraction, providing the energy for the power stroke. - It also powers the **SERCA pumps** for calcium reuptake, but the hydrolysis itself is not the rate-determining factor for relaxation—the Ca++ removal is. *During acetylcholine resynthesis* - **Acetylcholine resynthesis** occurs at the **neuromuscular junction** to replenish neurotransmitter stores after nerve stimulation. - This process is upstream of muscle fiber excitation and does not directly regulate the rate of calcium reuptake or muscle relaxation within the muscle cell.

Question 439: Which descending motor pathway primarily controls distal limb muscles?

A. Reticulospinal pathway
B. Vestibulospinal pathway
C. Rubrospinal pathway (Correct Answer)
D. Tectospinal pathway

Explanation: ***Rubrospinal pathway*** - The rubrospinal tract originates in the **red nucleus** and primarily contributes to the control of **flexor muscles**, especially those in the **distal extremities**. - It plays a role in **fine motor control** and **dexterity**, important for manipulating objects. *Reticulospinal pathway* - The reticulospinal tracts are involved in the control of **posture**, **gait**, and **proximal limb movements**. - They provide significant input to **axial and extensor muscles**, which contrast with the distal control function. *Vestibulospinal pathway* - The vestibulospinal tracts are critical for maintaining **balance** and **posture** by controlling **extensor muscles** in response to head movements and gravitational changes. - They primarily influence motor neurons that innervate the **trunk** and **proximal limbs**. *Tectospinal pathway* - The tectospinal tract is involved in coordinating **head and eye movements** in response to visual and auditory stimuli. - It primarily influences the muscles of the **neck** and **upper trunk** rather than distal limb control.

Question 440: Which of the following represents the site of reversal of polarity of myosin molecules in thick muscle filaments?

A. M line (Correct Answer)
B. Z line
C. I band
D. H zone

Explanation: ***M line*** - The **M line** is the central anchor point for thick filaments in a sarcomere, where myosin molecules are arranged in a **tail-to-tail fashion**. - This **tail-to-tail arrangement** at the M line means that the myosin heads on either side point outwards, creating a reversal of polarity where the molecules meet. *Z line* - The **Z line** defines the boundaries of a sarcomere and is where **actin (thin) filaments** are anchored. - It does not involve the arrangement or polarity reversal of myosin molecules. *I band* - The **I band** is the region of a sarcomere that contains only **thin (actin) filaments**. - Myosin molecules are not present in this region, thus no reversal of their polarity occurs here. *H zone* - The **H zone** is the central region of the A band that contains only **thick (myosin) filaments** when the muscle is relaxed. - While it contains myosin, the M line runs through its center, and it's at the M line where the polarity reversal occurs, not uniformly throughout the H zone.

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

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Cardiac Muscle Properties

Practice Questions

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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