Nerve and Muscle Physiology Practice Questions

Q: Proprioception is carried by which type of nerve fibers?

A alpha fibers. **Explanation:** Proprioception (the sense of body position and movement) requires the fastest possible transmission to the central nervous system to maintain balance and coordinate motor activity. This is achieved via **A-alpha (Aα) fibers**, which are the thickest and most heavily myelinated nerve fibers. **1. Why A-alpha is correct:** According to the Erlanger-Gasser classification, **A-alpha fibers** have the largest diameter (12–20 μm) and the fastest conduction velocity (70–120 m/s). They carry sensory information from **muscle spindles (Ia)** and **Golgi tendon organs (Ib)**, which are the primary receptors for proprioception. **2. Analysis of Incorrect Options:** * **A-beta (Aβ) fibers:** These are slightly smaller and slower than A-alpha. They primarily carry sensations of **fine touch, pressure, and vibration**. * **B fibers:** These are preganglionic autonomic fibers. They are small, myelinated, and have much slower conduction velocities than A-type fibers. * **C fibers:** These are the smallest, **unmyelinated** fibers. They carry "slow" pain (dull, aching), temperature, and crude touch. They have the slowest conduction velocity (0.5–2 m/s). **Clinical Pearls & High-Yield Facts:** * **Order of Susceptibility:** * **Hypoxia:** B > A > C (B fibers are most sensitive). * **Pressure:** A > B > C (A fibers are most sensitive; this is why your foot "falls asleep" when compressed). * **Local Anesthetics:** C > B > A (C fibers are most sensitive; pain is blocked before motor function). * **Fastest to Slowest:** Aα > Aβ > Aγ > Aδ > B > C. * **A-delta (Aδ) fibers** carry "fast" pain (sharp, localized) and cold temperature.

Q: Tetany may be a feature of the following conditions except?

Hyponatremia. **Explanation:** Tetany is a state of increased neuromuscular excitability characterized by carpopedal spasms, paresthesia, and laryngospasm. It is primarily driven by a decrease in the concentration of **ionized calcium ($Ca^{2+}$)** in the extracellular fluid. **Why Hyponatremia is the Correct Answer:** Hyponatremia (low serum sodium) does not cause tetany. In fact, severe hyponatremia typically leads to CNS symptoms like cerebral edema, seizures, and coma, but it does not increase neuromuscular irritability in the peripheral nerves. Therefore, it is the "except" in this list. **Analysis of Other Options:** * **Hyperventilation:** This leads to respiratory alkalosis. Alkalosis promotes the binding of ionized calcium to albumin, reducing the physiologically active $Ca^{2+}$ fraction, thereby triggering tetany. * **Hypokalemic Alkalosis:** Metabolic alkalosis (often seen in Conn’s syndrome or vomiting) similarly reduces ionized calcium levels [1]. While hypokalemia itself usually causes muscle weakness, the concomitant **alkalosis** is a classic trigger for tetany [1]. * **Hypomagnesemia:** Magnesium is a co-factor for adenylate cyclase; its deficiency leads to impaired PTH secretion and end-organ resistance to PTH, resulting in secondary hypocalcemia and tetany. **NEET-PG High-Yield Pearls:** 1. **Trousseau’s Sign:** Induction of carpal spasm by inflating a BP cuff above systolic pressure for 3 minutes (more sensitive than Chvostek’s). 2. **Chvostek’s Sign:** Tapping the facial nerve leads to twitching of facial muscles. 3. **The Ionized Calcium Rule:** Total calcium may be normal in alkalosis, but tetany occurs because the **ionized (free) fraction** decreases. 4. **Refractory Hypocalcemia:** If hypocalcemia does not respond to calcium gluconate, always check and correct **Magnesium** levels first.

Q: What is the chronaxie minimum in nerves?

Myelinated nerve. ### Explanation **Concept Overview:** Chronaxie is defined as the **minimum time** required to excite a tissue when using a stimulus intensity equal to twice the rheobase (the minimum current required for excitation). It is a measure of **excitability**: the shorter the chronaxie, the more excitable the tissue. **Why Myelinated Nerves are Correct:** Myelinated nerves (like Type A fibers) are designed for rapid signal transmission. Due to the presence of the myelin sheath and the Nodes of Ranvier, these fibers have a high density of voltage-gated sodium channels and low membrane capacitance. This makes them highly excitable, resulting in the **shortest chronaxie** among all nerve types. **Analysis of Incorrect Options:** * **Unmyelinated Nerves:** These fibers (like Type C fibers) lack myelin, leading to slower conduction velocities and lower excitability. Consequently, they have a **longer chronaxie** compared to myelinated fibers. * **Mixed Nerves:** A mixed nerve contains a variety of fibers (myelinated, unmyelinated, sensory, and motor). Its chronaxie would be an average or representative of its constituent parts, but it is not the "minimum" value. * **Sensory Nerves:** While many sensory nerves are myelinated, this category also includes unmyelinated fibers (e.g., slow pain). Therefore, "myelinated nerve" is a more precise physiological descriptor for minimum chronaxie. **High-Yield NEET-PG Pearls:** * **Excitability Relationship:** Chronaxie is inversely proportional to excitability ($Chronaxie \propto 1/Excitability$). * **Order of Chronaxie:** Myelinated Nerve < Unmyelinated Nerve < Skeletal Muscle < Cardiac Muscle < Smooth Muscle. * **Clinical Use:** Chronaxie is used in electrodiagnosis to assess nerve regeneration or muscle denervation (denervated muscles show a significantly increased chronaxie). * **Rheobase:** The minimum strength of current (intensity) required to evoke a response if applied for an infinite duration.

Q: What is true about an action potential?

A threshold stimulus is required for its generation.. ### Explanation **Correct Option: D. A threshold stimulus is required for its generation.** An action potential (AP) is an "all-or-none" electrical event. For an AP to occur, the membrane potential must be depolarized to a specific level known as the **threshold potential** (typically -55 mV in neurons). Once this threshold is reached, voltage-gated sodium channels open rapidly, triggering a self-propagating depolarization. If the stimulus is sub-threshold, no AP is generated. **Analysis of Incorrect Options:** * **A. It is a decremental phenomenon:** This is incorrect. Action potentials are **non-decremental**; they maintain a constant amplitude and shape as they propagate along the axon. Local potentials (like EPSPs or IPSPs), however, are decremental. * **B. It does not obey the all-or-none phenomenon:** This is incorrect. The AP strictly follows the **All-or-None Law**. If the threshold is met, a full-strength AP occurs; if not, nothing happens. Increasing the stimulus strength beyond the threshold does not increase the AP's amplitude. * **C. Potassium ions move from ECF to ICF:** This is incorrect. During the repolarization phase of an AP, voltage-gated **potassium channels open**, causing $K^+$ to move **out of the cell** (from ICF to ECF) down its electrochemical gradient. **High-Yield NEET-PG Pearls:** * **Depolarization phase:** Primarily due to $Na^+$ influx. * **Repolarization phase:** Primarily due to $K^+$ efflux. * **Absolute Refractory Period:** Occurs during the peak of AP when $Na^+$ channels are in an inactivated state; no second AP can be fired regardless of stimulus strength. * **Myelination:** Increases conduction velocity via **Saltatory Conduction** (jumping between Nodes of Ranvier).

Q: Z-lines are anchored to the sarcolemma by:

Desmin. **Explanation:** The correct answer is **Desmin**. In skeletal muscle, the structural integrity and spatial alignment of myofibrils are maintained by a complex network of intermediate filaments. **Desmin** is the primary intermediate filament that encircles the Z-lines of adjacent myofibrils, linking them to each other and anchoring the entire Z-disk complex to the sarcolemma (cell membrane) via costameres. This ensures that all myofibrils contract in synchrony and maintains the lateral registration of the sarcomeres. **Analysis of Incorrect Options:** * **Titin (Option A):** This is the largest known protein. It acts as a molecular spring, connecting the Z-line to the M-line, providing passive elasticity to the muscle and centering the myosin filaments. * **Dystrophin (Option B):** While it also links the cytoskeleton to the sarcolemma, it specifically connects **F-actin** (not the Z-line directly) to the dystroglycan complex in the membrane. Its deficiency leads to Duchenne Muscular Dystrophy. * **Actinin (Option D):** Specifically **α-actinin**, this protein is located *within* the Z-line. Its primary role is to anchor the plus ends of actin (thin) filaments to the Z-disk, rather than anchoring the Z-disk to the sarcolemma. **High-Yield Clinical Pearls for NEET-PG:** * **Costameres:** These are the functional units (containing desmin, vinculin, and dystrophin) that couple the sarcomere to the extracellular matrix. * **Nebulin:** Acts as a "molecular ruler" to regulate the length of actin filaments. * **Desmin-related Myopathy:** Mutations in the desmin gene lead to myofibrillar myopathies characterized by muscle weakness and cardiac conduction blocks.

Q: Which of the following are regulator proteins?

Both Troponin and Tropomyosin. In skeletal and cardiac muscle contraction, proteins are categorized based on their function into contractile, regulatory, and structural proteins. [2] **Understanding the Concept:** The correct answer is **Both Troponin and Tropomyosin** because these two proteins work together to regulate the interaction between actin and myosin. [2] * **Tropomyosin:** A long, rod-like protein that winds around the actin filament. In a resting state, it physically covers the myosin-binding sites on actin, preventing contraction. * **Troponin:** A complex of three subunits (TnT, TnI, and TnC). When calcium levels rise, calcium binds to **Troponin C**, causing a conformational change that pulls tropomyosin away from the binding sites, allowing the "power stroke" to occur. **Analysis of Options:** * **Option A (Troponin):** While correct, it is incomplete on its own as tropomyosin also serves a regulatory role. * **Option B (Tropomyosin):** Similarly, it is a regulatory protein but does not function independently of the troponin complex. * **Option D (None):** Incorrect, as both proteins are the primary regulators of the sliding filament mechanism. [2] **High-Yield NEET-PG Pearls:** 1. **Contractile Proteins:** Actin (thin filament) and Myosin (thick filament). [2] 2. **Structural Proteins:** Titin (largest protein, provides elasticity), Nebulin, and Dystrophin (defective in Duchenne Muscular Dystrophy). [1] 3. **Clinical Marker:** **Troponin I and T** are highly specific cardiac biomarkers used in the diagnosis of Myocardial Infarction (MI). 4. **Calcium Source:** In skeletal muscle, calcium is released from the Sarcoplasmic Reticulum; in cardiac muscle, it involves Calcium-Induced Calcium Release (CICR). [1]

Question 1

Hyponatremia has no effect on the resting membrane potential (RMP) because:

Accepted Answer

The equilibrium potential for Na+ is +60 mV.

Answer

The cell membrane is impermeable to Na+.

Answer

Extracellular fluid Na+ concentration is high.

Answer

Intracellular K+ concentration is high.

Question 2

Proprioception is carried by which type of nerve fibers?

Accepted Answer

A alpha fibers

Answer

A beta fibers

Answer

B fibers

Answer

C fibers

Question 3

Tetany may be a feature of the following conditions except?

Accepted Answer

Hyponatremia

Answer

Hyperventilation

Answer

Hypokalemic alkalosis

Answer

Hypomagnesemia

Question 4

What is the chronaxie minimum in nerves?

Accepted Answer

Myelinated nerve

Answer

Mixed nerves

Answer

Unmyelinated nerve

Answer

Sensory nerves

Question 5

What is true about an action potential?

Accepted Answer

A threshold stimulus is required for its generation.

Answer

It is a decremental phenomenon.

Answer

It does not obey the all-or-none phenomenon.

Answer

Potassium ions move from the extracellular fluid to the intracellular fluid.

Question 6

Z-lines are anchored to the sarcolemma by:

Accepted Answer

Desmin

Answer

Titin

Answer

Dystrophin

Answer

Actinin

Question 7

What is an auxotonic contraction of a muscle?

Accepted Answer

A contraction where the muscle length changes and the tension changes throughout the contraction.

Answer

A contraction where the tension developed changes when the length of the muscle changes.

Answer

A contraction where the muscle length is constant but tension changes.

Answer

A contraction where the muscle length changes but tension remains constant.

Question 8

Maximum number of sodium channels per square micrometer is present in which location?

Accepted Answer

Nodes of Ranvier

Answer

Cell body

Answer

Axon terminal

Answer

Surface of myelin

Question 9

Which of the following statements is true about myoglobin?

Accepted Answer

Myoglobin has a higher O2 affinity compared to that of hemoglobin.

Answer

Myoglobin binds to 4 moles of O2 per mole.

Answer

The myoglobin dissociation curve has a parabolic profile.

Answer

The myoglobin dissociation curve shows the Bohr effect.

Question 10

Which of the following are regulator proteins?

Accepted Answer

Both Troponin and Tropomyosin

Answer

Troponin

Answer

Tropomyosin

Answer

None of the above

Nerve and Muscle Physiology — MCQs

Nerve and Muscle Physiology — MCQs

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