Nerve and Muscle Physiology Practice Questions

Q: Which type of motor unit is of prime importance in generating the muscle power necessary for the maintenance of posture?

Low threshold, fatigue-resistant. ### Explanation **1. Why Option A is Correct:** The maintenance of posture requires muscles to remain contracted for prolonged periods without tiring. This task is performed by **Type I (Slow-Twitch) muscle fibers**, which form **low-threshold, fatigue-resistant** motor units. * **Low Threshold:** According to **Henneman’s Size Principle**, smaller motor neurons (which innervate Type I fibers) have a lower threshold for excitation and are recruited first. * **Fatigue-Resistant:** These fibers are rich in mitochondria and myoglobin (Red muscle), relying on aerobic metabolism to provide a steady supply of ATP, making them ideal for sustained activities like standing or sitting. **2. Why Other Options are Incorrect:** * **Option B & D (High threshold, fatigable/high force):** These describe **Type IIb (Fast-Twitch)** motor units. They are recruited only when high force or rapid movement is needed (e.g., sprinting or jumping). They rely on anaerobic glycolysis and fatigue rapidly, making them unsuitable for postural maintenance. * **Option C (Intrafusal, gamma controlled):** Intrafusal fibers are part of the **muscle spindle** and are responsible for sensing muscle length (proprioception) rather than generating the contractile power needed to maintain posture. **3. High-Yield Facts for NEET-PG:** * **Henneman’s Size Principle:** Motor units are recruited in order of increasing size (Small/Type I → Large/Type II). * **Soleus vs. Gastrocnemius:** The Soleus is a classic "postural muscle" dominated by Type I fibers, whereas the Gastrocnemius contains more Type II fibers for explosive movements. * **Myoglobin Content:** Type I fibers are "Red" (high myoglobin/oxidative), while Type IIb fibers are "White" (low myoglobin/glycolytic). * **ATPase Activity:** Type II fibers have high myosin ATPase activity, leading to faster contraction speeds compared to Type I.

Q: Which protein is shown in the thin filament as illustrated?

Troponin. ***Troponin*** - **Troponin** is a regulatory protein complex located on the **thin filament** (actin), consisting of **troponin C** (calcium-binding), **troponin I** (inhibitory), and **troponin T** (tropomyosin-binding). - Functions in **calcium-mediated muscle contraction** by regulating the interaction between **actin and myosin** through conformational changes in the **troponin-tropomyosin complex**. *Titin* - **Titin** is primarily associated with the **thick filament** (myosin) and acts as a **molecular spring** providing elasticity to muscle fibers. - It spans from the **Z-line to M-line** in the sarcomere but is not a component of the thin filament regulatory system. *Nebulin* - **Nebulin** is present on thin filaments but serves as a **molecular ruler** that determines **actin filament length** during muscle development. - Unlike troponin, it does not function as a **regulatory protein** for calcium-mediated contraction and is not part of the excitation-contraction coupling mechanism. *Dystrophin* - **Dystrophin** is a **cytoskeletal protein** that links intracellular **actin** to the **extracellular matrix** through the **dystrophin-glycoprotein complex**. - It provides **structural support** to muscle fibers but is not involved in the **thin filament regulatory mechanism** or direct contraction processes.

Q: What is true about the Golgi tendon organ?

Senses muscle tension. The **Golgi Tendon Organ (GTO)** is a high-threshold mechanoreceptor located in series with extrafusal muscle fibers at the muscle-tendon junction. ### **Explanation of the Correct Answer** **D. Senses muscle tension:** The primary function of the GTO is to monitor the **force or tension** generated within a muscle. When a muscle contracts (either isometrically or isotonically), the collagen fibers in the tendon are pulled taut, compressing the nerve endings of the **Ib afferent fibers**. This triggers the **inverse stretch reflex** (autogenic inhibition), which inhibits the agonist muscle to prevent damage from excessive tension. ### **Why Other Options are Incorrect** * **A. Senses dynamic length:** This is the function of the **Muscle Spindle** (specifically the nuclear bag fibers via Type Ia afferents). Spindles are arranged in *parallel* to sense length, while GTOs are in *series* to sense tension. * **B. Reciprocal innervation:** This term usually refers to the **Stretch Reflex**, where the agonist contracts and the antagonist is inhibited. The GTO mediates **autogenic inhibition**, where the muscle experiencing tension is inhibited. * **C. Stimulated by alpha motor neuron:** GTOs are **sensory receptors** (afferent). Alpha motor neurons are efferent fibers that stimulate extrafusal muscle contraction, which in turn *activates* the GTO, but they do not innervate the GTO itself. ### **High-Yield NEET-PG Pearls** * **Afferent Fiber:** GTO uses **Ib fibers** (fast-conducting, myelinated). * **Arrangement:** GTO is in **series**; Muscle Spindle is in **parallel**. * **Reflex Type:** GTO mediates the **Inverse Stretch Reflex** (disynaptic reflex). * **Function:** Acts as a protective mechanism to prevent avulsion or muscle tearing during heavy loading.

Q: Group A nerve fibers are most susceptible to which of the following?

Pressure. The susceptibility of nerve fibers to different insults depends on their diameter and myelination. This concept is a high-yield topic based on the **Erlanger-Gasser classification**. ### **Explanation** **Group A nerve fibers** are the thickest, most heavily myelinated fibers. Because they have the largest diameter, they are physically more vulnerable to mechanical compression. When **pressure** is applied to a nerve trunk, these large-diameter fibers are the first to undergo mechanical deformation and ischemia of their vasa nervorum, leading to a rapid conduction block. ### **Analysis of Other Options** * **B. Hypoxia:** **Group B fibers** (preganglionic autonomic fibers) are the most susceptible to hypoxia. Group A fibers are intermediate, and Group C fibers are the most resistant. * **C. Local Anesthesia:** **Group C fibers** (small, unmyelinated pain fibers) are the most susceptible to local anesthetics. Because they lack myelin and have a small surface area, the anesthetic can easily penetrate and block the sodium channels. * **D. Temperature:** While extreme temperatures can affect nerve conduction, it is not the primary differentiating factor used in the Erlanger-Gasser susceptibility hierarchy. ### **High-Yield NEET-PG Pearls** To remember the order of susceptibility (from most to least sensitive), use the mnemonic **"PHL"**: 1. **P**ressure: **A** > B > C (Think: "A" is big and easily squashed) 2. **H**ypoxia: **B** > A > C 3. **L**ocal Anesthesia: **C** > B > A (Think: "C" is small and easily drugged) **Clinical Correlation:** "Saturday Night Palsy" (radial nerve compression) primarily affects Group A motor and sensory fibers first, causing motor weakness and loss of touch before affecting pain sensation.

Q: In tetany, hyperexcitability is due to which of the following mechanisms?

Low calcium levels cause increased sodium permeability.. **Mechanism of Hypocalcemic Tetany** The correct answer is **A: Low calcium levels cause increased sodium permeability.** **Explanation:** In the extracellular fluid (ECF), calcium ions ($Ca^{2+}$) play a critical role in stabilizing the resting membrane potential of excitable tissues (nerve and muscle). Calcium ions normally bind to the external surface of voltage-gated sodium channels, increasing the voltage threshold required to open them. When ECF calcium levels drop (**hypocalcemia**), there are fewer positive charges to "mask" the negative charges on the cell membrane. This reduces the threshold for excitation, making the sodium channels much easier to open. Consequently, sodium permeability increases even with slight stimuli, leading to spontaneous depolarization and repetitive firing of action potentials. This manifests clinically as involuntary muscle contractions (tetany). **Analysis of Incorrect Options:** * **B & C:** These options are incorrect because tetany is primarily a phenomenon of the **cell membrane threshold** and sodium influx, rather than a direct inhibition of potassium release or intracellular calcium sequestration. * **D:** While intracellular calcium is required for muscle contraction (excitation-contraction coupling), the *hyperexcitability* (the tendency to fire) is triggered by the **extracellular** calcium deficit affecting sodium channel gating. **High-Yield NEET-PG Pearls:** * **Trousseau’s Sign:** Carpopedal spasm induced by inflating a BP cuff above systolic pressure (more sensitive than Chvostek's). * **Chvostek’s Sign:** Twitching of facial muscles upon tapping the facial nerve. * **ECG Finding:** Hypocalcemia characteristically causes **prolonged QT interval**. * **Rule of Thumb:** "Hypocalcemia excites, Hypercalcemia excites (the heart) but depresses (the nerves)."

Q: Which statement is true regarding Type-2 muscle fibers?

Increased Isoenzyme ATPase activity. ### Explanation **Correct Answer: C. Increased Isoenzyme ATPase activity** Skeletal muscle fibers are classified into Type 1 (Slow-twitch) and Type 2 (Fast-twitch) based on their metabolic profile and contraction speed. The speed of muscle contraction is directly proportional to the **Myosin ATPase activity**. Type 2 fibers possess a specific isoenzyme of myosin ATPase that hydrolyzes ATP rapidly, allowing for fast cross-bridge cycling and quick, powerful contractions. **Analysis of Options:** * **Option A & B (Incorrect):** These describe **Type 1 (Slow Oxidative) fibers**. Type 1 fibers are rich in myoglobin (giving them a red color) and mitochondria to support aerobic metabolism for endurance. Type 2 fibers have low myoglobin content and appear pale/white. * **Option D (Incorrect):** Type 2 fibers (specifically Type 2b) have a **high glycolytic capacity**, not moderate. They rely on anaerobic glycolysis for rapid energy production, making them prone to early fatigue due to lactic acid accumulation. --- ### High-Yield Clinical Pearls for NEET-PG: * **Type 1 Fibers (Slow-twitch):** "One Slow Red Ox" (Type **1**, **Slow**-twitch, **Red** color, **Ox**idative metabolism). Found in postural muscles like the **Soleus**. * **Type 2 Fibers (Fast-twitch):** "Two Fast White Sugar" (Type **2**, **Fast**-twitch, **White** color, **Sugar**/Glycolytic metabolism). Found in muscles used for sprinting or weightlifting, such as the **Gastrocnemius**. * **Intermediate Fibers (Type 2a):** These are fast-twitch but have both oxidative and glycolytic capacities (Fast Oxidative Glycolytic). * **Order of Recruitment:** According to **Henneman’s Size Principle**, smaller Type 1 motor units are recruited first, followed by larger Type 2 units as force requirements increase.

Question 1

Which type of motor unit is of prime importance in generating the muscle power necessary for the maintenance of posture?

Accepted Answer

Low threshold, fatigue-resistant

Answer

High threshold, fatigable

Answer

Intrafusal, gamma controlled

Answer

High threshold, high force

Question 2

Which protein is shown in the thin filament as illustrated?

Accepted Answer

Troponin

Answer

Titin

Answer

Nebulin

Answer

Dystrophin

Question 3

Unmyelinated nerve fibers differ from myelinated nerve fibers in that they:

Accepted Answer

Have no nodes of Ranvier

Answer

Have increased excitability

Answer

Have no power of regeneration

Answer

Are not associated with Schwann cells

Question 4

Which statement about rheobase is true?

Accepted Answer

The minimum strength of a stimulus required to produce an action potential.

Answer

The minimum time required to produce an action potential.

Answer

The maximum time required to produce an action potential.

Answer

The maximum strength of a stimulus required to produce an action potential.

Question 5

Absolute refractory period is due to which of the following?

Accepted Answer

Closure of inactive gates of sodium channel

Answer

Opening of calcium channels

Answer

Closure of potassium channels

Answer

Closure of active gates of sodium channel

Question 6

What is true about the Golgi tendon organ?

Accepted Answer

Senses muscle tension

Answer

Senses the dynamic length of the muscle

Answer

Is involved in reciprocal innervation

Answer

Is stimulated by the alpha motor neuron

Question 7

Group A nerve fibers are most susceptible to which of the following?

Accepted Answer

Pressure

Answer

Hypoxia

Answer

Local anesthesia

Answer

Temperature

Question 8

In tetany, hyperexcitability is due to which of the following mechanisms?

Accepted Answer

Low calcium levels cause increased sodium permeability.

Answer

Prevent potassium release.

Answer

Prevent sodium and intracellular calcium release.

Answer

Increased intracellular calcium produces the generation of an action potential.

Question 9

Which statement is true regarding Type-2 muscle fibers?

Accepted Answer

Increased Isoenzyme ATPase activity

Answer

Increased myoglobin content

Answer

Slow oxidative and red in color

Answer

Moderate glycolytic capacity

Question 10

Which of the following statements is true regarding total heat production during muscle contraction?

Accepted Answer

Activation heat is produced when a muscle is contracting.

Answer

Initial heat is due to basal metabolic processes.

Answer

Relaxation heat is present in both isotonic and isometric contractions.

Answer

Recovery heat of muscle is greater than the initial heat.

Nerve and Muscle Physiology — MCQs

Nerve and Muscle Physiology — MCQs

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