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: Duchenne's Muscular Dystrophy is a disease of which of the following?

Muscle contractile proteins. **Explanation:** **Duchenne’s Muscular Dystrophy (DMD)** is an X-linked recessive disorder caused by a mutation in the **DMD gene**, which encodes the protein **Dystrophin**. 1. **Why Option C is Correct:** Dystrophin is a vital structural protein that links the intracellular cytoskeleton (actin) of a muscle fiber to the surrounding extracellular matrix through the cell membrane (sarcolemma). While it is technically a "cytoskeletal" protein, in the context of standard medical examinations like NEET-PG, it is categorized under **muscle contractile/structural proteins** because its absence leads to mechanical instability during contraction, causing membrane tears, calcium influx, and eventual muscle fiber necrosis. 2. **Why Other Options are Incorrect:** * **Option A:** Diseases of the neuromuscular junction include Myasthenia Gravis (post-synaptic) and Lambert-Eaton Syndrome (pre-synaptic), not DMD. * **Option B:** While dystrophin interacts with the sarcolemma, it is primarily an intracellular protein. "Sarcolemmal proteins" usually refers to the Dystrophin-Glycoprotein Complex (DGC) or ion channels. * **Option D:** DMD is a primary myopathy (genetic destruction of muscle), not disuse atrophy. Disuse atrophy occurs when a muscle is healthy but not being stimulated (e.g., limb in a cast). **High-Yield Clinical Pearls for NEET-PG:** * **Inheritance:** X-linked recessive (most common and severe muscular dystrophy). * **Clinical Signs:** **Gower’s sign** (using hands to "climb up" the body to stand) and **Pseudohypertrophy of calves** (muscle replaced by fat/fibrosis). * **Diagnosis:** Elevated Creatine Kinase (CK-MM) levels; Muscle biopsy shows variation in fiber size; Genetic testing is the gold standard. * **Becker’s MD:** A milder form due to *truncated* dystrophin (DMD is a total *absence*).

Q: The spike in action potential of a nerve is due to which ion?

Sodium (Na+). **Explanation:** The "spike" of an action potential refers to the rapid **depolarization** phase. This occurs when a stimulus reaches the threshold potential (approx. -55mV), triggering the opening of **voltage-gated Sodium (Na+) channels**. 1. **Why Sodium (Na+) is correct:** According to the electrochemical gradient, Na+ ions are more concentrated outside the cell. When these channels open, there is a massive, rapid **influx of Na+** into the neuron. This influx reverses the membrane polarity from negative to positive (reaching up to +35mV), creating the characteristic "spike" on the oscilloscope. 2. **Why other options are incorrect:** * **Potassium (K+):** K+ is primarily responsible for **repolarization** and hyperpolarization. After the spike, Na+ channels close and voltage-gated K+ channels open, leading to an **efflux of K+** out of the cell to restore the resting membrane potential. * **Chloride (Cl-):** Cl- influx typically causes **hyperpolarization** (making the interior more negative), which inhibits the generation of an action potential (e.g., GABAergic inhibitory post-synaptic potentials). **High-Yield NEET-PG Pearls:** * **Tetrodotoxin (Pufferfish) & Saxitoxin:** Block voltage-gated Na+ channels, preventing the spike and causing paralysis. * **Local Anesthetics (Lidocaine):** Work by blocking these same Na+ channels from the internal side of the membrane. * **Overshoot:** The portion of the action potential where the membrane potential is positive (>0 mV). * **Resting Membrane Potential (RMP):** Primarily determined by **K+ permeability** via leak channels.

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.

Question 1

What is the primary function of a muscle spindle?

Accepted Answer

Provides feedback to maintain muscle length

Answer

Regulates the withdrawal reflex

Answer

Maintains muscle tone

Answer

Acts as a receptor for the inverse stretch reflex

Question 2

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 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

What is the resting membrane potential of a neuron?

Accepted Answer

-70 mV

Answer

-700 mV

Answer

-7 mV

Answer

-170 mV

Question 5

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 6

Duchenne's Muscular Dystrophy is a disease of which of the following?

Accepted Answer

Muscle contractile proteins

Answer

Neuromuscular junction

Answer

Sarcolemmal proteins

Answer

Disuse atrophy due to muscle weakness

Question 7

The spike in action potential of a nerve is due to which ion?

Accepted Answer

Sodium (Na+)

Answer

Potassium (K+)

Answer

Chloride (Cl-)

Answer

All of the above

Question 8

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 9

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 10

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

Accepted Answer

Pressure

Answer

Hypoxia

Answer

Local anesthesia

Answer

Temperature

Nerve and Muscle Physiology — MCQs

Nerve and Muscle Physiology — MCQs

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