Nerve and Muscle Physiology Practice Questions

Q: The major source of calcium for contraction of skeletal muscle is:

Sarcoplasmic reticulum (SR). **Explanation:** The correct answer is **Sarcoplasmic Reticulum (SR)**. In skeletal muscle, the process of excitation-contraction coupling relies almost exclusively on intracellular calcium stores. **1. Why Sarcoplasmic Reticulum is correct:** Skeletal muscle fibers contain an extensive network of Sarcoplasmic Reticulum (SR) that acts as a specialized reservoir for calcium ions ($Ca^{2+}$). When an action potential travels down the **T-tubules**, it activates **Dihydropyridine (DHP) receptors**. These receptors are mechanically linked to **Ryanodine receptors (RyR1)** on the SR membrane. This mechanical coupling triggers the massive release of $Ca^{2+}$ from the SR cisternae into the sarcoplasm, which then binds to **Troponin C** to initiate contraction. **2. Why other options are incorrect:** * **Extracellular Fluid (ECF):** Unlike cardiac or smooth muscle, skeletal muscle contraction is **not** dependent on ECF calcium. The DHP receptor acts as a voltage sensor, not a primary calcium channel for influx. * **Cytosol:** The cytosol is where calcium acts, but it is not the *source*. In a resting state, cytosolic calcium levels are kept extremely low to allow for muscle relaxation. * **Mitochondria:** While mitochondria can sequester some calcium, their primary role is ATP production; they do not provide the calcium required for the contractile cycle. **High-Yield NEET-PG Pearls:** * **Calsequestrin:** The protein inside the SR that binds and buffers calcium, allowing for high-capacity storage. * **SERCA Pump:** The $Ca^{2+}$-ATPase pump responsible for pumping calcium back into the SR to induce muscle relaxation. * **Malignant Hyperthermia:** A clinical condition caused by a mutation in the **Ryanodine receptor (RyR1)**, leading to excessive calcium release from the SR in response to certain anesthetics.

Q: Which type of muscle exercise involves the muscle changing length against a constant load?

Isotonic exercise. **Explanation:** The correct answer is **Isotonic exercise**. This type of muscle contraction occurs when the **tension remains constant** while the **muscle length changes** (either shortening or lengthening) to move a load. The term is derived from the Greek words *"iso"* (same) and *"tonos"* (tension). Isotonic exercises are further divided into **concentric** (muscle shortens, e.g., upward phase of a bicep curl) and **eccentric** (muscle lengthens under tension, e.g., controlled lowering of a weight). **Why other options are incorrect:** * **Isometric exercise:** In this type, the **length of the muscle remains constant** while the **tension increases**. No external work is performed (Work = Force × Displacement; here displacement is zero). An example is pushing against a stationary wall. * **Aerobic exercise:** This refers to the metabolic pathway used (oxidative phosphorylation) rather than the mechanics of muscle length or tension. While many isotonic exercises are aerobic, the terms are not synonymous. **High-Yield NEET-PG Pearls:** 1. **Isokinetic exercise:** A type of exercise where the muscle contracts at a **constant velocity** throughout the entire range of motion (requires specialized equipment). 2. **Energy Expenditure:** Isotonic contractions perform external work and consume more energy compared to isometric contractions. 3. **Clinical Correlation:** Isometric exercises cause a more significant rise in **mean arterial pressure (MAP)** due to the compression of blood vessels within the contracting muscle, leading to increased peripheral resistance. 4. **Golgi Tendon Organs:** These are sensitive to changes in muscle **tension** (prominent in isometric), whereas **Muscle Spindles** are sensitive to changes in muscle **length** (prominent in isotonic).

Q: Following traumatic peripheral nerve transection, re-growth usually occurs at which of the following rates?

1 mm per day. **Explanation:** The correct answer is **1 mm per day (Option B)**. Following a peripheral nerve injury (transection), the distal segment undergoes **Wallerian degeneration**, while the proximal segment undergoes **regeneration**. Once the axonal sprouts cross the lesion site and enter the distal endoneurial tubes, they grow toward the target organ. This regenerative process is driven by slow axonal transport and typically occurs at a rate of **1 mm/day** (or approximately 1 inch per month). **Analysis of Incorrect Options:** * **Option A (0.1 mm/day):** This rate is too slow. While initial "die-back" and the latent period before sprouting may delay visible progress, the actual growth rate is significantly higher. * **Option C (5 mm/day):** This is an overestimation. While some experimental conditions or very young patients might show slightly faster rates, the standard physiological average used in clinical practice is 1 mm/day. * **Option D (1 cm/day):** This is physiologically impossible for human nerve regeneration. Such a rapid rate would lead to recovery in days for injuries that typically take months. **High-Yield Facts for NEET-PG:** * **Hoffmann-Tinel Sign:** A clinical test where distal percussion over the regenerating nerve elicits a tingling sensation. The distal-most point of tingling indicates the extent of axonal regrowth, allowing clinicians to track the 1 mm/day progress. * **Factors affecting regeneration:** Regeneration is faster in **crush injuries** (Sunderland Grade 2) than in complete transections because the endoneurial sheath remains intact to guide the axons. * **Proximal vs. Distal:** Regeneration is generally faster in proximal segments compared to distal segments. * **CNS vs. PNS:** Regeneration occurs in the PNS (aided by Schwann cells) but is virtually absent in the CNS due to inhibitory factors like Nogo-A and the absence of a basement membrane.

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.

Question 1

The major source of calcium for contraction of skeletal muscle is:

Accepted Answer

Sarcoplasmic reticulum (SR)

Answer

Extracellular fluid (ECF)

Answer

Cytosol

Answer

Mitochondria

Question 2

After-hyperpolarization during nerve conduction is due to:

Accepted Answer

Slow entry of K+

Answer

Slow entry of Na+

Answer

Pumping of Na+ outside

Answer

Pumping of K+ outside

Question 3

Which type of muscle exercise involves the muscle changing length against a constant load?

Accepted Answer

Isotonic exercise

Answer

Isometric exercise

Answer

Aerobic exercise

Answer

None of the above

Question 4

Following traumatic peripheral nerve transection, re-growth usually occurs at which of the following rates?

Accepted Answer

1 mm per day

Answer

0.1 mm per day

Answer

5 mm per day

Answer

1 cm per day

Question 5

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 6

Which protein is shown in the thin filament as illustrated?

Accepted Answer

Troponin

Answer

Titin

Answer

Nebulin

Answer

Dystrophin

Question 7

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 8

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 9

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 10

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

Nerve and Muscle Physiology — MCQs

Nerve and Muscle Physiology — MCQs

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