Nerve and Muscle Physiology Practice Questions

Q: Which of the following muscles has the maximum density of white muscle fibers?

Extraocular muscles. **Explanation:** The classification of muscle fibers into **Red (Type I)** and **White (Type II)** is based on their metabolic profile and contraction speed. White muscle fibers (Type II) are characterized by high myosin ATPase activity, low myoglobin content, and a reliance on anaerobic glycolysis. They are designed for **rapid, short bursts of activity** but fatigue easily. **Why Extraocular Muscles are correct:** The extraocular muscles (EOMs) require the fastest contraction speeds in the human body to perform saccadic eye movements. Consequently, they possess the highest density of **Type II (White) fibers**. These muscles are specialized for precision and extreme velocity rather than sustained weight-bearing or posture, making them the classic example of white muscle predominance. **Analysis of Incorrect Options:** * **Calf Muscle (Gastrocnemius/Soleus):** The Soleus is a classic "Red" muscle (Type I) used for maintaining standing posture. While the Gastrocnemius has mixed fibers, it does not match the white fiber density of the EOMs. * **Back Muscles (Erector Spinae):** These are "Antigravity" muscles. They consist predominantly of Type I (Red) fibers because they must remain contracted for long periods to maintain posture without fatiguing. * **Hip Muscles (Gluteus Maximus):** These are large, powerful muscles used for locomotion and posture. They contain a high proportion of Type I fibers to support sustained activity. **High-Yield Clinical Pearls for NEET-PG:** * **Type I (Red):** "One Slow Red Ox" — Type **I**, **Slow**-twitch, **Red** (high myoglobin), **Ox**idative phosphorylation (high mitochondria). Found in marathon runners. * **Type II (White):** Fast-twitch, glycolytic, low mitochondria. Found in sprinters and extraocular muscles. * **Laryngeal muscles** also have a high density of fast-twitch fibers, second only to extraocular muscles.

Q: Energy in muscles is stored as:

Phosphocreatine. **Explanation:** The primary form of energy **storage** in skeletal muscle is **Phosphocreatine (Creatine Phosphate)**. While ATP is the direct source of energy for muscle contraction, its concentration within the muscle cell is very low—sufficient only to sustain maximal contraction for about 1 to 2 seconds. To maintain activity, muscles store high-energy phosphate bonds in the form of Phosphocreatine, which is present at concentrations **5 to 6 times higher than ATP**. * **Why Phosphocreatine is correct:** It acts as a "buffer" or a reservoir of high-energy phosphate. When ATP is hydrolyzed to ADP during contraction, the enzyme **Creatine Kinase (CK)** rapidly transfers a phosphate group from Phosphocreatine back to ADP to reconstitute ATP. This system (the Phosphagen system) provides energy for short bursts of maximal effort (approx. 5–8 seconds). * **Why ATP is incorrect:** ATP is the immediate energy *currency*, not the storage form. Because ATP is a potent allosteric effector of many enzymes, storing it in high concentrations would disrupt cellular metabolism. * **Why ADP is incorrect:** ADP is a metabolic byproduct of energy utilization. High levels of ADP signal a low-energy state in the cell and trigger pathways like glycolysis to produce more ATP. **High-Yield Clinical Pearls for NEET-PG:** 1. **Lohmann’s Reaction:** The reversible transfer of phosphate between creatine and ATP catalyzed by Creatine Kinase. 2. **Creatinine:** A waste product formed by the non-enzymatic breakdown of phosphocreatine. Its excretion rate is relatively constant and proportional to total muscle mass. 3. **Order of Energy Source Utilization:** ATP (1-2s) → Phosphocreatine (5-8s) → Muscle Glycogen (Anaerobic glycolysis) → Oxidative phosphorylation.

Q: Actin's active site is covered by?

Tropomyosin. ### Explanation **Correct Option: B. Tropomyosin** In a resting muscle fiber, the interaction between actin and myosin is physically prevented to allow for relaxation. The **active sites (myosin-binding sites)** on the filamentous (F) actin strands are covered by **tropomyosin**. Tropomyosin is a long, rod-shaped protein that wraps spirally around the actin filament, specifically masking the sites where myosin heads would otherwise attach to initiate contraction. **Analysis of Options:** * **A. Myosin:** This is the thick filament. It possesses "heads" that seek to bind to actin to form cross-bridges. It does not cover the site; it is the protein being blocked. * **C. Troponin:** This is a complex of three regulatory proteins (I, T, and C) attached to tropomyosin. While Troponin I helps inhibit the binding, it is the **tropomyosin molecule itself** that physically lies over and masks the active site. Troponin acts as the "lock" that moves the tropomyosin "bar" when calcium binds to Troponin C. * **D. Desmin:** This is an intermediate filament found near the Z-line. Its primary role is structural—linking myofibrils together and anchoring them to the sarcolemma—rather than regulating the actin-myosin interface. **High-Yield NEET-PG Pearls:** * **The Troponin Complex:** * **Troponin T:** Binds the complex to **T**ropomyosin. * **Troponin I:** **I**nhibits the actin-myosin interaction. * **Troponin C:** Binds **C**alcium ions (requires 4 $Ca^{2+}$ ions to trigger a conformational change). * **Mechanism of Contraction:** When $Ca^{2+}$ binds to Troponin C, it causes a conformational change that pulls tropomyosin deeper into the actin groove, uncovering the active sites. This is known as the **"Dual-control mechanism."** * **Relaxation:** Occurs when $Ca^{2+}$ is pumped back into the Sarcoplasmic Reticulum (via SERCA), causing tropomyosin to return to its original masking position.

Q: Which of the following muscle fiber types is brown in color?

Type II A. **Explanation:** The classification of skeletal muscle fibers is based on their metabolic profile, contraction speed, and myoglobin content. **Type IIA fibers (Fast-twitch oxidative-glycolytic)** are known as **"Brown fibers"** or "Intermediate fibers." They occupy a middle ground between Type I and Type IIB. They contain a high concentration of myoglobin and mitochondria (similar to Type I) but also possess high glycolytic capacity. The combination of high myoglobin content and specific enzymatic activity gives them a distinct brownish appearance under certain staining conditions. **Analysis of Incorrect Options:** * **Option A: Type I (Slow-twitch):** These are known as **"Red fibers."** They have the highest myoglobin content and mitochondrial density, specialized for endurance and aerobic metabolism. * **Option C: Type IIB (Fast-twitch glycolytic):** These are known as **"White fibers."** They have low myoglobin and mitochondrial content, relying primarily on anaerobic glycolysis for short bursts of power. * **Option D: Type III:** This is a **distractor.** In standard mammalian skeletal muscle classification, fibers are categorized into Types I, IIA, IIB, and sometimes IID/X. Type III is not a recognized skeletal muscle fiber type in this context. **High-Yield NEET-PG Pearls:** * **Mnemonic for Type I:** **"One Slow Red Ox"** (Type **I**, **Slow**-twitch, **Red** color, **Ox**idative metabolism). * **Fatigability:** Type I is fatigue-resistant; Type IIA is relatively resistant; Type IIB fatigues rapidly. * **Myosin ATPase activity:** Low in Type I, High in Type II. * **Order of Recruitment (Henneman’s Size Principle):** Type I fibers are recruited first, followed by Type IIA, and finally Type IIB for maximal force.

Q: Which nerve fibres have the largest diameter?

Somatic motor A alpha fibres. **Explanation:** The classification of nerve fibers is based on the **Erlanger-Gasser classification**, which categorizes fibers according to their diameter, conduction velocity, and functions. **1. Why A Alpha (Aα) is correct:** Type A alpha fibers are the largest in diameter (**12–20 μm**) and possess the thickest myelin sheath. According to the principles of neurophysiology, conduction velocity is directly proportional to fiber diameter. Therefore, Aα fibers are the fastest (70–120 m/s). They function as **somatic motor fibers** (to extrafusal muscle fibers) and **proprioceptive afferents** (from muscle spindles and Golgi tendon organs). **2. Analysis of Incorrect Options:** * **Type B fibers:** These are preganglionic autonomic fibers. They are myelinated but significantly smaller in diameter (**<3 μm**) than any Type A fiber. * **Type C fibers:** These are the smallest (**0.4–1.2 μm**) and are **unmyelinated**. They have the slowest conduction velocity and carry dull pain, temperature, and postganglionic autonomic signals. * **A gamma (Aγ) fibers:** While these are Type A myelinated fibers, they are smaller (**3–6 μm**) than Aα. They supply the intrafusal fibers of the muscle spindle. **High-Yield Facts for NEET-PG:** * **Order of Diameter/Velocity:** Aα > Aβ > Aγ > Aδ > B > C. * **Sensitivity to Local Anesthetics:** Type C fibers are the most sensitive, while Type A fibers are the least sensitive (Size principle: smaller fibers are blocked first). * **Sensitivity to Pressure:** Type A fibers are the most sensitive to pressure (e.g., a limb "falling asleep"). * **Sensitivity to Hypoxia:** Type B fibers are the most sensitive to oxygen deprivation. * **Fastest Conduction:** Somatic motor (Aα); **Slowest Conduction:** Dorsal root/Pain (C).

Q: Which of the following acts as a muscle spring?

Titin. **Explanation:** **1. Why Titin is the Correct Answer:** Titin (also known as connectin) is a giant protein that functions as a molecular spring within the sarcomere. It extends from the Z-disk to the M-line. The segment of titin located in the I-band is highly elastic and foldable. When a muscle is stretched, titin develops passive tension, and when the stretch is released, it acts like a spring to return the sarcomere to its resting length. This provides **passive elasticity** [1] to the muscle and ensures the thick filaments (myosin) remain centered within the sarcomere during contraction and relaxation. **2. Why the Other Options are Incorrect:** * **Alpha-actinin:** This is a structural protein located specifically at the **Z-disk**. Its primary role is to anchor the thin (actin) filaments to the Z-disk, maintaining the structural framework of the sarcomere [1]. It does not possess elastic properties and does not act as a spring. * **Both/None:** Since only titin possesses the unique elastic domain required to function as a spring, these options are incorrect. **3. NEET-PG High-Yield Pearls:** * **Largest Protein:** Titin is the largest known single polypeptide chain in the human body. * **Nebulin:** Often confused with titin, nebulin acts as a "molecular ruler" to regulate the length of actin filaments; it is non-elastic. * **Dystrophin:** Connects the cytoskeleton of a muscle fiber to the surrounding extracellular matrix; its deficiency leads to Duchenne Muscular Dystrophy. * **Desmin:** An intermediate filament that links Z-disks of adjacent myofibrils together.

Q: Conduction along a membrane is dependent on all of the following EXCEPT?

Fe2+. **Explanation:** The conduction of electrical impulses along a cell membrane (Action Potential) is primarily governed by the movement of specific ions through voltage-gated channels, a process described by the **Hodgkin-Huxley model**. **Why Fe2+ is the Correct Answer:** Iron (**Fe2+**) is a trace element essential for oxygen transport (hemoglobin) and electron transport (cytochromes), but it plays **no direct role** in the acute generation or propagation of an action potential across a nerve or muscle membrane. It does not have specific voltage-gated channels that contribute to membrane excitability. **Analysis of Other Options:** * **Na+ (Sodium):** Crucial for the **depolarization** phase. The rapid influx of Na+ through voltage-gated sodium channels is responsible for the upstroke of the action potential. * **K+ (Potassium):** Essential for the **repolarization** phase and maintaining the Resting Membrane Potential (RMP). The efflux of K+ restores the negative charge inside the cell. * **Ca2+ (Calcium):** Plays a dual role. In cardiac and smooth muscle, it contributes to the action potential plateau/depolarization. More importantly, extracellular Ca2+ levels stabilize the membrane; **hypocalcemia** increases excitability (tetany) by lowering the threshold for Na+ channel activation. **High-Yield Clinical Pearls for NEET-PG:** * **RMP** is primarily determined by **K+** (due to high resting permeability). * **Depolarization** is primarily determined by **Na+**. * **Hyperkalemia** initially increases excitability but eventually leads to inactivation of Na+ channels, causing paralysis or cardiac arrest. * **Local Anesthetics (e.g., Lidocaine)** work by blocking voltage-gated **Na+ channels**, thereby halting conduction.

Question 1

What is the nerve impulse that initiates muscle contraction?

Accepted Answer

It causes the opening of calcium channels, leading to an increase in calcium ion concentration within the sarcomere.

Answer

It causes both the plasma membrane and the T-tubules to undergo hyperpolarization.

Answer

It inhibits sodium entry into the sarcomere.

Answer

It is inhibited by the binding of acetylcholine to receptors in the sarcoplasmic reticulum.

Question 2

Which of the following muscles has the maximum density of white muscle fibers?

Accepted Answer

Extraocular muscles

Answer

Calf muscle

Answer

Back muscles

Answer

Hip muscles

Question 3

When energy is derived from creatine phosphate to cause muscle contraction, what is the first step in this transfer of energy?

Accepted Answer

The energy of the creatine phosphate is used to convert ADP to ATP

Answer

The creatine phosphate transfers its energy to the cross bridges to cause them to become locked

Answer

The creatine phosphate causes the power stroke of the cross bridges

Answer

The creatine phosphate transfers its energy to the actin filament

Question 4

Energy in muscles is stored as:

Accepted Answer

Phosphocreatine

Answer

ATP

Answer

ADP

Answer

None of the above

Question 5

Active tension in a muscle depends upon which of the following factors?

Accepted Answer

Number of motor units recruited

Answer

Number of muscle fibers

Answer

Aerobic capacity of the muscle

Answer

Length of the muscle fiber

Question 6

Actin's active site is covered by?

Accepted Answer

Tropomyosin

Answer

Myosin

Answer

Troponin

Answer

Desmin

Question 7

Which of the following muscle fiber types is brown in color?

Accepted Answer

Type II A

Answer

Type I

Answer

Type II B

Answer

Type III

Question 8

Which nerve fibres have the largest diameter?

Accepted Answer

Somatic motor A alpha fibres

Answer

Preganglionic autonomic type B fibres

Answer

Dorsal root C fibres

Answer

Motor fibres A gamma fibres

Question 9

Which of the following acts as a muscle spring?

Accepted Answer

Titin

Answer

Alpha actinin

Answer

Both

Answer

None

Question 10

Conduction along a membrane is dependent on all of the following EXCEPT?

Accepted Answer

Fe2+

Answer

Ca2+

Answer

K+

Answer

Na+

Nerve and Muscle Physiology — MCQs

Nerve and Muscle Physiology — MCQs

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