Nerve and Muscle Physiology Practice Questions

Q: Sclerostin is produced by which type of bone cell?

Osteocytes. **Explanation:** **Correct Answer: A. Osteocytes** Sclerostin is a glycoprotein primarily secreted by **mature osteocytes** (cells embedded within the mineralized bone matrix). It acts as a potent negative regulator of bone formation. Mechanistically, sclerostin binds to LRP5/6 receptors on the surface of osteoblasts, thereby inhibiting the **Wnt signaling pathway**. This inhibition prevents osteoblast proliferation and differentiation, leading to decreased bone formation. When bone experiences mechanical loading, sclerostin production decreases, allowing Wnt signaling to trigger bone deposition. **Why other options are incorrect:** * **B. Osteoblasts:** While osteoblasts are the targets of sclerostin action, they do not produce it. Osteoblasts are responsible for bone matrix synthesis (osteoid). * **C. Osteoclasts:** These are myeloid-derived cells responsible for bone resorption. They do not produce sclerostin; however, sclerostin indirectly promotes their activity by increasing the RANKL/Osteoprotegerin (OPG) ratio. * **D. Chondrocytes:** These are cells found in cartilage. While they share a mesenchymal origin with bone cells, they are not the source of sclerostin in the context of bone remodeling. **High-Yield Clinical Pearls for NEET-PG:** * **Romosozumab:** A monoclonal antibody against sclerostin used in the treatment of severe osteoporosis. It has a dual effect: increasing bone formation and decreasing bone resorption. * **Van Buchem Disease & Sclerosteosis:** Rare genetic conditions caused by a deficiency in sclerostin, leading to excessive bone overgrowth (hyperostosis). * **The "Master Regulator":** Osteocytes are now considered the "master regulators" of bone remodeling because they sense mechanical strain and coordinate both osteoblast and osteoclast activity via sclerostin and RANKL.

Q: Which of the proteins connect Z lines to M lines?

Titin. **Explanation:** **1. Why Titin is the Correct Answer:** Titin (also known as connectin) is the largest known protein in the human body. It acts as a molecular spring that extends from the **Z-disk to the M-line** within the sarcomere. Its primary functions are to provide structural scaffolding, maintain the central position of the thick (myosin) filaments during contraction, and contribute to the passive elasticity of the muscle. By anchoring the thick filaments to the Z-lines, it ensures the sarcomere returns to its original length after being stretched. **2. Why Other Options are Incorrect:** * **Nebulin:** This is a large protein that wraps around the **thin (actin) filaments**. It acts as a "molecular ruler" to regulate the length of actin filaments during assembly but does not reach the M-line. * **Actin:** These are the primary components of the **thin filaments**. They are anchored to the Z-lines (via alpha-actinin) and extend toward the center of the sarcomere but do not connect to the M-line. * **Myosin:** These are the primary components of the **thick filaments**. While they are centered at the M-line, they do not directly attach to the Z-lines; they rely on Titin for that connection. **3. NEET-PG High-Yield Pearls:** * **Dystrophin:** Connects the cytoskeleton of a muscle fiber to the surrounding extracellular matrix through the cell membrane. Deficiency leads to Duchenne Muscular Dystrophy. * **Alpha-actinin:** The protein that anchors actin filaments to the Z-line. * **Desmin:** An intermediate filament that links Z-disks of adjacent myofibrils together, ensuring synchronized contraction. * **M-line proteins:** Include **Myomesin** and **M-protein**, which hold the thick filaments in a hexagonal lattice.

Q: The energy for muscle contraction is supplied by the enzymatic hydrolysis of ATP by ATPase present in which component?

Heavy meromyosin. **Explanation:** The fundamental unit of muscle contraction involves the interaction between actin and myosin. Myosin II, the protein forming thick filaments, is composed of two heavy chains and four light chains. When treated with the enzyme trypsin, myosin is cleaved into two fragments: **Heavy Meromyosin (HMM)** and **Light Meromyosin (LMM)**. **Why Heavy Meromyosin (HMM) is correct:** HMM contains the globular heads (S1 subfragment) and the short neck/hinge region (S2 subfragment). The **myosin head** is the functional center of the molecule; it possesses two critical binding sites: 1. An **actin-binding site**. 2. An **ATP-binding site** with intrinsic **ATPase activity**. This ATPase enzyme hydrolyzes ATP into ADP and inorganic phosphate, releasing the energy required for the "power stroke" and cross-bridge cycling. **Why the other options are incorrect:** * **Light Meromyosin (LMM):** This represents the long, rod-like tail of the myosin molecule. It provides structural stability and helps in the assembly of the thick filament but lacks enzymatic (ATPase) activity. * **Tropomyosin:** A regulatory protein that wraps around actin filaments. In a resting state, it physically covers the myosin-binding sites on actin, preventing contraction. * **Troponin:** A complex of three subunits (I, T, and C) that regulates the position of tropomyosin based on calcium concentration. It does not possess ATPase activity. **High-Yield NEET-PG Pearls:** * **S1 Fragment:** The specific part of HMM that contains the ATPase activity. * **Rate-limiting step:** The release of Pi (inorganic phosphate) from the myosin head initiates the power stroke. * **Rigor Mortis:** Occurs because ATP is required for the *detachment* of the myosin head from actin; without ATP, the cross-bridge remains locked. * **Fenn Effect:** The observation that a muscle generates more heat when it performs work, correlating to increased ATP hydrolysis.

Q: Which of the following contains the site of attachment for calcium ions that initiates muscle contraction?

Troponin C. **Explanation:** Muscle contraction is initiated by the **Excitation-Contraction (E-C) coupling** process, where an increase in intracellular calcium ions ($Ca^{2+}$) acts as the primary trigger. **1. Why Troponin C is Correct:** The troponin complex is a regulatory protein associated with the actin (thin) filament. It consists of three subunits, each with a specific function. **Troponin C (TnC)** is the specific subunit that contains four binding sites for calcium. When $Ca^{2+}$ binds to Troponin C, it induces a conformational change in the entire troponin-tropomyosin complex. This shift moves tropomyosin away from the active sites on the actin filament, allowing the myosin heads to bind and initiate the cross-bridge cycle. **2. Analysis of Incorrect Options:** * **Troponin I (TnI):** The "I" stands for **Inhibitory**. It binds to actin and inhibits the interaction between actin and myosin by physically blocking the binding site. * **Troponin T (TnT):** The "T" stands for **Tropomyosin**. Its primary role is to anchor the troponin complex to the tropomyosin molecule. * **Myosin:** This is the thick filament. While it has binding sites for ATP and Actin, it does not have the primary regulatory binding site for calcium to initiate contraction. **3. High-Yield Clinical Pearls for NEET-PG:** * **Cardiac Biomarkers:** Troponin I and T are highly specific markers for myocardial infarction (MI). Troponin C is not used clinically because it is identical in both skeletal and cardiac muscle. * **The "Power Stroke":** This occurs when ADP and inorganic phosphate are released from the myosin head, not when calcium binds. * **Relaxation:** Muscle relaxation occurs when $Ca^{2+}$ is pumped back into the Sarcoplasmic Reticulum via the **SERCA pump** (an active process requiring ATP).

Q: Which of the following steps is NOT involved in muscle contraction following a nerve impulse?

Dopamine release at the NMJ. **Explanation:** The process of muscle contraction, known as **Excitation-Contraction Coupling**, relies on a specific sequence of electrochemical events. **Why Option B is Correct:** The neurotransmitter responsible for signal transmission at the Neuromuscular Junction (NMJ) is **Acetylcholine (ACh)**, not dopamine. When an action potential reaches the motor nerve terminal, voltage-gated calcium channels open, leading to the exocytosis of ACh into the synaptic cleft. Dopamine is a neurotransmitter primarily involved in the Central Nervous System (e.g., basal ganglia) and does not play a role in peripheral skeletal muscle contraction. **Why Other Options are Incorrect:** * **Option A:** An action potential must cross the NMJ via ACh binding to nicotinic receptors to initiate depolarization of the sarcolemma. * **Option C:** Depolarization travels down **T-tubules**, activating DHP receptors, which triggers the **Ryanodine receptors** to release stored Calcium from the **Sarcoplasmic Reticulum (SR)**. * **Option D:** Once released, Calcium binds to **Troponin C**, causing a conformational change in Tropomyosin. This exposes the active sites on actin, allowing the "attractive forces" (cross-bridge formation) between actin and myosin to occur. **High-Yield Clinical Pearls for NEET-PG:** * **Lambert-Eaton Syndrome:** Antibodies against *pre-synaptic* voltage-gated calcium channels. * **Myasthenia Gravis:** Antibodies against *post-synaptic* NMJ nicotinic ACh receptors. * **Malignant Hyperthermia:** Caused by a mutation in the **Ryanodine receptor (RYR1)**, leading to excessive calcium release. * **Rigor Mortis:** Occurs because ATP is required for the *detachment* of myosin from actin; without ATP, the cross-bridge remains locked.

Question 1

What potential would develop from the movement and eventual equilibrium of Na+, K+, and Cl- across the muscle membrane?

Accepted Answer

- 86 mV

Answer

- 94 mV

Answer

- 89 mV

Answer

+ 61 mV

Question 2

Which of the following is a true statement about the latch bridge mechanism?

Accepted Answer

Sustained contraction of smooth muscle with low energy consumption

Answer

Binding of tropomyosin to actin

Answer

Variability of tension at a particular length

Answer

None of the above

Question 3

Sclerostin is produced by which type of bone cell?

Accepted Answer

Osteocytes

Answer

Osteoblasts

Answer

Osteoclasts

Answer

Chondrocytes

Question 4

In which type of nerve fibers is conduction maximally blocked by pressure?

Accepted Answer

A alpha

Answer

A beta

Answer

A gamma

Answer

C

Question 5

Which of the proteins connect Z lines to M lines?

Accepted Answer

Titin

Answer

Nebulin

Answer

Actin

Answer

Myosin

Question 6

What is meant by resting muscle length?

Accepted Answer

The length of the muscle fiber at the onset of contraction, which is required to attain maximum active tension.

Answer

The length of actin and myosin filaments before contraction.

Answer

Approximately 4 micrometers.

Answer

The length at which the muscle has minimal tension.

Question 7

The energy for muscle contraction is supplied by the enzymatic hydrolysis of ATP by ATPase present in which component?

Accepted Answer

Heavy meromyosin

Answer

Light meromyosin

Answer

Tropomyosin

Answer

Troponin

Question 8

All of the following functions are associated with myelination except?

Accepted Answer

Decreases the release of neurotransmitter from the nerve endings

Answer

Decreases energy expenditure

Answer

Increases speed of conduction

Answer

Provides protective covering for the axon

Question 9

Which of the following contains the site of attachment for calcium ions that initiates muscle contraction?

Accepted Answer

Troponin C

Answer

Troponin I

Answer

Troponin T

Answer

Myosin

Question 10

Which of the following steps is NOT involved in muscle contraction following a nerve impulse?

Accepted Answer

Dopamine release at the NMJ

Answer

Action potential across the neuromuscular junction (NMJ)

Answer

Release of calcium from the sarcoplasmic reticulum

Answer

Calcium initiates attractive forces between actin and myosin filaments

Nerve and Muscle Physiology — MCQs

Nerve and Muscle Physiology — MCQs

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