Skeletal Muscle Contraction — MCQs

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Skeletal Muscle Contraction — MCQs

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The following smooth muscle relaxants act by affecting calcium release except:

Arrange the following parts of sarcomere from periphery to center. 1. Z line 2. M line 3. A band 4. H zone

Tetanic contraction is due to accumulation of?

Latent period of muscle twitch is 10 milliseconds, contraction period is 40 milliseconds, and the relaxation time is 50 milliseconds. What would be the tetanizing frequency?

Histology of Myositis ossificans most closely mimics which of the following?

Sarcomere is the area between which two Z lines?

In the context of muscle physiology, which structure is described as a threadlike component that extends along the length of a muscle fiber?

The most significant immediate result of lowered serum calcium is

Protein connecting Z-lines to M-lines is:

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Skeletal Muscle Contraction Indian Medical PG Practice Questions and MCQs

Question 1: The following smooth muscle relaxants act by affecting calcium release except:

A. Dantrolene
B. Nifedipine
C. Prazosin (Correct Answer)
D. Verapamil

Explanation: ***Prazosin*** - Prazosin is an **alpha-1 adrenergic receptor antagonist** that causes smooth muscle relaxation by blocking the vasoconstrictive effects of norepinephrine, **not by directly affecting calcium release or calcium channels** [4]. - Its mechanism involves preventing receptor-mediated vasoconstriction through downstream signaling pathways, which are distinct from direct calcium channel modulation or calcium release from intracellular stores. - **This is the correct answer** as it does not act by affecting calcium handling. *Dantrolene* - Dantrolene acts by inhibiting the release of **calcium from the sarcoplasmic reticulum** in muscle cells by blocking **ryanodine receptors (RyR1)**. - **Important note:** Dantrolene is primarily a **skeletal muscle relaxant** used for malignant hyperthermia and spasticity, not a typical smooth muscle relaxant. - While it does affect calcium release, its primary therapeutic action is on skeletal muscle, making its inclusion in "smooth muscle relaxants" questionable. *Nifedipine* - Nifedipine is a **dihydropyridine calcium channel blocker** that inhibits the influx of extracellular calcium into smooth muscle cells [2]. - By blocking **L-type voltage-gated calcium channels**, it reduces intracellular calcium availability, leading to smooth muscle relaxation, particularly in vascular smooth muscle [1], [3]. *Verapamil* - Verapamil is a **non-dihydropyridine calcium channel blocker** that also inhibits the influx of extracellular calcium into smooth muscle cells [3]. - It primarily affects **L-type calcium channels** in both cardiac and smooth muscle, leading to vasodilation and reduced cardiac contractility [1], [2].

Question 2: Arrange the following parts of sarcomere from periphery to center. 1. Z line 2. M line 3. A band 4. H zone

A. 2,3,4,1
B. 4,2,3,1
C. 3,1,4,2
D. 1,3,4,2 (Correct Answer)

Explanation: ***1,3,4,2*** - The **Z line** is found at the **periphery** of the sarcomere, defining its boundaries and anchoring the **actin filaments**. - Moving inwards, the **A band** is next, representing the entire length of the **myosin filament**, which may also overlap with actin. - The **H zone** is located within the A band, comprising only **myosin filaments** without actin overlap. - Finally, the **M line** is at the **center** of the sarcomere, bisecting the H zone and anchoring the myosin filaments. *2,3,4,1* - This sequence is incorrect because the **M line** is at the **center** and the **Z line** is at the **periphery**, which is the reverse of the expected order for from periphery to center. - Such an arrangement would place the innermost structure first and outermost last, not reflecting the correct spatial organisation. *4,2,3,1* - This order is incorrect as the **H zone** and **M line** are more central, while the **Z line** is peripheral. - Placing structures like the H zone and M line at the beginning does not align with arrangement from periphery to center. *3,1,4,2* - This option is incorrect because the **A band** includes both actin and myosin filaments, while the **Z line** is at the periphery of the sarcomere. - The given order does not represent a progression from the periphery to the center of the sarcomere.

Question 3: Tetanic contraction is due to accumulation of?

A. Na+
B. K+
C. Ca2+ (Correct Answer)
D. Cl-

Explanation: ***Ca2+*** - **Tetanic contraction** results from a rapid succession of muscle stimulations, leading to the sustained elevation of **intracellular calcium (Ca2+)** levels. - This persistent high Ca2+ concentration in the sarcoplasm allows for continuous binding to **troponin**, maintaining the activation of cross-bridge cycling. *Na+* - **Sodium (Na+)** influx is primarily responsible for the **depolarization** of the muscle cell membrane, leading to an **action potential**. - While essential for initiating the contraction, Na+ accumulation itself does not directly cause the sustained high Ca2+ levels characteristic of tetany. *K+* - **Potassium (K+)** efflux is crucial for the **repolarization** of the muscle cell membrane after an action potential. - Accumulation of K+ in the extracellular space can contribute to muscle fatigue and reduce excitability, but it does not directly lead to tetanic contraction. *Cl-* - **Chloride (Cl-)** ions play a significant role in stabilizing the resting membrane potential and contributing to muscle **repolarization**, particularly in skeletal muscle. - While important for muscle function, changes in Cl- concentration do not directly cause the sustained Ca2+ release required for tetanic contraction.

Question 4: Latent period of muscle twitch is 10 milliseconds, contraction period is 40 milliseconds, and the relaxation time is 50 milliseconds. What would be the tetanizing frequency?

A. 50 Hz
B. 75 Hz
C. 25 Hz (Correct Answer)
D. 100 Hz

Explanation: ***25 Hz*** - **Tetanizing frequency** is the minimum stimulation frequency required to produce tetanus (sustained muscle contraction without complete relaxation between stimuli). - For **incomplete tetanus** to occur, the next stimulus must arrive during the relaxation phase, before the muscle fully relaxes. - The critical time window is the **latent period + contraction period** = 10 ms + 40 ms = 50 ms. However, to ensure summation occurs reliably during relaxation, stimuli typically arrive at a slightly higher frequency. - **Practical tetanizing frequency** = approximately 1/(40 ms) = **25 Hz**, which ensures stimuli arrive during the latter part of contraction or early relaxation phase, producing sustained tension. - This frequency allows sufficient overlap for tetanic fusion while accounting for the physiological requirements of the muscle twitch cycle. *50 Hz* - This frequency (one stimulus every 20 ms) would produce a **complete tetanus** with no visible relaxation between stimuli. - This is higher than the minimum tetanizing frequency required for this muscle with its 100 ms total twitch duration. - While this would produce tetanus, it exceeds the minimum frequency needed. *75 Hz* - This very high frequency (one stimulus every 13.3 ms) would produce a **smooth, complete tetanus**. - This is approximately 3 times the minimum tetanizing frequency and represents excessive stimulation. - Such high frequencies are well beyond what is needed to prevent relaxation in this muscle. *100 Hz* - This extremely high frequency (one stimulus every 10 ms, equivalent to the latent period alone) would produce **maximal tetanic fusion**. - This is 4 times the minimum tetanizing frequency needed for this muscle. - While physiologically possible, this represents supramaximal stimulation frequency for tetanus production in this scenario.

Question 5: Histology of Myositis ossificans most closely mimics which of the following?

A. Giant Cell Tumor
B. Ewing's tumor
C. Osteochondroma
D. Osteosarcoma (Correct Answer)

Explanation: ***Osteosarcoma*** - Histologically, myositis ossificans can mimic **osteosarcoma**, as both conditions show features of **ossification and bone formation** [1]. - **Age** and **location** may also help differentiate, but careful histological examination reveals similarities. *Osteochondroma* - Characterized by a **cartilaginous cap** overlying the bone, which is not present in myositis ossificans [2]. - Usually develops in patients with a **pedunculated or sessile base**, rather than the heterogeneous ossification seen in myositis ossificans. *Ewing's tumour* - This is a **malignant round cell tumor** that predominantly affects children and young adults, with specific **translocation** genetic features. - Histologically, it shows a **small, blue cell appearance**, distinct from the fibroblastic reaction and bone formation seen in myositis ossificans. *GCT* - Granulosa cell tumors (GCT) are primarily **gynecological**, with no significant **bony changes** as seen in myositis ossificans. - Histologically, GCTs show **multinucleated giant cells** and are associated with **subarticular bone lesions**, unlike the heterotopic ossification seen in myositis ossificans. **References:** [1] Cross SS. Underwood's Pathology: A Clinical Approach. 6th ed. Common Clinical Problems From Osteoarticular And Connective Tissue Disease, pp. 673-674. [2] Cross SS. Underwood's Pathology: A Clinical Approach. 6th ed. Common Clinical Problems From Osteoarticular And Connective Tissue Disease, pp. 672-673.

Question 6: Sarcomere is the area between which two Z lines?

A. Two adjacent Z lines (Correct Answer)
B. A band and I band
C. Two H zones
D. Two consecutive I bands

Explanation: ***Two adjacent Z lines*** - A **sarcomere** is defined as the fundamental contractile unit of muscle, extending from one **Z line** to the next. - The **Z lines** anchor the **actin (thin) filaments**, and their proximity helps delineate the functional sarcomere unit. *Two consecutive I bands* - The **I band** contains only **thin (actin) filaments** and is bisected by a Z line. It is not a boundary that defines a sarcomere. - A sarcomere encompasses parts of two I bands, plus an A band in the middle. *A band and I band* - The **A band** contains **thick (myosin) filaments** and overlapping thin filaments, while the **I band** contains only thin filaments. - These bands are components within a sarcomere, not boundaries that define its extent. *Two H zones* - The **H zone** is a region within the A band that contains only **thick (myosin) filaments** and is visible in relaxed muscle. - It is located in the center of the A band and does not serve as a boundary for the entire sarcomere.

Question 7: In the context of muscle physiology, which structure is described as a threadlike component that extends along the length of a muscle fiber?

A. Sarcomere
B. Sarcolemma
C. Myofibril (Correct Answer)
D. Myofilament

Explanation: ***Myofibril*** - A **myofibril** is a cylindrical organelle that runs longitudinally inside a muscle fiber and contains **contractile proteins**. - Myofibrils are composed of repeating units called **sarcomeres**, which are the fundamental units of muscle contraction. *Sarcomere* - A **sarcomere** is the basic contractile unit of a muscle fiber, extending from one Z-disc to the next. - While it is a key component for muscle contraction, it is a **segment within a myofibril**, not the threadlike component that extends the entire length of the fiber. *Sarcolemma* - The **sarcolemma** is the cell membrane of a muscle fiber, responsible for transmitting nerve impulses to the muscle cell. - It encloses the muscle fiber but is not an internal, threadlike contractile component. *Myofilament* - **Myofilaments** are the individual protein filaments (actin and myosin) that make up a sarcomere within a myofibril. - They are the **smallest contractile elements**, but they are not the threadlike structure that extends along the entire muscle fiber.

Question 8: The most significant immediate result of lowered serum calcium is

A. Decalcification of bones
B. Decalcification of teeth
C. Weakened heart action
D. Hyperirritability of nerves and muscles (Correct Answer)

Explanation: ***Hyperirritability of nerves and muscles*** - Lowered serum calcium (hypocalcemia) decreases the threshold potential of excitable cells, leading to **increased neuronal and muscular excitability**. - This can manifest as **tetany**, muscle cramps, paresthesias, and in severe cases, seizures. *Decalcification of bones* - **Chronic hypocalcemia** can lead to secondary hyperparathyroidism, which may eventually cause bone decalcification. - This is a **long-term effect**, not an immediate significant result of acutely lowered serum calcium. *Decalcification of teeth* - Tooth decalcification is primarily associated with **fluoride deficiency**, poor oral hygiene, or acidic erosion, not directly with acute systemic hypocalcemia. - The calcium in teeth is **highly stable** and less readily mobilized than bone calcium in response to acute serum calcium changes. *Weakened heart action* - While severe **hypocalcemia can impair myocardial contractility** and lead to a weakened heart action, it is often preceded or accompanied by significant neuromuscular symptoms. - **Hyperkalemia** is more classically associated with immediate life-threatening cardiac dysfunction, while hypocalcemia primarily affects nerve and muscle excitability first.

Question 9: Protein connecting Z-lines to M-lines is:

A. Kinin
B. Desmin
C. Titin (Correct Answer)
D. Actin

Explanation: ***Titin*** - **Titin** is a giant protein that extends from the **Z-disc to the M-line** in the sarcomere, acting as a molecular spring. - It maintains the **structural integrity** of the sarcomere and provides passive elasticity to muscles. *Kinin* - **Kinin** is a protein involved in **inflammation and blood pressure regulation**, not a structural component of muscle sarcomeres. - Examples include **bradykinin**, which mediates pain and vasodilation. *Desmin* - **Desmin** is an **intermediate filament protein** that forms a scaffold around the Z-discs, linking myofibrils together. - While it connects myofibrils, it does not directly span between the Z-line and M-line within a single sarcomere. *Actin* - **Actin** is a primary component of **thin filaments** in the sarcomere and participates in muscle contraction, but it does not connect the Z-line to the M-line. - Thin filaments are anchored at the **Z-disc** but only extend partway into the A-band.

Question 10: The image shows:

A. Skeletal muscle (Correct Answer)
B. Cardiac muscle
C. Smooth muscle
D. Compact bone

Explanation: ***Skeletal muscle*** - The image clearly displays **striations** (alternating light and dark bands) and **multinucleated cells** with peripherally located nuclei, which are characteristic features of skeletal muscle tissue. - Skeletal muscle fibers are also typically **long and unbranched**, as seen in the linear arrangement in the image. *Cardiac muscle* - Cardiac muscle also exhibits striations, but it is characterized by **branched fibers** and the presence of **intercalated discs**, neither of which are visible here. - Cardiac muscle cells are typically uni- or binucleated, with centrally located nuclei. *Smooth muscle* - Smooth muscle tissue lacks striations and is composed of **spindle-shaped cells** with a single, centrally located nucleus. - It does not present the organized, linear fascicles seen in the image. *Compact bone* - Compact bone tissue is characterized by **osteons** (Haversian systems) with central canals, lacunae containing osteocytes, and lamellae, which are entirely different from the cellular structure shown. - Bone tissue is **rigid and calcified**, unlike the contractile tissue depicted.

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