Test: Muscular System - NEET MCQ

In the context of muscular contraction, a cross-bridge refers to the attachment of myosin with actin within the muscle cell.
All muscle types contract by cross-bridge cycling i.e. repeated attachment of actin and myosin within the cell.

Hence, a cross-bridge is formed during muscle contraction. 

Sliding Filament Theory of Contraction:
During muscle contraction, the thin actin filaments slide over the thick myosin filament. The actin filament slides over myosin filament thus reduces the length of the sarcomere and contracts the muscle fibre.
When Calcium is present the blocked active site of the actin clears.

A sarcomere is the basic contractile unit of muscle fiber.

Each sarcomere is composed of two main protein filaments, actin and myosin, which are the active structures responsible for muscular contraction.

Skeletal muscles are also known as voluntary muscles. They have striations and are also known as striated muscles. They are involved in changing of body posture but not in the movement of food through the oesophagus.

• Cardiac muscles are the muscles of the heart.
• Cardiac muscles are involuntary muscles that work continuously to pump blood throughout the body.
• They are branched and are striated in appearance.

Fascia is a layer of connective tissue that surrounds the fascicles or muscle bundles in a muscle. It is made out of collagen. Each muscle bundle contains a number of muscle fibres or muscle cells.

In all myosins, the head domain is a specialized ATP-ase that is able to couple the hydrolysis of ATP with motion. A critical feature of the myosin ATP-ase activity is that it is actin-activated. In the absence of actin, solutions of myosin slowly convert ATP into ADP and phosphate.

Each muscle cell or muscle fibre contains filaments in its sarcoplasm which are arranged in a parallel manner. These filaments are known as myofibrils and they have alternate dark and light bands.

Cardiac muscle is found only in the walls of the heart. When cardiac muscle contracts, the heartbeats and pumps blood.

• Cardiac muscle contains many contains a great many mitochondria, which produce ATP for energy. This helps the heart resist fatigue.
• Contractions of cardiac muscle are involuntary, like those of smooth muscle.
• Cardiac muscle, like skeletal muscle, is arranged in bundles, so it appears striated, or striped.

Hence, option D is correct.

- Statement i is correct because muscle contraction is initiated by a neural signal from the CNS through a motor neuron.

- Statement ii is correct as the neuromuscular junction is indeed where the motor neuron connects to the muscle fibre.

- Statement iii is incorrect because calcium ions do not bind to myosin; they bind to troponin, which then allows myosin to attach to actin.

- Statement iv is correct as troponin binds calcium, causing a conformational change that exposes the active sites on actin.

Thus, the correct statements are i, ii, and iv.

- Statement i is correct because the I-band is indeed composed of actin filaments, which are thinner than myosin filaments.

- Statement ii is incorrect; cardiac muscles are involuntary and are not directly controlled by the nervous system.

- Statement iii is correct as the sarcomere is defined as the segment between two Z-lines and is the functional unit of contraction.

- Statement iv is incorrect; myosin filaments are actually thicker than actin filaments.

Thus, the correct statements are i and iii, making Option A the correct answer.

- Statement i is correct: Each actin filament is indeed composed of two helically wound 'F' actins.

- Statement ii is also correct: 'G' (Globular) actins serve as monomers that polymerize to form filamentous 'F' actins.

- Statement iii is incorrect: Tropomyosin runs parallel to 'F' actins and interacts with them, playing a crucial role in muscle contraction by regulating access to the binding sites on actin for myosin.

- Statement iv is correct: Troponin is distributed at regular intervals along the tropomyosin and is essential for regulating the binding sites for myosin on actin filaments during muscle contraction.

Thus, the correct statements are i, ii and iv, making Option c the right answer.

Each actin (thin) filament is made of two ‘F’ (filamentous) actins helically wound to each other. Each ‘F’ actin is a polymer of monomeric ‘G’ (Globular) actins. Two filaments of another protein, tropomyosin also run close to the ‘F’ actins throughout its length. A complex protein Troponin is distributed at regular intervals on the tropomyosin. In the resting state a subunit of troponin masks the active binding sites for myosin on the actin filaments: Reference from NCERT

- Statement i is correct. Myosin filaments are indeed composed of multiple monomeric proteins called Meromyosins.

- Statement ii is incorrect. The heavy meromyosin (HMM) refers specifically to the globular head and short arm, while the light meromyosin (LMM) refers to the tail.

- Statement iii is correct. The cross arm of the myosin filament does project outwards at a regular distance and angle.

- Statement iv is incorrect. The globular head of myosin is an active ATPase enzyme with binding sites for ATP and active sites for actin.

Thus, the correct statements are i and iii, making the correct answer Option A.

• The Assertion (A) is false because myasthenia gravis does not primarily involve the degeneration of muscle fibers; rather, it is an autoimmune disorder affecting the communication at the neuromuscular junction.
• The Reason (R) is true as it correctly describes the mechanism of myasthenia gravis, which indeed leads to fatigue and weakness in skeletal muscles due to the disruption at the neuromuscular junction.
• Therefore, since the Assertion is false and the Reason is true, the correct answer is Option C: If Assertion is true but Reason is false.

- Assertion (A) is true: Muscle fibers are indeed classified into red and white fibers based on myoglobin content.

- Reason (R) is also true: Red fibers do contain more myoglobin, which allows for better oxygen storage and supports aerobic metabolism.

- Since the reason provided accurately explains why muscle fibers are classified this way, the correct option is Option A.