Test: Muscular System - NEET MCQ

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.

Line in NCERT: "These cross bridges pull the actin filaments causing them to slide over the myosin filaments and thereby causing contraction."

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.

Line in NCERT: "The portion of the myofibril between two successive 'Z' lines is considered as the functional unit of contraction and is called a sarcomere."

• 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 If Assertion is false and Reason is true

Sarcomere Components

• A-Band (I): Represents the length of the myosin filaments, including areas where actin and myosin overlap, and appears as the darker region.
• I-Band (II): Contains only actin filaments and is the lighter area that appears to shorten during muscle contraction.
• Sarcomere (III): The entire unit from one Z-line to the next.
• H-Zone (IV): The lighter region in the middle of the A-band where only myosin filaments are present when the muscle is relaxed.
• Myosin (V): The thick filament.
• Actin, Troponin, Tropomyosin (VI): Components of the thin filaments.
• Z-line (VII): The dark line to which actin filaments are attached, marking the boundary of each sarcomere.

Label Correspondence

• A: Appears to be the Z-line, marking the boundary of the sarcomere.
• B: Thick filaments (myosin) as seen in the center part of the sarcomere.
• C: Contains elements of both myosin and actin filaments, typical of the A-band.
• D: Shows the I-band, where only actin filaments are present.
• E: Another part of the A-band with overlapping actin and myosin filaments.
• F: The entire unit from one Z-line to the next Z-line, representing the sarcomere.
• G: The lighter middle region within the A-band where only myosin filaments are visible, representing the H-zone.

Evaluating the Options

• Option (1):
  • I - E: Correct, A-band includes overlapping areas of actin and myosin.
  • II - D: Correct, I-band with only actin.
  • III - F: Correct, entire unit from one Z-line to the next.
  • IV - G: Correct, H-zone with only myosin visible.
  • V - B: Correct, central part with myosin filaments.
  • VI- C: Incorrect, C includes actin and myosin, not just actin, troponin, and tropomyosin.
  • VII - A: Correct, Z-line.

• 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.

Line in NCERT: "Based on appearance, cardiac muscles are striated."

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.

Line in NCERT: "The globular head is an active ATPase enzyme and has binding sites for ATP and active sites for actin."

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.

Line in NCERT: "Each myofibril has alternate dark and light bands on it."

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.

Line in NCERT: "Based on appearance, cardiac muscles are striated. They are involuntary in nature as the nervous system does not control their activities directly."

- 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.

Line in NCERT: "Muscle contraction is initiated by a signal sent by the central nervous system (CNS) via a motor neuron. The junction between a motor neuron and the sarcolemma of the muscle fibre is called the neuromuscular junction or motor-end plate. This causes the release of Ca** from sarcoplasmic reticulum. Ca** activates actin which binds to the myosin head to form a cross bridge."

- 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.

Line in NCERT: "The light bands contain actin and is called I-band or Isotropic band, whereas the dark band called 'A' or Anisotropic band contains myosin." "Each functional unit of contraction and is called a sarcomere (Figure 17.2)." "Actin filaments are thinner as compared to the myosin filaments, hence are commonly called thin and thick filaments respectively."

- 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.

Line in NCERT: "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."

- 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.

Line in NCERT: "Each myosin (thick) filament is also a polymerised protein. Many monomeric proteins called Meromyosins constitute one thick filament. Each meromyosin has two important parts, a globular head with a short arm and a tail, the former being called the heavy meromyosin (HMM) and the latter, the light meromyosin (LMM). The HMM component, i.e.; the head and short arm projects outwards at regular distance and angle from each other from the surface of a polymerised myosin filament and is known as cross arm."

- 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.

Line in NCERT: "Muscles are classified as Red and White fibres based primarily on the amount of red coloured myoglobin pigment in them."

The sequence begins when a motor neuron's signal generates an action potential in the muscle fiber. This event triggers the release of calcium ions from the sarcoplasmic reticulum. These calcium ions then activate actin, allowing it to bind to myosin heads, forming cross bridges. The formation of these cross bridges enables the actin filaments to slide over the myosin filaments, causing the muscle to contract. Once contraction is completed, calcium ions are reabsorbed into the sarcoplasmic reticulum, actin becomes inactivated, cross bridges are broken, and the muscle relaxes.

Topic in NCERT: Muscle Contraction

Line in NCERT: "Upon an action potential in the muscle fiber, calcium ions are released from the sarcoplasmic reticulum, activating actin to bind with myosin and form cross bridges, resulting in muscle contraction."