In the striated muscles, the functional unit of contractile system is

Correct Answer: C

Rationale: In striated muscles (skeletal and cardiac), the sarcomere is the functional contractile unit, spanning from one Z line to the next. It contains actin and myosin filaments that slide during contraction, shortening the sarcomere to produce force, as per the sliding filament theory. The Z band anchors actin but isn't the unit itself. Cross bridges are myosin heads interacting with actin, a mechanism within the sarcomere, not the unit. A myofibril is a larger structure of many sarcomeres. The sarcomere's role as the basic repeating segment driving contraction makes it the best answer, central to understanding muscle mechanics and striation patterns.

Correct Answer: A

Rationale: Antagonists are muscles with opposing actions, like biceps flexing the elbow and triceps extending it, ensuring balanced movement and control. Agonists (prime movers) drive the main action, contracting together. Synergists assist agonists, stabilizing or refining motion. Prime movers overlap with agonists, not opposites. Antagonists' counteraction is unique, relaxing when agonists contract, distinct from cooperative or leading roles, essential for coordinated, reversible motion in the musculoskeletal system.

Correct Answer: B

Rationale: External intercostals lift ribs during inspiration, expanding the chest for inhalation. Internal intercostals depress ribs in forced expiration, aiding exhalation. Opposite pairing misaligns their roles. Trunk flexion or extension involves abdominals or back muscles, not intercostals. Their respiratory functions distinguish them, critical for breathing mechanics, unlike trunk movement roles.

Correct Answer: B

Rationale: Medial thigh muscles, like adductor longus, pull the thigh toward the midline, adducting it, as in crossing legs. Abduction moves it outward, a lateral thigh role. Leg extension (knee) or flexion involves quadriceps or hamstrings, not medial focus. Adduction defines their action, distinct from lateral, knee, or bending functions, key for thigh stability.

Correct Answer: C

Rationale: At the neuromuscular junction, acetylcholine release initiates muscle contraction by binding to receptors on the muscle membrane, causing depolarization and an action potential that propagates along the fiber. This triggers calcium release from the sarcoplasmic reticulum, which binds to troponin, shifting tropomyosin to expose myosin-binding sites on actin, enabling contraction. These events form a direct cascade from acetylcholine's action. However, ATP binding to the myosin head occurs later in the cross-bridge cycle, detaching myosin from actin after a power stroke, a process not directly initiated by acetylcholine but part of the contraction-relaxation cycle powered by ATP hydrolysis. This distinction highlights that while calcium and actin exposure are immediate downstream effects, ATP's role is a subsequent step, reliant on energy dynamics rather than the initial synaptic signal, making it the exception in this sequence.