How many muscles are there in the body of human?

Correct Answer: C

Rationale: The human body contains approximately 640 muscles, a widely accepted estimate in anatomy reflecting skeletal muscles primarily, which vary slightly by individual due to small accessory muscles. This number excludes smooth and cardiac muscles' microscopic units but focuses on distinct, named skeletal muscles enabling movement. Fewer, like 340, undercounts the extensive network across limbs, torso, and face. Higher figures like 860 might exaggerate by including minor variants or non-skeletal types, but 640 aligns with standard texts. It accounts for paired muscles (e.g., biceps) and smaller ones (e.g., in the ear), balancing precision with practicality. This figure underscores the muscular system's complexity, supporting diverse functions from walking to facial expressions, and provides a reliable benchmark for anatomical study, distinguishing it from overly broad or narrow estimates.

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: D

Rationale: When ADP and inorganic phosphate (P) release from myosin's head after ATP hydrolysis, the cross-bridge bends, pulling actin toward the sarcomere's center the power stroke. This single event encompasses bridge bending, filament pulling, and the stroke itself, driving contraction. Each aspect isn't separate; they're facets of one action. Bending alone omits the pull, pulling alone skips mechanics, and power-stroke alone undersells the full process. All occur simultaneously as myosin resets, distinguishing this step from prior ATP binding or later detachment, critical for the sliding filament cycle and muscle shortening.

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.