Extensions of the sarcolemma that go deep into the muscle fiber are the

Correct Answer: A

Rationale: Within a muscle fiber, the sarcolemma, the cell membrane, extends inward as transverse tubules, known as T-tubules. These structures penetrate deep into the fiber, ensuring rapid transmission of action potentials from the nerve impulse to trigger contraction. This allows synchronized calcium release across the fiber, critical for muscle function. The sarcoplasmic reticulum, while closely associated, is a separate organelle storing calcium, not an extension of the sarcolemma. Myofibrils are the contractile units, composed of actin and myosin, but they don't extend from the membrane. Sarcomeres are segments of myofibrils, defining the contractile zone, not membrane extensions. T-tubules' role in signal conduction distinguishes them, enabling efficient, uniform contraction, unlike the storage, structural, or organizational roles of the others, aligning with their anatomical and physiological purpose in muscle activation.

Correct Answer: D

Rationale: The tibialis anterior's name reflects its location anterior (front) of the tibia in the lower leg. Muscle names often denote position, here indicating its role in dorsiflexing the foot from the shin's front. Size (e.g., maximus) or shape (e.g., deltoid) apply elsewhere, not here. Fiber direction (e.g., oblique) isn't the focus location is. This positional naming distinguishes it, aligning with anatomical convention for identifying muscle placement and function.

Correct Answer: A

Rationale: The diaphragm, a dome-shaped muscle below the lungs, contracts to flatten, expanding the thoracic cavity for inspiration breathing's primary driver. External intercostals assist, lifting ribs, but diaphragm's action is dominant. Internal intercostals aid expiration, secondary to inspiration. Rectus abdominis flexes the trunk, not breathing. Diaphragm's central role in volume change sets it apart, essential for respiratory rhythm.

Correct Answer: D

Rationale: The quadriceps group rectus femoris, vastus muscles lies on the anterior thigh, extending the knee and flexing the hip, as in kicking. Abductors are lateral, adductors medial, hamstrings posterior. Quadriceps' anterior placement and extension role distinguish them, vital for leg straightening, unlike positional or opposing groups.

Correct Answer: C

Rationale: During muscle contraction, the cross-bridge cycle drives filament sliding. After myosin binds to actin and performs a power stroke pulling actin toward the sarcomere's center ATP binds to the myosin head. This binding reduces myosin's affinity for actin, breaking the cross-bridge and detaching myosin, allowing it to reset for another cycle. ATP is then hydrolyzed to ADP and phosphate, cocking the head into a high-energy state to reattach to actin. Calcium ions, managed by the sarcoplasmic reticulum, regulate actin exposure, not myosin detachment. Acetylcholine release occurs at the neuromuscular junction, unrelated to ATP's role here. Hydrolysis energizes myosin but doesn't cause attachment binding does the detachment. This ATP-driven detachment is critical for repeated contractions, distinguishing it from initiation or calcium-related steps, and underscores its mechanical role in muscle dynamics.