Which of the events below is the FIRST to occur prior to a muscle cell contracting?

Correct Answer: D

Rationale: Contraction begins with a neural signal. Calcium release from the sarcoplasmic reticulum into the cytoplasm is the first intracellular step, triggered by sarcolemma depolarization. This calcium binds troponin, exposing actin sites, followed by ATP binding and hydrolysis to drive myosin movement. ADP detachment occurs later in the cycle. Calcium's release initiates the cascade, distinct from subsequent biochemical steps, ensuring contraction aligns with nerve input, a critical sequence in muscle physiology.

Correct Answer: B

Rationale: Sternocleidomastoid names origin (sternum, clavicle) and insertion (mastoid process), not just location, unlike biceps brachii (arm), rectus abdominus (abdomen), and flexor carpi radialis (wrist). Its attachment-based name distinguishes it from purely locational terms, aiding precise anatomical identification over regional descriptors.

Correct Answer: B

Rationale: Myosin, a protein molecule (160 nm), forms thick myofilaments within sarcomeres, inside myofibrils, within muscle fibres (cells), grouped into fasciculi. As a molecular component, myosin is the smallest unit, distinguishing it from larger structural assemblies, foundational to contraction mechanics.

Correct Answer: B

Rationale: Thick myofilaments in sarcomeres are composed of myosin, a motor protein with heads forming cross-bridges to actin during contraction. Actin forms thin myofilaments with tropomyosin and troponin, while acetylcholine is a neurotransmitter, not a filament component. Myosin's bundled structure, comprising about 300 molecules per filament, drives the sliding filament mechanism, distinguishing it as the thick filament's core protein, critical for muscle force generation and movement.

Correct Answer: A

Rationale: Gluteus maximus, medius, and minimus are named for size largest, middle, smallest among buttock muscles. Shape (e.g., trapezius), whimsy (e.g., sartorius), or fibre direction (e.g., rectus) aren't indicated. Size-based naming distinguishes them, reflecting relative mass critical for hip function and anatomical clarity.