The neurotransmitter that excites the muscle fiber is

Correct Answer: D

Rationale: Acetylcholine (Ach), released by somatic motor neurons, excites skeletal muscle fibers by binding nicotinic receptors, opening Na⁺ channels to depolarize the sarcolemma, initiating contraction. GABA, inhibitory in the CNS, doesn't excite muscle. Na⁺ is an ion, not a neurotransmitter it enters post-Ach binding. Ca²⁺ triggers contraction internally, not as a neurotransmitter. Ach's excitatory role at the neuromuscular junction distinguishes it, key for signal transmission, unlike inhibitory, ionic, or intracellular agents.

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: A

Rationale: Biceps brachii encodes two origins ( bi- ) two tendon heads on the scapula and location ( brachii ) on the arm's humerus. Action (flexion) isn't specified, nor is size (e.g., maximus) or shape (e.g., deltoid). This origin-location naming, common in muscles like triceps, aids anatomical mapping, distinguishing it from action- or shape-based names, reflecting skeletal muscle's attachment-based classification.

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.