The extracellular receptor that binds the neurotransmitter on the muscle fiber is

Correct Answer: C

Rationale: A cholinergic receptor, specifically the nicotinic subtype, on the muscle fiber's sarcolemma binds acetylcholine, opening Na⁺ channels to depolarize and contract the muscle. Adrenergic receptors (alpha/beta) bind catecholamines (e.g., norepinephrine) in smooth muscle or cardiac contexts, not skeletal. Beta receptors, a subset, are irrelevant here. Tyrosine kinase receptors (e.g., for growth factors) don't mediate neurotransmitter action. Cholinergic receptors' specificity for acetylcholine distinguishes them, critical for skeletal muscle excitation, unlike adrenergic or signaling receptors.

Correct Answer: C

Rationale: A sarcomere is the contractile unit of muscle, a segment of a myofibril between Z-lines, containing overlapping actin and myosin myofilaments. It's not a cancer (sarcoma), cytoplasm (sarcoplasm), or membrane (sarcolemma), but a structural section where sliding filaments shorten during contraction. Defining myofibril organization, sarcomeres' repetitive nature enables muscle shortening, distinguishing them from pathological or cellular envelope terms, central to understanding muscle mechanics at the microscopic level.

Correct Answer: A

Rationale: The deltoid, in the upper arm's shoulder region, is a standard intramuscular injection site due to its accessible, thick muscle mass, minimizing nerve damage risk. Gluteus maximus is avoided to protect the sciatic nerve (gluteus medius is preferred), vastus medialis isn't typical, and latissimus dorsi, on the back, lacks suitable mass and access. Deltoid's prominence and safety distinguish it in clinical practice, key for effective drug delivery.

Correct Answer: A

Rationale: Both cardiac and skeletal muscle cells display striations banded patterns from sarcomere alignment unlike smooth muscle. Intercalated discs, branching, and involuntary control are cardiac-specific, aiding heart synchronization and automaticity. Striations reflect shared contractile machinery, distinguishing them from non-striated smooth muscle, key to their histological and functional similarity.

Correct Answer: D

Rationale: Skeletal muscle is striated (sarcomere-banded), voluntary (somatic control), and multinucleate (syncytial), unlike smooth (unstriated, involuntary, uninucleate) or cardiac (striated, involuntary, uninucleate) muscle. This trio defines its role in conscious movement, distinguishing it in structure and function, key to its physiological classification.