Which type of muscle primarily initiates contraction via an external nerve signal?

Skeletal muscle primarily initiates contraction through an external nerve signal, specifically via motor neurons that release acetylcholine at the neuromuscular junction. This interaction leads to the depolarization of the muscle membrane, followed by a cascade of events that culminate in muscle contraction. Skeletal muscles are controlled voluntarily and are responsible for movements of the body as well as posture.

In contrast, smooth muscle and cardiac muscle have different modes of initiation. Smooth muscle contraction is typically regulated by the autonomic nervous system, hormones, and local tissue factors rather than direct nerve signals. Cardiac muscle, while also striated, generates its own rhythmic contractions through specialized pacemaker cells in the sinoatrial node, independent of external nervous stimulation. Striated muscle is a broader term that includes both skeletal and cardiac muscle, which means it does not specifically address the external nerve signal component. Thus, the distinctive feature of skeletal muscle’s reliance on direct nerve signals makes it the correct answer.