What is the contractile unit of a skeletal muscle that runs in sequence along the myofibril?

The contractile unit of a skeletal muscle is the sarcomere, which is essential for muscle contraction and is organized in a repeating pattern along the length of the myofibril. Each sarcomere is defined by the presence of distinct boundaries called Z-lines, which anchor the thin filaments (actin) and help delineate the functional section of the muscle fiber.

Within the sarcomere, the interplay between thick filaments (myosin) and thin filaments (actin) during contraction leads to the shortening of the sarcomere, which ultimately results in muscle contraction. This segmental organization allows for the effective propagation of contraction along the muscle fiber, contributing to the overall muscle contraction process.

In contrast, the sarcoplasmic reticulum is a specialized form of the endoplasmic reticulum that stores calcium ions, playing a role in muscle contraction but not acting as the contractile unit itself. A fascicule refers to a bundle of muscle fibers within a skeletal muscle, which does not represent the smallest functional unit of contraction. Actomyosin refers to the complex of actin and myosin within muscle cells essential to contraction but does not define the structural unit; instead, it describes the proteins involved in the contractile process.