Final answer:
The membranous structures that run along the Z lines in skeletal muscle are sarcomeres, which are the functional units of the muscle fiber and consist of organized actin and myosin filaments.
Step-by-step explanation:
The membranous structures that run along the Z lines in skeletal muscle are termed the sarcomeres. The sarcomere is the functional unit of the muscle fiber, encapsulated within the myofibril that spans the entire length of the muscle fiber and connects to the sarcolemma at its ends. It is defined as the region between two consecutive Z lines, and it contains an array of both thin (actin) and thick (myosin) filaments organized in a highly structured manner, allowing for muscle contraction.
The T tubules, or transverse tubules, are extensions of the sarcolemma that penetrate into the cell's interior and facilitate the conduction of electrical impulses, which trigger the release of calcium (Ca2+) from the sarcoplasmic reticulum. However, these T tubules are not the structures that directly run along the Z lines.