What and where is the Z-Line?

Skeletal muscle consists of muscle fibers, which in turn, are composed of myofibrils. The myofibril is organized into thick and thin filaments composed of 10,000 functional units called sarcomeres. A view of muscle at the level of the light microscope shows striations or bands. A band is actually the Z line, which marks the boundary between adjacent sarcomeres. The Z lines contain proteins called actinins, which connect thin filaments of adjacent sarcomeres. Another protein, titin, connects thick filaments to the attachment sites at the Z line. Intermediate filaments that connect adjacent myofibrils surround each Z line.