The structure indicated is the Z-disc/Z-line formed between adjacent sarcomeres.

A sarcomere is the name given to the basic unit of muscle, composed of sliding protein filaments of actin and myosin. Myosin filaments are thick, are thin. These two filaments overlap each other, and their ability to slide past each other forms the basis of the theory of muscle contraction and relaxation.

Under electron microscopy, the overlapping pattern of these thick and thin protein filaments gives rise to darker and lighter areas (bands), as well as distinct lines. The thinner actin filaments are all bound to the Z-line, which makes up the boundary of the sarcomere. Z-lines therefore, are found between adjacent sarcomeres, and a sarcomere is thus defined as the muscle unit that is found between Z-lines.

The sarcomere consist of the following bands and zones:

• I-band – light area around the Z-line consisting of thin actin filaments not overlapped by thick filaments
• A-band – dark area which spans the length of the thick filament
• H-zone – light zone in the centre of the a-band where the thick filaments are not overlapped by the thin filaments

The sarcomere consist of two lines:

• Z-line – forms periphery of sarcomere where thin actin filaments attach
• M-line – found inside H-zone and forms the middle of the sarcomere.