The ‘I’ band, also known as the isotropic band, is one of the two bands in a sarcomere, which is the basic unit of a muscle fiber. The I band is composed of thin filaments of the protein actin and appears lighter in color under the microscope. The ‘Z’ line is present in the center of each ‘I’ band and bisects it. It is a narrow, plate-shaped structure made up of proteins that anchors the thin filaments (actin) and defines the boundaries of a sarcomere. The ‘Z’ line also plays a critical role in the contraction of the muscle fiber by enabling the thin filaments to slide past the thick filaments (myosin).