Structure of Vertebrate Skeletal Muscles

Vertebrate skeletal muscles are composed of a large number of long, cylindrical and multinucleated cells, called muscle fibres arranged parallel to each other. The fibres contain longitudinally arranged elements called myofilaments. The myofilaments are organised into myofibrils. The fibres measure between 0.1 to 0.01 mm in diameter and several centimetres long. The myofibrils have characteristic cross striations called Z-lines, which are repeated at regular intervals. The region between two Z-lines is called a sarcomere, which is a functional unit of a myofibril. Thus a myofibril consists of longitudinally repeating sarcomeres. The Z-lines of adjacent myofibrils are lined up with each other, forming alternating A-bands and I-bands. There is a lighter region in the middle of the A-band called the H-Zone. These bands appear continuous for all the fibrils of a muscle fibre and it is this alignment of banding that gives the fibre its striated appearance.

Figure:  Schematic diagram of striated (skeletal) muscle and its components