 Dear students, in this topic we shall discuss the structure of muscles. Each muscle is composed of thousands of muscle cells. These muscle cells are called myofibers. These myofibers are thin, long, cylindrical and multi-nucleated cells. These cells in a muscle are arranged in groups or bundles which are called fascicles. Each muscle cell or myofibre is about 5 to 100 micrometre in diameter and may be many centimetres long. Muscle cells are usually called plasma membranes but these membranes are called lema. Muscle cells are called cytoplasm or sarcoplasm and its endoplasmic reticulum is called sarcoplasmic reticulum. Within each muscle cell there are many myofibers which are thinner in diameter and run in parallel fashion. Muscle cells are long cells and run in long threads and they are called myofibrils. Myofibrils are in diameter from 1 to 2 micrometre and run in length of the entire muscle cell. Dear students, every myofibril is itself composed of myofilaments. These myofilaments are of two types, thin filaments which are made up of protein actin and thick filaments which are composed of protein myocin. Dear students, this thick and thin filaments arrangement in myofibril produces a particular thin and thick band pattern in muscle cells which alternates light bands from dark colored bands. As a result of this, muscle cells produce striped appearance or striations. Every repeating unit of light and dark band is known as sarcomere and sarcomere is the functional unit of contraction in a muscle. Dear students, now we shall discuss the detail of structure of sarcomere. There are two types of bands in sarcomere, dark band and light band. Every dark band in the sarcomere is called the A band. This A band is anisotropic that is it can polarize visible light. The portion of this dark band is comparatively lighter in color. This lighter strip is called H zone. This H zone itself has a darker line which is called M line. This is the structure of dark band. Now we shall discuss the structure of the light band in sarcomere. We call it I band. This band is isotropic that is it is non-polarizing. It does not polarize the visible light. In this light band, in the center there is a midline which is called the Z line. One sarcomere is attached to the other sarcomere. Similarly, if one sarcomere is starting from the Z line, then the other one is ending from the Z line. These two Z lines that come up and down, the successive two Z lines are called sarcomere. And the sarcomere are attached to each other. They are in a line. And the long one is called myofibril.