 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 LIMA. Muscle cells are called cytoplasm or sarcoplasm and its endoplasmic reticulum is called sarcoplasmic reticulum. Within each muscle cell there are many myofibrils which are thinner in diameter and run in parallel fashion. Muscle cells are long cells and they run in long threads and they are called myofibrils. Myofibrils run from 1 to 2 micrometre in diameter and they run in the length of the muscle cell. Dear students, every myofibril is itself composed of myofilaments. These myofilaments are of two types, one thin filaments which are made up of protein actin and the other thick filaments which are composed of protein myocin. Dear students, this thick and thin filaments arrangement in myofibrils produces a particular thin and thick pattern in muscle cells in which light bands alternate from dark colored bands. Due to this, muscle cells produce striped appearance or striations. Every repeating unit of light and dark band is known as sarcomere. This sarcomere is actually 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 the dark band is comparatively lighter color. The portion of the lighter color is called the edge zone. This edge zone itself has a darker line which is called M line. This is the structure of dark band. So we have seen the dark band of sarcomere. Now we shall see the structure of the light band in sarcomere. We call the light band as 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. In this way one sarcomere starts with one Z line and ends with the other. These two Z lines come up and down. The successive two Z lines are called sarcomere. And the sarcomere are attached to each other. And one line starts with the sarcomere.