Surrounding the whole muscle is a layer of connective tissue, epimysium that encloses the individual fascicules together. The individual muscle fibers are made up of filamentous bundles that run along the length of the fiber. Most of the interior of the fibre consists of the protein filaments which constitute the contractile apparatus, grouped together in bundles called myofibrils. Each myofibril consists of a repeating unit, known as a sarcomere. The alignment of the sarcomeres between adjacent muscle fibers is responsible for the characteristic striations in the striated skeletal muscle fibers. The sarcomere is the fundamental contractile unit of the skeletal muscles. When a muscle fiber is viewed by polarized light, the sarcomeres are seen as alternating dark and light zones. Some regions appear dark because they refract the polarized light. This property is called anisotropy, and the corresponding band is known as an A band. The light regions do not refract polarized light and are called isotropic and are denoted as I bands. Each I band is divided by a characteristic line known as a Z line, and the unit between successive Z lines is the sarcomere .
Under electron microscopic examination with high magnification, the A bands are seen to be composed of thick filaments arranged in a regular order. The I bands consist of thin filaments. When the muscle is in the resting state, that is, when there is no shortening of the fibers due to contraction, a pale area can be seen in the center of the A band. This is known as the H zone, and it corresponds to the region where the thick and thin filaments do not overlap, which otherwise is the case throughout the muscle fiber architecture. In the center of each H zone, there is a line called M line. It is in this line, links are formed between adjacent thick filaments. The principal protein of the A bands is myosin, while that of I bands is actin. The interaction between these proteins is fundamental to the contractile process in the skeletal muscle at the ultrastructural and molecular levels. There are two types of actin filaments. One is globular in shape, known as globular or G actin subunits, and the other is filamentous of F actin. The actin filaments of the I band are made by joining many G actin subunits together by polymerization to form F actin. The F actin, in turn, is stabilized by binding to the Z line. The thick filaments are made up of an assembly of myosin molecules together. Each myosin molecule consists of two heavy chains. Each of these heavy chains has two light chains associated with a head region that is globular. The junction between the head region and the long tail contains a hinge. This hinge allows the myosin to generate the force required for muscle contraction. The tail regions of the myosin molecules associate together to form the thick filaments. Each thick filament consists of several hundred myosin molecules .
The sliding filament theory explains muscle contraction, and the structure of skeletal muscle provides important clues to the mechanism of contraction. The width of the A bands or thick filament areas in striated muscle remains constant, regardless of the length of the entire muscle fiber, while the width of the I bands or the thin-filament areas varies