Structure Of Skeletal Muscle

A striated muscle consists of numerous physiological units called muscle fibers or muscle cells. They are cylindrical in shape and arranged parallel to each other. They are between 10 to 100 (µm) in diameter, several centimeters long and multinucleate. The nuclei are located near the surface of each fiber. Bundles of muscle fibers are enclosed by collagen fibers and connective tissue, collagen also occurs between fibers. At the ends of the muscle the collagen and connective tissue forms tendons which attach the muscle to skeletal elements. 

Structure Of Skeletal Muscle

Each muscle fiber is enclosed by a membrane, the sarcolemma. This is very similar in structure to a typical plasma membrane. Within the muscle fibers are numerous thin myofibrils (myo, muscle, fibra, thread) which possess characteristic cross striations. The myofibrils are 1-2 µm in diameter that run in paralleled fashion and extend entire length of the cell. Each myofibril is composed of two types of proteinaceous myofilaments (or microfilament) actin and myosin. Numerous mitochondria are interposed between the myofibrils. The cytoplasm of the myofibril is called sarcoplasm and contains a network of internal membranes termed as sarcoplasmic reticulum.

The Anatomy Of Muscle From The Gross Structure Of An Arm Polarizing 

Under a light microscope only the striated nature of the myofibrils can be observed. This is seen as a regular alternation of light and dark bands called the I and A bands respectively, traversed by thin, dark lines. Electron microscope studies clearly indicate that the bands are due to the regular arrangement of actin (thin filaments) and myosin (thick filaments). Traversing the middle of each I band is a dark line called the Z line. (Z for zwish means between) The section of myofibril between two Z lines is called a sarcomere, which is a contractile unit. From the Z line actin filaments extend in both directions, whilst in the centre of the sarcomere (Gk. sarkos, flesh, and meros part) are found myosin filaments. In certain regions of the sarcomere, actin and myosin filaments overlap. Where they do, transverse sections in these regions indicate that six actin filaments surround each myosin filament. This arrangement of actin and myosin filaments results in a number of other bands being recognizable in the sarcomere. Myosin and actin filaments constitute the A band because they are anisotropic that they can polarize visible light. Actin filaments alone constitute the I band which are isotropic or polarizing.

The center of the A band is lighter than its other regions in a relaxed sarcomere as there is no overlap between actin and myosin in this region. It is called the W tone. (H stands for “hele” means bright). The H zone itself may be bisected by a dark fine, the M line. The M line joins adjacent myosin filaments together at a point halfway along their length.