Muscle tissues
Muscle tissues are responsible for movements of the body and for change in the size and shape of its internal organs. Muscle cells contain contractile proteins – actin, myosin and others.
There are two principal types of muscle tissues: striated muscle and smooth muscle. Striated muscle tissue is subclassified on the basis of its location: skeletal and cardiac muscle tissues.
SKELETAL MUSCLE
Skeletal muscle is made up of long, cylindrical fibres. Each fibres is really a symplastum with hundreds of nuclei along its length.
The nuclei are elongated and lie just under the sarcolemma (Gr: sarkos – meat), covering the fibers.
The thick sarcolemma consists of basal or external lamina and internal lamina. External lamina similar to the basal lamina of epithelium and internal lamina is usual plasma membrane of the cell.
The cytoplasm (which is called the sarcoplasm) is filled with numerous longitudinal fibrils, that are called myofibrils.
The transverse striations of the fibers are seen as alternate dark and light bands. The dark bands are called A-bands (anisotropic, birefringent in polarised light). The light bands are called I-bands (isotropic, monorefringent). There is a thin dark line, called the Z-band (disc, line), running across the middle of each I-band. A lighter band called the H-band traverses the centre of A-band. There is a thin dark line called M-line, running across the H-band.
The part of a muscle fibril situated between two successive Z-bands is called a sarcomere. Its formula: S = 1/2I + A +1/2I
Sarcomere is the functional unit of the striated fibers.
Ultrastructure of myofibril
Each myofibril is composed of bundles of myofilaments. There are two types of myofilaments: thin filaments, composed primarily of the protein actin, and thick ones, composed of protein myosin.
Z-disc is complicated network, which appears as a zigzag line. The thin, actin filaments, attach to the Z-line and extend into the A-band to the edge of the H-zone.
Thick filaments are placed between thin filaments and don’t extend to the Z-discs. They are linked with thin filaments by bridges.
Therefore the I-band consists of actin filaments alone and the A-band consist of both actin and myosin filaments. The H-band represents the part of the A-band into which actin filaments do not extend. The M-band is produced by fine interconnections between adjacent myosin filaments.
An actin filament is really composed of two subfilaments that are twisted round each other. Each subfilament is a chain of globular (rounded) actin molecules and fibrillar tropomyosin molecules. So actin filaments have appearance like beads on a string.
Myosin filament is made up of large number of myosin molecules. Myosin molecules consist of a long tail and a rounded head. A myosin filament is a "bundle" of the tails of such molecules. The myosin molecules aggregate tail to tail with a special globular proteins, forming M-line. The heads project outwards and establish bonds (bridges) with actin filaments. The myosin posses adenosine triphosphatase activity.