Skeletal Muscle:

Each skeletal muscle is made up of a number of muscle bundles or fascicles connected by a sheath of connective tissue called fascia. And each muscle bundle is made up of a number of muscle fibres. Each muscle fibre is enclosed by sarcolemma enclosing sarcoplasm, which contains many nuclei and endoplasmic reticulum (store house of Calcium ion). Sarcoplasm contains a large number of parallelly arranged filaments called myofilaments or myofibrils, which contain light and dark band on it.

The light band (I-band or Isotropic band) contains actin (thin filament) and the dark band (A-band or Anisotropic band) contains myosin. actins are held together in the center by an elastic fibre called “Z” line and myosin filaments are held together in the middle by a thin fibrous membrane called “M” line. The portion of myofibrils between two successive “Z” line is the called sarcomere. In resting phase ends of actin filaments partially overlap the ends of myosin filaments.
Actin: An actin is made up of two “F” (filamentous) actin, polymer of monomeric “G” (globular) protein, helically wounded around each other. Two filaments of another protein, tropomyosin, on which complex protein troponin is distributed at regular intervals, also run close to the two “F” actins. During resting state, subunit of troponin masks the active binding site for the myosin on the actin filaments.

Myosin: Each actin filament is made up of monemeric protein meromyosin. Meromyosin has two parts: a globular head and a short arm-also called heacy meromyosin (HMM), cross arm- and a tail also called light meromyosin (LMM). The globular head is an active ATPase and has active binding sites for ATP and active sites for actin.