Structure of Skeletal Muscle

Consists of bundles of muscle fibres (muscle cells) called fascicles enclosed in fibrous (collagen) connective tissue cover that finally  emerge as tendons at both ends by which they attach with bones. Each muscle fibres is a long, cylindrical, coenocytic (multinucleated) cell. Its plasmalemma, (sarcolemma) forms T-tubules by transverse invagination at intervals. The cytoplasm (sarcoplasm) is rich in mitochondria (sarcosome) and myofibrils. E.R. is greatly reduced as vesicles called   S.R. Striations (alternate dark and light bands) are formed due to particular arrangement pattern of actin and myosin filaments—as revealed in electron microscopic structure of a myofibril.

Fig. Organisation of skeletal muscle, from the gross to the molecular level

Lengthwise each myofibril consists of many sarcomeres, which is plate like zig-zag structure through which thin filaments pass. It is the functional unit of muscle and constitute the area between two well defined Z-lines (Z-disc or Krause’s membrane or Dobies membrane). Dark band (Anisotropic band or A-band) consists of both actin (thin) and myosin (thick) filaments.  Light band (Isotropic band or I-band) consists of only actin filaments.
H-zone or Hensen’s disc within dark band consists of only myosin part.
During contraction the actin filaments slide over myosin filament as explained in sliding filament theory of muscle contraction by A.F. Huxley (1953) later supported by H.E. Huxley (1954).

Fig. Ultrastructure of Myosin and actin filaments

Actin, the filament (F-actin) is formed by polymerization of G-actin monomers. It has the site, occupied by regulatory protein troponin and tropomyosin, for the attachment of myosin.
Tropomyosin is rod shaped fibrous protein helically wrapped around the actin filament along its groove.
Troponin is small globular protein with 3 components: troponin-c, troponin-t and troponin-i masking the site on F-actin for attachment of myosin.
Myosin is a versatile protein with both fibrous and globular parts. The fibrous part (tail) aggregate to form the filament while globular part, project out at both ends as cross bridges or heads. Head is formed of HMM (heavy meromyosin) consisting S-1 (Sub-fragment -1) and the neck (or hinge). Tail part is formed of LMM (light meromyosin). Head acts as ATPase which after connecting with actin hydrolyse ATP.