Muscle is a specialized tissue of mesodermal origin that has special properties like excitability, contractility, extensibility, and elasticity.
Skeletal muscle consists of numerous physiological units called muscle fibres of muscle cells. Each muscle fiber is elongated and syncytial (multinucleate). Its membrane is called sarcolemma and its cytoplasm is called sarcoplasm. Each fire contains many thin and rod-like myofibrils with characteristic cross striations. The myofibrils are arranged in a number of section, called sarcomeres.
The sarcomeres are delineated by a very thin and comparatively dense Z-line (Krause’s membrane). A dark anisotropic band (A-band) is present in the centre of the sarcomere. Adjacent to this lies a light isotropic band (I-band). At the centre of the A-band, a comparatively less dark zone called H-zone (Henson zone) is present.
In the centre of H-zone, M-line is present. The Z-line is located at the centre of the I-band. The part of the myofibril between two successive Z-lines, constitutes the functional unit of myofibril i.e., sarcomere.
Each sarcomere is a bundle of fine longitudinal myofilaments of two types:
(i) Primary myofilaments: These are thicker and are formed of the protein myosin which is a polymerized protein made of many monomeric proteins called meromyosins. Each meromyosin has two important parts – a globular head with a short arm, and a tail; the former is termed heavy meromyosin (HMM) and the latter called light meromyosin (LMM).
The HMM component projects outwards at an angle from myosin filament at a regular distance, and is known as the cross arm. The globular head has an active ATPase enzyme which has sites for attachment with ATP and actin.
(ii) Secondary myofilaments: These are thinner and composed of the protein actin, having with it two regulatory proteins: tropomyosin and troponin. Two filaments of tropomyosin protein run along the length of F-actin. Also, a complex troponin protein of three globular peptides is distributed at regular intervals on the tropomyosin.
In the resting stage of muscle fibre, a subunit of troponin masks the active sites for myosin on the actin filaments. Partial overlapping of the primary myofilaments by the secondary myofilaments imparts dark appearance to the A bands.
The skeletal muscles are of two types – Red and White muscles. The red muscles contain a very high amount of red pigment, myoglobin, while the white muscles have a very low amount of myoglobin. Little lactic acid accumulates in aerobic respiration, thus, enabling the red muscle fibres to carry on slow and sustained contractions for long periods without fatigue e.g., extensor muscles of the back.
White muscles derive energy for their fast contractions by anaerobic oxidation, accumulate lactic acid during strenuous work and soon get fatigued e.g., muscles moving eyeballs.