The cellular properties of skeletal muscle fibers are best appreciated in longitudinal sections. The fibers are delimited by the plasma membrane or sarcolemma, which is covered with an external slide (colored by a periodic acid reaction). Thin, elongated nuclei are usually arranged under the sarcolemma spirally at a distance of 10-50 μm. On myotendinous compounds, muscle fibers have many nuclei located in the center. The nuclei of the satellite cells are positioned between the sarcolemma and the external slide. The fibers contain hundreds of longitudinally aligned myofibrils, which consist of repetitive sarcomeres. The characteristic transverse crossing of skeletal muscle fibers results from the parallel alignment of ligaments in neighboring myofibrils. The largest bands, which are designated by their appearance in polarized light, are the A bands (anisotropic or birefringent, appear light) and the I bands (iostropic, appear dark). The I bands, consisting of thin myofilaments, are halved by Z lines (disks, bands) that form the end of each sarcomere; the A bands, consisting of thick filaments, are halved by the less birefringent H bands. The tape pattern, named after the appearance in polarized light, is reversed when examined by phase contrast optical microscopy, optical microscopy with conventional optics on sections colored with the usual cationic dyes, or transmission electron microscopy. The evolution of contractile muscles has given the higher organisms of the animal kingdom the ability to be mobile in their environment. There are three types of muscles: skeleton, heart and smooth.
Myocytes are the cellular unit of muscle structure and contain high concentrations of specialized proteins that use chemical energy to generate mechanical force in the form of cell contraction. Skeletal and cardiac muscles are called striped muscles because of the visible organization of repetitive units of contractile filaments, known as sarcomas, into cylindrical bundles called myofibrils. In mature muscle fibers (striated muscle myocytes), most of the cell volume is occupied by myofibrill, leaving little room for nuclei and the associated Golgi system, mitochondria, sarcoplasmic reticulum (SR; the specialized endoplasmic reticulum of striated muscle), glycogramide granules, and other organelles/structures. In contrast, smooth muscles have large amounts of actin and myosin filaments that are not organized into sarcomeres. Unlike striated muscle cells, which are postmitotic, smooth muscle cells can multiply under physiological and pathological conditions. The striped muscles are regulated by Ca2+, which is released by the SR and binds to troponin (Tn) on the actin filament. This event releases tropomyosin (Tm) from its position, which blocks the interaction of myosin heads with actin. However, smooth muscle does not contain Tn, and contraction is regulated by the level of phosphorylation of the myosin regulating light chain (RLC). Skeletal muscle contains three different layers of connective tissue (Figure 1): cytokinesis. After the completion of mitosis (nuclear division), a contractile ring composed of filaments of actin and myosin II divides the cell into two parts.
The tropomyosin protein covers the myosin binding sites of actin molecules in the muscle cell. For a muscle cell to contract, tropomyosin must be moved to reveal the binding sites on the actin. Calcium ions bind to troponin C molecules (which are distributed throughout the tropomyosin protein) and alter the structure of tropomyosin, forcing it to expose the binding site of the transverse bridge to actin. Tm is present in smooth muscle in about the same ratio to actin as in skeletal muscle and is also associated with actin filaments. However, smooth muscles lack Tn proteins, which are necessary to ensure the regulation of thin filaments in skeletal and cardiac muscles. Instead, thin smooth muscle filaments may contain caldesmon or calponin, which appear to compete for occupation on thin filaments (Makuch et al. 1991). Although the exact physiological roles of these two proteins in smooth muscle are not yet clear, they appear to play modulatory functions by finely adjusting the contractile properties of smooth muscle. The contractile apparatus of smooth muscles. (A) Schematic representation of the key components of the energy-producing protein network in mammalian smooth muscle. (B) The organization and rearrangement of the cytoskeleton of the smooth muscle cell during contraction.
The contraction of skeletal muscles is triggered by nerve impulses that stimulate the release of Ca2+ from the sarcoplasmic reticulum – a specialized network of inner membranes, similar to the endoplasmic reticulum, which stores high concentrations of Ca2+ ions. The release of Ca2+ from the sarcoplasmic reticulum increases the concentration of Ca2+ in the cytosol by about 10-7 to 10-5 M. The increase in Ca2+ concentration signals muscle contraction via the action of two accessory proteins bound to actin filaments: tropomyosin and troponin (Figure 11.25). Tropomyosin is a fibrous protein that binds longitudinally along the groove of actin filaments. In striated muscle, each tropomyosin molecule is bound to troponin, which is a complex of three polypeptides: troponin C (Ca2+ bond), troponin I (inhibitory), and troponin T (tropomyosin binding). When the concentration of Ca2 + is low, the troponin complex with tropomyosin blocks the interaction of actin and myosin, so that the muscle does not contract. At high concentrations, the binding of Ca2+ to troponin C shifts the position of the complex, relieving this inhibition and allowing contraction to continue. Clark, M.
Milestone 3 (1954): Sliding filament model for muscle contraction. Filaments of muscle slippage. Nature Reviews Molecular Cell Biology 9, s6–s7 (2008) doi:10.1038/nrm2581. The myofibrils of smooth muscle cells are not arranged in sarcomeres. .