What Is the Contractile Unit of Striated Muscle
SarkolemmaThe muscular cell membrane is called sarcolemma and the cytoplasm, the sarcoplasm. The sarcolemma of myocytes contains many intussusceptions (pits) called transverse tubules, which are usually perpendicular to the length of the myocyte. The transverse tubules play an important role in the supply of Ca+ ions to myocytes, which are crucial for muscle contraction. 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).
A sarcoma is the basic contractile unit of muscle fiber. Each sarcomere consists of two main protein filaments – actin and myosin – which are the active structures responsible for muscle contraction. The most popular model that describes muscle contraction is called slippery thread theory. In this theory, the active force is generated when the actin filaments slide beyond the myosin filaments, resulting in a contraction of an individual sarcomere. The rapid skeletal myosin gene locus (on chromosome 17p13 in humans) comprises six different severe chain genes: embryonic, perinatal (or neonatal), rapid type IIa, rapid type IIx (or IId), rapid type IIb and extraocular (Mahdavi et al., 1986; Weiss et al., 1999). Isoform IIb is abundant in the muscles of small mammals, especially rodents, but is not expressed by human myocytes. Since the original type I, IIa and IIb nomenclature was derived from histochemical characterization (Brooke and Kaiser, 1970), it was not clear whether the human fibers called IIb actually expressed the isoform later called IIx (DeNardi et al. 1993). Embryonic and perinatal isoforms are expressed during muscle development and repressed in adults during muscle regeneration after injury (Mahdavi et al. 1986). The other heavy chains are expressed in adult muscles, the order indicated above reflecting the increasing speed (shortening rate and actin-activated ATPase activity) of the myosins they form.
In mammals, an additional myosin-heavy chain called „superfast“ is strongly expressed in the jaw muscles of most mammals, but not in humans, where this gene does not code for a functional protein due to a two-base deletion/frame shift (Stedman et al. 2004). Slow-twitch skeletal muscles (type I fiber) express the same β-heart myosin isoform as heart cells. Expression of the heavy chain of myosin α-cardiac is much rarer in skeletal muscle and has only been shown in the muscles of the head and neck. Comparison of the properties of the three types of mammalian muscles The main difference between myofibrils and sarcomas is that myofibrils are the contracting units of the muscles, while sarcomeres are the small repetitive units of the myofibrillus. Skeletal muscles are made up of myofibrils. . The repetitive unit of skeletal muscle is myofibrill. At low concentrations of free Ca2+, Tn restricts the position of Tm on the actin filament, and the Tn-Tm complex sterically blocks actin-myosin interactions (Xu et al. 1999).
The ability of Ca2+ to activate contraction (i.e., to enable actin-myosin interaction) when released by SR is mediated by binding to the Ca2+ aminoterminal binding site(s) of low-affinity TnC within the Tn complex. Note that there are two isoforms of TnC in striated muscles – fast TnC with two functional sites of Ca2+ aminoterminal binding and slow/cardiac TnC with one functional site – resulting in different sensitivities to Ca2+. Binding Ca2+ to the low-affinity TnC binding site(s) releases Tm from its steric blocking position, allowing myosin to interact with actin and generate strength. .