Myofibrils are made up of thick, thin myofilaments that give the muscle its striped appearance. The thick filaments are made of myosin, and the thin filaments are mainly actin, as well as two other muscle proteins, tropomyosin and troponin. Muscle cells are designed to generate strength and movement. There are three types of mammalian muscles – skeleton, heart and smooth. Skeletal muscles are attached to the bones and move them relative to each other. The heart muscle includes the heart, which pumps blood through the vascular system. Skeletal and cardiac muscles are called striped muscles because the filaments of actin and myosin that cause them to contract are organized into repetitive arrangements called sarcomeres, which have a microscopic striped appearance. Smooth muscle does not contain sarcomers, but uses the contraction of filaments of actin and myosin to narrow blood vessels and move the contents of hollow organs in the body. Here, we examine the main molecular organization of the three types of muscles and their contractile regulation through signaling mechanisms and discuss their main structural and functional similarities that indicate the possible evolutionary relationships between cell types. Like the Z line, the M line has a complex composition that includes M protein, myomesin and creatine kinase. In particular, myomesin is important as a structural binder between thick filaments and therefore serves as the most important crosslinking protein of the M line (Lange et al.
2005). The extent of cross-linking of thick filaments by the M-band depends on the type of fiber, as it is more extensive in fast-twitch fibers, somewhat incomplete in slow-twitch fibers, and absent in slow-twitch tonic fibers of all vertebrates. In addition, some mammalian extraocular muscles do not really have an M line. Closer to the sarcolemma, the M and Z lines of myofibrils are peripherally associated with obscurine and ankyrin, which are involved in the interaction between myofibrils and cytoskeleton. Smooth muscle actin includes the thin filament network which, compared to skeletal muscle actin in skeletal muscle, can be twice as high as concentration in smooth muscle compared to myosin content (Gabella 1984). Thin filaments are richly distributed throughout the cell, usually aligned parallel to the longitudinal axis of the cell, and sometimes appear grouped. The length of the thin filament is on average longer than in skeletal muscle. Some actin filaments are anchored to “dense bands” on the plasma membrane, others end in cytoplasmic “dense bodies” (Fig.
4). A network of intermediate filaments consisting of desmin (sometimes replaced by vimentin in some smooth muscle cells) connects the membrane and dense cytoplasmic bodies (Stromer and Bendayan 1988) and strengthens the thin filament system. Both smooth and tense body types contain actinin α. These similarities in protein content and function support the idea that the intermediate filamentous network/dense body network is essentially a dispersed Z line – or, more likely, Z lines that emerged as an aggregate of dense bodies during the evolution of striated muscles. 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. Titin is a huge, extensible protein that extends from the Z line on half of the I band on the thick filament and ends at the M line.
The extensibility of the titin molecule and therefore the intrinsic passive tension of myofibers and cardiomyocytes is attributed to the tension region of band I. In skeletal muscle, this region (of aminoterminus) consists of a proximal immunoglobulin (Ig) domain, a variable number of Ig domains, an N2-A segment, a PEVK domain, and a distal series of Ig domains before it reaches the edge of the A band. The heart muscle contains a rigid N2-B segment between the proximal and distal regions of the Ig domains and may or may not have a complementary and more conformal N2-A segment. Therefore, the isoforms of cardiac titin are called “N2-B” (absence of N2-A) or “N2-BA” (contains N2-A). The spring properties of the I-band region have two phases: low strain forces straighten the irregular chain of Ig domains, and higher forces stretch the PEVK domain, which further interacts with the thin filament (Linke and Kruger 2010). The contractile functional unit of myofibrils is called sarcomeres, which is about 1.6 to 2.0 μm long. The most notable functional requirement of skeletal muscle is the ability to produce rapid bursts of force and/or movements under the control of the nervous system. The movement itself should not only be fast, but also the contractile activity should turn on and off immediately. The heart muscle needs similar circumstances to maintain blood circulation and blood flow based on an organism`s energy needs. In contrast, smooth muscles perform functions in the body that last more or less continuously, often requiring very little movement (as in the case of vasoconstriction to maintain blood pressure or direct blood flow), but rather requiring the maintenance of stable tonic forces. In smooth muscles where movement is important for function (for example. B, intestinal peristalsis), movements can be quite large in terms of muscle size, but are usually several hundred times slower than in skeletal muscle.
To fulfill these different functional roles, striated and smooth muscles have developed different structural, physiological and biochemical profiles. Figure 1 shows the sarcomaer, which is the basic contractile unit of the striated muscle. Sarcomeres are organized in series to form a myofibril. The sarcomere is defined as an expanse from the Z line to the Z line (described in detail below), only a few microns long, and consists of an A band containing myosin filaments (“thick”) flanked by two half-bands I consisting of actin filaments (“thin”). This A-band is the central region of the sarcomere, composed mainly of filaments of myosin, the energy-generating motor protein of skeletal, cardiac and smooth muscles (reviewed by Sweeney and Holzbaur 2016). Muscular myosin, now called myosin II or conventional myosin, was the first of many members discovered in the myosin motor protein superfamily (Odronitz and Kollmar 2007). An additional non-muscular form of myosin II contributes to cytokinesis and cellular locomotion of amoebae, fungi and animal cells (Bresnick 1999) and may play an important role in smooth muscle cells. The myofibrils of the striated muscles (skeleton and heart) consist of sarcomeres in series. Both ends are delimited by Z-lines (also called Z-bands or Z-discs), thin disks characterized by high protein density, high refractive index, and high electron density. A series of bands – ordered half-band I, band A, half band I, which have differential luminous optical properties and electron-optical properties due to their structural components – are located between the Z lines (which halve the continuous I bands).