What Is the Basic Contracting Unit of a Skeletal Muscle
Botulinum toxin is a remedy that alters neuromuscular function. This toxin, produced by C. botulinum, prevents the release of ACh from the presynaptic membrane of the motor neuron. Therefore, skeletal muscles cannot contract, which leads to flaccid paralysis.  β-adrenergic heart and PKA activation also lead to phosphorylation of phospholamban, a subunit of the SR Ca2+ SERCA pump. Phosphorylated phospholamban does not inhibit the pump, allowing for faster removal of Ca2+ from the cytosol in the SR (Wegener et al. 1989). This further accelerates relaxation, but also leads to the SR being charged with more Ca2+, so that despite the reduced systolic interval, more Ca2+ is available for release. Because skeletal muscle cells are long and cylindrical, they are commonly referred to as muscle fibers. Skeletal muscle fibers can be large enough for human cells, with diameters of up to 100 μm and lengths of up to 30 cm (11.8 in.) in the sartorius of the thigh. In early development, embryonic myoblasts, each with its own nucleus, fuse with up to hundreds of other myoblasts to form multinucleated skeletal muscle fibers.
Multiple nuclei mean several copies of genes that allow the production of the large amounts of proteins and enzymes needed for muscle contraction. The process of contraction of filament slip can only occur when the myosin binding sites on the actin filaments are exposed by a series of steps that begin with the entry of Ca++ into the sarcoplasm. Tropomyosin wraps around the chains of the actin filament and covers the myosin binding sites to prevent actin from binding to myosin. The troponin-tropomyosin complex uses the binding of calcium ions to TnC to regulate when myosin heads form transverse bridges with actin filaments. The formation of transverse bridges and the sliding of filaments occur when calcium is present, and the signaling process that leads to calcium release and muscle contraction is called excitation-contraction coupling. When a muscle contracts, the power of the movement is transmitted by the tendon that pulls on the bone to create skeletal movements. Muscle cells specialize in contraction. Muscles allow movements such as walking and also facilitate physical processes such as breathing and digestion. The body contains three types of muscle tissue: skeletal muscle, heart muscle, and smooth muscle (Figure 19.33).
As in skeletal muscle, the binding of Ca2+ to Tn C regulates the actin-myosin interaction in the heart muscle. However, cardiomyocytes have developed mechanisms to modify the regulation of thin filaments in order to quickly adjust contractility properties in response to immediate stimuli. This is important because each heart cell contracts with each heartbeat, so any change in the force generated by the heart must be achieved by changing the force output of each cell. The heart has the ability to change its performance, and the speed at which it operates on a beat-to-beat basis. In addition, in response to chronically altered conditions such as high blood pressure or damage caused by coronary occlusion or repeated acute stimuli, such as training, the heart adapts for several days by rebuilding its cellular components. Watch this animation of the muscular contraction of the sacred bridge. In the Z line, the beard end of each actin filament (each closed by Z cap) is connected to the spiny ends of the four thin filaments closest to the neighboring sarcoma by cross-connections of most of the time of α-actinin, a major component of the Z lines, which can create a zigzag pattern visible in simple Z lines. The thickness of the Z line varies from one species to another, depending on the length and arrangement of the connecting elements. In fish, for example, the Z line is very narrow, with the limbs forming a simple pattern woven into cross-sections.
Frogs, on the other hand, have longer connecting bonds that take two different cross-section configurations depending on the sarcomeric state, resulting in a wider Z-line. The Z-lines in mammalian muscles are thick due to the longitudinal overlap of thin filaments of two adjacent sarcomeres with two or more stages of periodicly arranged cross-connections that connect them. Typically, fibers that maintain prolonged contractile activity, such as the heart muscle, have thicker-width Z-lines, with more connection planes between thin filaments. Skeletal muscles have an abundant supply of blood vessels and nerves. This is directly related to the primary function of skeletal muscle, contraction. Before a skeletal muscle fiber can contract, it must receive an impulse from a nerve cell. In general, an artery and at least one vein accompany any nerve that enters the epimysium of a skeletal muscle. The branches of the nerve and blood vessels follow the connective tissue components of the muscle of a nerve cell and with one or more small blood vessels called capillaries. The number of transverse bridges formed between actin and myosin determines how much tension a muscle fiber can create. Transverse bridges can only form where thick, thin filaments overlap, allowing myosin to bind to actin.
As more transverse bridges form, more myosin will pull on the actin and more tension will be generated. In addition to structural and energy-generating roles, sarcomeres are involved in signaling processes that provide cellular feedback in response to contraction-based stimuli. As a transmitter of the force generated by the actin-myosin interaction, the Z-line is an ideal location for force exploration mechanisms that allow myocytes to adapt to load needs. In fact, Z-line structural proteins with signaling potential include members of the myotiline, FATZ, and Enigma protein families (by Nandelstadh et al. 2009). The myotilin family includes myotiline, palladine and myopalladin, all of which contain Ig domains and bind α-actinin, filamine and FATZ. The FATZ family includes three forms of FATZ (also known as cassarcin and myozenin), where FATZ-1 and FATZ-3 are expressed in fast-twitch muscles and FATZ-2 in slow contractions and heart muscles. Binding partners in the FATZ family include myotiline, filamine, telethonin, α-actinin and cypher.
Proteins of the Enigma family contain an aminoterminal PDZ domain and 0 to 3 LIM domains at the carboxyl end. Cypher (also known as ZASP or Oracle) is the most studied puzzle member (Faulkner et al. 1999; Passier et al., 2000; Zhou et al. 2001) and could serve as a strut by binding to α-actinin via its PDZ domain, and could be involved in signaling as a binding partner to protein kinase C via its LIM domains. Cypher is essential for maintaining Z-band structure and muscle integrity (Zhou et al. . .