Fast-twitch fibers are good for fast movements such as jumping or sprinting, which require short-term rapid muscle contractions. Unlike slow-twitch fibers, fast-twitch fibers depend on anaerobic respiration (glycolysis alone) to produce two ATP molecules per glucose molecule. Although it is much less effective than aerobic breathing, it is ideal for rapid movement spurts because it is not limited by oxygen demand. Lactate (lactic acid), a byproduct of anaerobic respiration, accumulates in muscle tissue, reduces pH (makes it more acidic and creates the tingling sensation in the muscles during exercise). This inhibits subsequent anaerobic respiration. While this may seem counterintuitive, it is a feedback cycle to protect muscles from overwork and the resulting damage. The heart muscle is the muscle of the heart. It contracts, is regulated autonomously and must continue to contract rhythmically throughout the life of the organism. Therefore, it has peculiarities. The lower myosin content and incomplete polymerization of myosin result in a much lower density of thick smooth muscle filaments than in skeletal and cardiac muscles. The thick filaments of smooth muscles are distributed more or less evenly in the cell, approximately parallel to the longitudinal axis. The increase in actin content leads to a final ratio of thick to thin filaments of about 1:15 in smooth muscles, as opposed to 1:2 in skeletal and cardiac muscles (Gabella 1984). Skeletal muscle connects to the skeletal system primarily through tendons to maintain posture and control movement.

For example, the contraction of the biceps muscle, attached to the shoulder blade and radius, will lift the forearm. Some skeletal muscles can adhere directly to other muscles or to the skin, as seen on the face, where many muscles control facial expression. 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. In skeletal muscle, contraction is stimulated by electrical impulses transmitted by motor nerves. Contractions of the heart and smooth muscles are stimulated by the internal cells of the pacemaker that regularly contract and spread the contractions to other muscle cells with which they are in contact. All skeletal muscles and many smooth muscle contractions are facilitated by the neurotransmitter acetylcholine. An important feature of actin filament formation is the intrinsic structural and functional polarity caused by the uniform orientation of actin monomers in the filament and the non-identical domains that make up monomers, giving each end different properties. Direct evidence of this polarity was demonstrated by the complete decoration of actin filaments with myosin heads, resulting in filaments apparently covered with „arrowheads“. These arrowheads point away from the Z line and therefore coincide with a directional dependence of the myosin-actin interaction for the sliding of filaments during muscle shortening. Unlike striated muscles, smooth muscles can maintain contractions in the very long term.

Smooth muscles can also stretch and maintain their contractile function, which striated muscle cannot. An extracellular matrix secreted by myocytes increases the elasticity of smooth muscles. The matrix consists of elastin, collagen and other stretchy fibers. The ability to stretch and contract is an important feature of smooth muscle in organs such as the stomach and uterus (Figure (PageIndex{9})), both of which must stretch significantly if they perform their normal functions. There are two main classes of these thin filament regulatory mechanisms that allow heart muscle cells to alter their force response to a particular cytosolic concentration of Ca2+ – dependence on the length of Ca2+ sensitivity and neuroendocrine control. Dependence on the length of calcium sensitivity, as shown by permeabilized heart cells, involves an increase in force generation by submaximal stimulation of Ca2+ with increasing length of the sarcomere (Kentish et al. 1986). Although the maximum force activated by Ca2+ does not change, the activation threshold concentration changes and changes the ratio of strength to Ca2+ concentration. The basis of the mechanism is ambiguous, because a change depending on the length of the Ca2+ bond to Tn C does not depend on the cardiac isoform of Tn C (Moss et al. 1991), although some evidence suggests that titin is involved in this phenomenon (Le Guennec et al.

2000). This mechanism should contribute to the relationships between length and tension (cellular level) or pressure-volume (ventricular level). Muscle tissue is soft tissue that makes up the different types of muscles in most animals and gives muscles the ability to contract. It is also known as myopropulsive tissue. Muscle tissue is formed during embryonic development in a process known as myogenesis. Muscle tissue contains special contractile proteins called actin and myosin, which contract and relax to induce exercise. Among many other muscle proteins, there are two regulatory proteins, troponin and tropomyosin. The Z-lines consist of a dense network of proteins that cross-link thin actin filaments and transmit force along the myofibrils of skeletal and cardiac muscles. The actin filaments are held in a precise orthogonal arrangement in the Z lines, which is significantly different from the hexagonal arrangement of the overlapping region of the A-band. The transition between orthogonal and hexagonal arrangements occurs gradually with longer (i.e.

dormant) sarcomere lengths – however, it becomes increasingly abrupt and even problematic at shorter lengths when the sarcomere is shortened so that the edges of the A-band are very close to the Z-lines due to hypercontraction. Skeletal muscle contains various fibers that allow for both fast short-term contractions and slower, reproducible long-term contractions. The heart muscle is an involuntary striated muscle located in the walls and histological base of the heart, especially the myocardium. Heart muscle cells (also called cardiomyocytes or myocardiocytes) mainly contain only one nucleus, although there are populations with two or four nuclei. [10] [11] [Page needed] The myocardium is the muscle tissue of the heart and forms a thick intermediate layer between the outer epicardial layer and the inner endocardial layer. .