Contraction and Relaxation

The thin filaments and the heads of the thick filaments reverses half way between the Z lines. Therefore, the two thin filaments which interact with one thick filament are drawn toward each other, resulting in a shortening of the distance between the Z lines. 

When the myosin heads release ADR and Pi and become detached from the thin filaments (Fig. 12.11c), the heads are ready to take up a fresh charge of ATP (Fig. 12.lid). If the Cd + concentration in the sarcoplasm remains high, the ATP will again hydrolyze and the interaction of the myosin heads with the thin filament is repeated (Fig. 12.11a). However, if the Ca + concentration drops in the meantime, no ATP hydrolysis occurs, tropomyosin again blocks the access of myosin heads to the actin binding sites 

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The cells of the latter three types contain only a single nucleus and are called myocytes. The cells of skeletal muscle are long and multinucleate and are referred to as muscle fibers. At the microscopic level, skeletal muscle and cardiac muscle display alternating light and dark bands, and for this reason are often referred to as striated muscles. The different types of muscle cells vary widely in structure, size, and function. In addition, the times required for contractions and relaxations by various muscle types vary considerably. The fastest responses (on the order of milliseconds) are observed for fast-twitch skeletal... 

Hypothyroid myopathy occurs in about 30% of patients with hypothyroidism irrespective of its cause. Muscle pain, cramps, and stiffness may be seen, and are often exacerbated by cold weather. Pseudomyotonic features of delayed muscle contraction and relaxation are common. Myoedema (the mounding phenomenon) is due to the painless, electrically silent contracture produced on direct percussion. Muscle biopsy often shows a predominance of type 1 (slow-twitch) fibers, again analogous to that seen in experimental hypothyroidism (Figure 22). Muscle hypertrophy with weakness and slowness of movement occurs in the Debre-Semelaigne syndrome seen in severely hypothyroid children, and Hoffman s syndrome is a similar condition seen in adults with hypothyroidism, but is also accompanied by painful spasms. 

Resilin has a remarkably high fatigue lifetime (probably >500 million cycles) and our aim is to reproduce this desirable mechanical property in synthetic materials derived from our studies of resilin structure and function. We believe that recombinant resilin-like materials may be used, in the future, in the medical device field as components of prosthetic implants, including spinal disks and synthetic arteries. Spinal disks, for example, must survive for at least 100 million cycles of contraction and relaxation [30]. 

The major biochemical events occurring during one cycle of muscle contraction and relaxation can be represented in the five steps shown in Figure 49-6 ... 

The contraction of muscles from all sources occurs by the general mechanism described above. Muscles from different organisms and from different cells and tissues within the same organism may have different molecular mechanisms responsible for the regulation of their contraction and relaxation. In all systems, plays a key regulatory role. There are two general mechanisms of regulation of muscle contraction actin-based and myosin-based. The former operates in skeletal and cardiac muscle, the latter in smooth muscle. 

Thus, Ca " controls skeletal muscle contraction and relaxation by an allosteric mechanism mediated by TpC, Tpl, TpT, tropomyosin, and F-actin. 

Table 49-1 summarizes the overall events in contraction and relaxation of skeletal muscle. 

Cameron, N.E. and Cotter, M.A. (1992) Impaired contraction and relaxation in aorta from streptozocin-diabetic rats role of polyol pathway. Diabetologja 35, 1011-1019. 

Sodium and potassium are used for the electrochemical transfer of signals in the nervous system. The contraction and relaxation of muscles are regulated by an interplay of calcium and... 

Although skeletal muscle comprises the bulk of muscle tissue in the body, smooth muscle is far more important in terms of homeostasis. Most smooth muscle is found in the walls of tubes and hollow organs. Contraction and relaxation of the smooth muscle in these tissues regulates the movement of substances within them. For example, contraction of the smooth muscle in the wall of a blood vessel narrows the diameter of the vessel and leads to a decrease in the flow of blood through it. Contraction of the smooth muscle in the wall of the stomach exerts pressure on its contents and pushes these substances forward into the small intestine. Smooth muscle functions at a subconscious level and is involuntary. It is innervated by the autonomic nervous system, which regulates its activity. 

Bronchial smooth muscle tone. Changes in bronchial smooth muscle tone are particularly important in the bronchioles compared to the bronchi. Recall that the walls of the bronchioles consist almost entirely of smooth muscle. Contraction and relaxation of this muscle has a marked effect on the internal radius of the airway. An increase in bronchial smooth muscle tone, or bron-choconstriction, narrows the lumen of the airway and increases resistance to... 

Ultrarapid computed tomography may minimize artifact from heart motion during contraction and relaxation and provides a semiquantitative assessment of calcium content in coronary arteries. 

Smooth muscle is distributed throughout the body, largely around hollow structures such as blood vessels, the gastrointestinal tract and the genitourinary system. Normal function requires that the smooth muscles contract and relax at appropriate times, and abnormalities of contraction underlie such important pathologies as hypertension, incontinence and abnormal childbirth. Since contraction is initiated by an increase of cytoplasmic Ca2+ concentration then normal function requires appropriate Ca2+ handling. 

Perez GJ, Bonev AD, Nelson MT 2001 Micromolar Ca2+ from sparks activates Ca2+-sensitive K+ channels in rat cerebral artery smooth muscle. Am J Physiol Cell Physiol 281 C1769-C1775 Rembold CM 1992 Regulation of contraction and relaxation in arterial smooth muscle. Hypertension 20 129-137... 

Uterine muscle contains both a- and (3-adrenoceptors, which mediate contraction and relaxation, respectively. The response of the human uterus to catecholamines is variable and depends on the endocrine balance of the individual at the time of amine administration (see Chapter 62). During the last stage of pregnancy and during parturition, epinephrine inhibits the uterine muscle, as does isoproterenol norepinephrine contracts the uterus. 

Normal function, then, requires organ system integrity (Fig. 51.1). Muscular and vascular apparatuses are equally important to sensory and motor innervation (Elbadawi, 1973). In the bladder, for example, muscular function includes storage (distensibility, or compliance) as well as elimination (coordinated muscular contractions) and therefore requires appropriate sphincter tone, contraction, and relaxation. Neurovascular bundles provide critical vascular and neural input and output for all pelvic subunits (Daniel et ah, 1983). 

The biological importance of nitrogen monoxide (NO) as a messenger substance was originally recognized in coimection with contraction and relaxation of blood vessels. In the meantime, it has become clear that NO is a universal messenger substance that takes part in diverse forms in intercellular and intracellular commimication. Practically every cell in mammals is subject to regulation by NO in one form or another. 

The regulation of smooth muscle contraction and relaxation is shown schematically in Figure 12-1. The mechanisms of action of the major types of vasodilators are listed in Table 11-2. As shown in Figures 12-1 and 12-2, drugs may relax vascular smooth muscle in... 

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