Muscles contraction

An actin filament is about 1 jim long (see Fig. 9.4). It consists of two threads (F-actin) wound about each other and composed of globular monomers of actin maintaining their polarity. Fine tropomyosin threads, onto which troponin molecules are attached at regular intervals, run into the niches. The 

Change of angle a, a, corresponding to a consumption of energyrelative motion As of myosin-acdn myosin-actin rigor complex 

The myosin filaments, which are approximately twice as thick as actin filaments, are about 1.5 pm long and consist of several hundred myosin molecules connected in parallel, each with a braced head at the end. These heads are located at a distance of 426 A from one another in whorls of three. 

Excitation of a muscle fiber (or an entire muscle) under increasing extension results in a maximum of the relative stress (maximum stress set to 100%) when plotted against the relative length (unstimulated length set to 100%) in the vicinity of the unstimulated length (points 2 and 3 in Fig. 9.5). Under compression, the configuration is distorted and the stress that can be devel- 

Descriptions of a muscle contraction length-stress relationship and fila-configuration 

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Calcium plays an important part in structure-building in living organisms, perhaps mainly because of its ability to link together phosphate-containing materials. Calcium ions in the cell play a vital part in muscle contraction. 

Description of Method. Creatine is an organic acid found in muscle tissue that supplies energy for muscle contractions. One of its metabolic products is creatinine, which is excreted in urine. Because the concentration of creatinine in urine and serum is an important indication of renal function, rapid methods for its analysis are clinically important. In this method the rate of reaction between creatinine and picrate in an alkaline medium is used to determine the concentration of creatinine in urine. Under the conditions of the analysis, the reaction is first-order in picrate, creatinine, and hydroxide. 

As an activator of the phosphokinases, magnesium is essential in energy-requiring biological processes, such as activation of amino acids, acetate, and succinate synthesis of proteins, fats, coen2ymes, and nucleic acids generation and transmission of nerve impulses and muscle contraction (67). 

Morphine has certain undesirable side effects. Among these are respiratory depression, nausea, and vomiting, depression of the cough reflex, cardiovascular depression and hypotension, smooth muscle contraction (constipation), and histamine release (93). Morphine s onset of action, duration, and low therapeutic indices have prompted a search for a more effective opiate iv anesthetic. Extreme simplification of the complex morphine molecule has resulted in anilido —piperidines, the fentanyl class of extremely potent opiate iv anesthetics (118,119). 

Pig. 2. Proposed mechanism of inbition of smooth muscle contraction by P2" gonists, where AMP is adenosine monophosphate, cAMP is cycHc-3 5 adenosine monophosphate, ATP is adenosine triphosphate, and -P is an attached phosphate. 

The human body has more than 600 muscles. The body s movement is performed by muscle contractions, which are stimulated by the nervous system. This system links muscle tissue to the spinal cord and brain. The network of nerve cells which carries the brain s signals directs the flow of muscular energy. Most muscular activity occurs beyond the range of the conscious mind. The body, working through the neuromuscular network, manages... 

Proteins can be broadly classified into fibrous and globular. Many fibrous proteins serve a stmctural role (11). CC-Keratin has been described. Fibroin, the primary protein in silk, has -sheets packed one on top of another. CoUagen, found in connective tissue, has a triple-hehcal stmcture. Other fibrous proteins have a motile function. Skeletal muscle fibers are made up of thick filaments consisting of the protein myosin, and thin filaments consisting of actin, troponin, and tropomyosin. Muscle contraction is achieved when these filaments sHde past each other. Microtubules and flagellin are proteins responsible for the motion of ciUa and bacterial dageUa. 

Disease States. Rickets is the most common disease associated with vitamin D deficiency. Many other disease states have been shown to be related to vitamin D. These can iavolve a lack of the vitamin, deficient synthesis of the metaboUtes from the vitamin, deficient control mechanisms, or defective organ receptors. The control of calcium and phosphoms is essential ia the maintenance of normal cellular biochemistry, eg, muscle contraction, nerve conduction, and enzyme function. The vitamin D metaboUtes also have a function ia cell proliferation. They iateract with other factors and receptors to regulate gene transcription. 

Soluble Compounds. The mechanism of barium toxicity is related to its ability to substitute for calcium in muscle contraction. Toxicity results from stimulation of smooth muscles of the gastrointestinal tract, the cardiac muscle, and the voluntary muscles, resulting in paralysis (47). Skeletal, arterial, intestinal, and bronchial muscle all seem to be affected by barium. 

Biological functions of Ca(Il) ion are numerous but maybe classified ia one of three categories the formation of soHd skeletal material such as bone, teeth, and shell the stabilizing of proteia conformational stmcture and the most varied, the abiUty of Ca(Il) to trigger certain physiological activities such as muscle contraction and the release of hormones (qv). 

Calcium is the trigger behind the muscle contraction process (24,25). Neural stimulation activates the release of stored Ca(Il) resulting in a dramatic increase in free calcium ion levels. The subsequent binding of Ca(Il) resulting in a dramatic increase in free calcium ion levels. The subsequent binding of Ca(Il) to the muscle protein troponin C provides the impetus for a conformational change in the troponin complex and sets off successive events resulting in muscle contraction. 

Calcium and Vascular Smooth Muscle Contraction. Calcium acts on a number of sites associated with the control of the cytoplasmic calcium concentration. Vascular smooth muscle contraction can be initiated by the opening of the slow calcium channel aUowing influx of extraceUular calcium through the sarcolemmal membrane into the cytoplasmic compartment. The iatraceUnlar calcium concentration increases to 1 x 10 Af, a threshold concentration necessary to initiate contraction. 

In the presence of calcium, the primary contractile protein, myosin, is phosphorylated by the myosin light-chain kinase initiating the subsequent actin-activation of the myosin adenosine triphosphate activity and resulting in muscle contraction. Removal of calcium inactivates the kinase and allows the myosin light chain to dephosphorylate myosin which results in muscle relaxation. Therefore the general biochemical mechanism for the muscle contractile process is dependent on the avaUabUity of a sufficient intraceUular calcium concentration. 

Phosphodiesterase Inhibitors. Because of the complexity of the biochemical processes involved in cardiac muscle contraction, investigators have looked at these pathways for other means of dmg intervention for CHF. One of the areas of investigation involves increased cycHc adenosine monophosphate [60-92-4] (cAMP) through inhibition of phosphodiesterase [9025-82-5] (PDE). This class of compounds includes amrinone, considered beneficial for CHF because of positive inotropic and vasodilator activity. The mechanism of inotropic action involves the inhibition of PDE, which in turn inhibits the intracellular hydrolysis of cAMP (130). In cascade fashion, cAMP-catalyzed phosphorylation of sarcolemmal calcium-channels follows, activating the calcium pump (131). A series of synthetic moieties including the bipyridines, amrinone and milrinone, piroximone and enoximone, [77671-31-9], C22H22N2O2S, all of which have been shown to improve cardiac contractiUty in short-term studies, were developed (132,133). These dmgs... 

Another mechanism in initiating the contraction is agonist-induced contraction. It results from the hydrolysis of membrane phosphatidylinositol and the formation of inositol triphosphate (IP3)- IP3 in turn triggers the release of intracellular calcium from the sarcoplasmic reticulum and the influx of more extracellular calcium. The third mechanism in triggering the smooth muscle contraction is the increase of calcium influx through the receptor-operated channels. The increased cytosolic calcium enhances the binding to the protein, calmodulin [73298-54-1]. 

Muscle fibers contain myosin and actin which slide against each other during muscle contraction... 

Figure 14.11 The sliding filament model of muscle contraction. The actin (red) and myosin (green) filaments slide past each other without shortening.

Figure 14.12 The swinging cross-bridge model of muscle contraction driven by ATP hydrolysis, (a) A myosin cross-bridge (green) binds tightly in a 45 conformation to actin (red), (b) The myosin cross-bridge is released from the actin and undergoes a conformational change to a 90 conformation (c), which then rebinds to actin (d). The myosin cross-bridge then reverts back to its 45° conformation (a), causing the actin and myosin filaments to slide past each other. This whole cycle is then repeated.

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