Contractile element

Because there are no sarcomeres in smooth muscle, there are no Z lines. Instead, the actin filaments are attached to dense bodies. These structures, which contain the same protein as Z lines, are positioned throughout the cytoplasm of the smooth muscle cell as well as attached to the internal surface of the plasma membrane. Myosin filaments are associated with the actin filaments, forming contractile bundles oriented in a diagonal manner. This arrangement forms a diamond-shaped lattice of contractile elements throughout the cytoplasm. Consequently, the interaction of actin and myosin during contraction causes the cell to become shorter and wider. 

The contractile elements in smooth muscle are not organized into sarcomeres. Furthermore, the resting length of smooth muscle is much shorter than its optimal length. In other words, this muscle can be significantly stretched and the amount of tension developed may actually increase because the muscle is closer to its optimal length. Finally, thick filaments are longer in smooth muscle than they are in skeletal muscle. As a result, overlap... 

Myoepithelial cells are contractile elements found in salivary, sweat, and mammary glands that show a combined smooth muscle and epithelial phenotype (Foschini et al. 2000). In the normal breast, the ductal and acinar units are lined by two cell layers the inner layer lining the lumen and an outer layer of myoepithelial cells. An intact myoepithelial cells layer is seen in both benign and in situ lesions, whereas loss of the myoepithelial cells layer is considered the rule for the diagnosis of invasive cancer (Kalof et al. 2004 and citations therein). 

Fig. 2. Macroscopic and microscopic structure of muscle (a) Entire muscle and its cross-section with fatty septa, (b) Fascicle with several muscle fibres (cells). A layer of fat along the fascicle is indicated, (c) Striated myofibre corresponding with one single muscle cell containing several nuclei. The lengths of a myofibre can be several tens of centimetres, (d) Myofibril inside a myocyte. It is one contractile element and contains actin and myosin and further proteins important for the muscular function, (e) Electron myograph of human skeletal muscle showing the band structure caused by the contractile myofilaments in the sarcomeres. One nucleus (Nu) and small glycogen granules (arrow, size <0.1 pm) are indicated.

Any dialogue on meat flavor development and deterioration requires a brief discussion of muscle structure. Muscle has a highly compact and complex multicellular structural organization (Figure 2). Individual muscle cells contain numerous mitochondria and nuclei. They also contain contractile elements as the bulk of their structure. While the sarcoplasm of muscle (the aqueous non-organellar component) is small compared to the cytoplasm of non-muscle cells, it does have a highly evolved system of membranes called the SR/L representing an acronym for sarcoplasmic reticulum/lysosomal membrane system (11). The SR/L surrounds each contractile element (Fig. 9-13 in 12 Fig. 7-10 in 13). The close proximity of the SR/L to the contractile proteins situates the proteins in a location that is optimal for their hydrolysis by lysosomal hydrolases (12, 13). 

Ion channel modulation represents another approach to positive inotropy [13]. Sodium channel modulators increase Na+ influx and prolong the plateau phase of the action potential sodium/calcium exchange then leads to an increase in the level of calcium available to the contractile elements, thus increasing the force of cardiac contraction [13,14]. Synthetic compounds such as DPI 201-106 and BDF 9148 (Figure 1) increase the mean open time of the sodium channel by inhibiting channel inactivation [15]. Importantly, BDF 9148 remains an effective positive inotropic compound even in severely failing human myocardium [16] and in rat models of cardiovascular disease [17]. Modulators of calcium and potassium channel activities also function as positive inotropes [13], but in the remainder of this article we shall focus on sodium channel modulators. 

Ca2+ -free media do not seem to be related to changes in cellular Ca2+ regulation but to an inhibitory effect on the regulation of contractile proteins, an effect probably related to the decreased sensitivity of contractile elements to Ca2+ that apparently resulted from the inhibitory effects of quercetin on protein kinases [110]. 

S100A1 is the most abundant S100 protein found in striated muscle and predominates in myocardial tissue (Kato and Kimura, 1985). Besides its cytoplasmic occurrence, S100A1 was reported in these cells to associate with the sarcolemma, sarcoplasmatic reticulum (SR), contractile filaments, intercalated discs, outer mitochondrial membrane and other intracellular membrane stmctures (Arcuri et al., 2002 Donato et al., 1989 Haimoto and Kato, 1988 Sorci et al., 1999). However, the exact location of S100A1 on the contractile elements of the sarcomere is still controversial (Maco et al., 2001 Zimmer, 1991). 

Cardiac dilatation and hypertrophy — taking advantage of the Frank-Starling relationship to utilize more contractile elements. 

Contractile elements of the podocyte foot processes, which may influence the hydraulic permeability of the glomerular capillary wall, may be regulated via vasoactive hormones. Receptors for some vasoactive hormones, for example, en-dothelin (R4), atrial natriuretic peptide (S9), nitric oxide (K22), and angiotensin n (Yl), have been described on the podocyte surface. 

Muscle cells Actin and myosin Contractile elements... 

Stoner et al. (1974) have proposed a mechanical model for postmortem striated muscle it is shown in Figure 8-28. The model is a combination of the Voigt model with a four-element viscoelastic model. The former includes a contractile element (CE), which is the force generator. The element SE is a spring that is passively elongated by the shortening of the CE and thus develops an... 

The SA node lies in the right atrium, in an epicardial location and functions as the primary source of the electrical impulse formation. It consists of specialized muscle cells, smaller than ventricular cells containing few contractile elements. Due to its location, the action potential generated traverses the atria first, causing the primary contraction. The action potential is relayed to the AV node, the conduit for the electrification of the ventricles. The signal must pass through the AV node, otherwise the atria and the ventricles are electrically insulated from each other. A time delay in transmission is needed for the atria to be emptied of blood. 

Intracellular transport of bile acids mainly takes place through the cytoskeleton and intracellular structures (Golgi apparatus, endoplasmic reticulum). Here, too, cholestatic factors can prove to be damaging. Microfilaments are contractile elements not only is the intracellular transport of the bile acids disturbed, but the peristaltic activity of the canaliculi (so-called paralytic cholestasis within the lolsules) is also reduced if the functional capacity of those microfilaments becomes diminished. 

The role of Ca" " in the function of cell microskeleton, represented by microtubules and microfiliment, has been well documented (14,106,107), The contractile elements of this system are in some way responsible for the mobility of microorganelles and transport of granules to the membrane as well as excitability of the plasma membrane itself. Zinc may compete with calcium and thereby inhibit the calcium effect. 

Gerthoffer, W.T. (1991). Regulation of the contractile element of airway smooth muscle. Am. J. Physiol. 261 (Lung Cell Mol. Physiol. 5), L15-L28. 

Nishimura, J. and van Breemen, C. (1989). Direct regulation of smooth muscle contractile elements by second messenger. Biochem. Biophys. Res. Comm. 163, 929-935. 

Proteins serve in many roles (e.g., as enzymes, hormones, receptors, antibodies, structural components, transporters of other compounds, and contractile elements in muscle). 

In the experience of the authors, desmin may be detected as well, although others have disagreed on that point.224 The ultrastructural and biochemical attributes of myofi-broblastic contractile elements are essentially identical to those of smooth muscle cells. Notably, the peculiar intra-cytoplasmic inclusions that are present in DF correspond to aggregates of immunoreactive actin filaments.222-229 Fibrosarcoma of the skin has been reported in association with burn scars, surgical scars, smallpox vaccinations, and other injection sites.221 This exceedingly uncommon neoplasm is composed exclusively of fibroblastic elements and stains only for vimentin, to the exclusion of the other specialized mesenchymal... 

The stored Ca + can be released by either caffeine orlPg. Many studies (e.g., Endo eta/., 1977 Saida and Nonomura, 1978 Itoh et al, 1982a,b Saida, 1982) used the transient tension response elicited by the released Ca2+ as a measure of Ca + uptake into the SR. In order to observe a contractile effect, a solution with a weak Ca + buffering capacity (0.1 mM EGTA) has to be used so that Ca + released from the store, instead of being bound by the buffer system, is made available to the contractile elements. The limitations of this index of Ca + uptake have been discussed previously (Meisheri et al, 1985 Yamamoto and van Breemen,... 

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