Actin-myosin muscle system

The actin-myosin muscle system provides the performance target for electroactive polymer actuators [3, 4]. The process is driven chemically by the energy change from hydrolysis of the polyphosphate bond as ATP (adenosine triphosphate) binds to myosin and is converted to ADP (adenosine diphosphate) and phosphate. A simple chemical analogy suggests that muscle-like gels should be feasible but it is now clear that the task is much harder than it seems. 

Of the several kinase activities which are important in smooth muscle, myosin light chain kinase, MLCK, is the one responsible for activation of the actin-myosin system to in vivo levels. MLCK is present in the other nonmuscle cell types which have the actin-myosin contractile system and all of these are probably activated in a manner similar to smooth muscle rather than by way of the Ca -troponin mechanism of striated muscle. MLCK from smooth muscle is about 130 kDa and is rather variable in shape. It is present in smooth muscle in 1-4 pM concentrations and binds with an equally high affinity to both myosin and actin. Thus, most MLCK molecules are bound to actin. Myosin light chain serine-19 is the primary target of smooth muscle myosin light chain kinase. 

Muscles and Other Actin-Myosin Contractile Systems Actin and Myosin... 

Inside the typical smooth muscle cell, the cytoplasmic filaments course around the nuclei filling most of the cytoplasm between the nuclei and the plasma membrane. There are two filamentous systems in the smooth muscle cell which run lengthwise through the cell. The first is the more intensively studied actin-myosin sliding filament system. This is the system to which a consensus of investigators attribute most of the active mechanical properties of smooth muscle. It will be discussed in detail below. The second system is the intermediate filament system which to an unknown degree runs in parallel to the actin-myosin system and whose functional role has not yet been completely agreed upon. The intermediate filaments are so named because their diameters are intermediate between those of myosin and actin. These very stable filaments are functionally associated with various protein cytoarchitectural structures, microtubular systems, and desmosomes. Various proteins may participate in the formation of intermediate filaments, e.g., vimentin. 

The analytic validity of an abstract parallel elastic component rests on an assumption. On the basis of its presumed separate physical basis, it is ordinarily taken that the resistance to stretch present at rest is still there during activation. In short, it is in parallel with the filaments which generate active force. This assumption is especially attractive since the actin-myosin system has no demonstrable resistance to stretch in skeletal muscle. However, one should keep in mind, for example, that in smooth muscle cells there is an intracellular filament system which runs in parallel with the actin-myosin system, the intermediate filament system composed of an entirely different set of proteins, (vimentin, desmin, etc.), whose mechanical properties are essentially unknown. Moreover, as already mentioned, different smooth muscles have different extracellular volumes and different kinds of filaments between the cells. 

Correlated with the wide distribution of the actin-myosin system found in smooth muscle, MLCK is also found in neural, epithelial, connective, and blood... 

The general picture of muscle contraction in the heart resembles that of skeletal muscle. Cardiac muscle, like skeletal muscle, is striated and uses the actin-myosin-tropomyosin-troponin system described above. Unlike skeletal muscle, cardiac muscle exhibits intrinsic rhyth-micity, and individual myocytes communicate with each other because of its syncytial nature. The T tubular system is more developed in cardiac muscle, whereas the sarcoplasmic reticulum is less extensive and consequently the intracellular supply of Ca for contraction is less. Cardiac muscle thus relies on extracellular Ca for contraction if isolated cardiac muscle is deprived of Ca, it ceases to beat within approximately 1 minute, whereas skeletal muscle can continue to contract without an extraceUular source of Ca +. Cyclic AMP plays a more prominent role in cardiac than in skeletal muscle. It modulates intracellular levels of Ca through the activation of protein kinases these enzymes phosphorylate various transport proteins in the sarcolemma and sarcoplasmic reticulum and also in the troponin-tropomyosin regulatory complex, affecting intracellular levels of Ca or responses to it. There is a rough correlation between the phosphorylation of Tpl and the increased contraction of cardiac muscle induced by catecholamines. This may account for the inotropic effects (increased contractility) of P-adrenergic compounds on the heart. Some differences among skeletal, cardiac, and smooth muscle are summarized in... 

While all muscles contain actin, myosin, and tropomyosin, only vertebrate striated muscles contain the troponin system. Thus, the mechanisms that regulate contraction must differ in various contractile systems. 

The vigor of contraction of heart muscle is determined by several processes that lead to the movement of actin and myosin filaments in the cardiac sarcomere (Figure 13-1). Ultimately, contraction results from the interaction of activator calcium (during systole) with the actin-troponin-tropomyosin system, thereby releasing the actin-myosin interaction. This calcium is released from the sarcoplasmic reticulum (SR). The amount released depends on the amount stored in the SR and on the amount of trigger calcium that enters the cell during the plateau of the action potential. 

In summary, interaction between actin and myosin in vertebrate and invertebrate muscle systems requires a critical level of calcium. It is tentatively suggested that interaction between myosin and actin is prevented by blocking sites on actin in the case of vertebrate muscles, whereas in the case of molluscan muscles it is the sites on myosin which are blocked in the absence of calcium. The acidic amino acids seem to be of utmost importance in the regulation process. 

A structural dilemma lies in combining a phase concept of the molecular arrangement with the molecular description of the contraction process. It is known that the movements of different myosin heads on a particular myosin thread occur asynchronously, which is evident for example, from X-ray diffraction experiments. That means that a force is generated constantly with time. The coordination of movement between every muscle cell (down to the individual actin/myosin molecule) is controlled by the excitation via the tubuli system, which is a cubosome type of membrane system, described in the previous chapter. 

Constraints imposed by caldesmon that disrupt potentiation may not block myosin docking on actin. This process of modulating ATPase could in turn lead to or permit the development of the latch-state of tension maintenance displayed by tonic smooth muscles and observed at low Ca + concentration. This phenomenon probably involves stable actin-myosin binding and is associated with low actomyosin ATPase activity (Hai and Murphy, 1988 McDaniel et al, 1990). As envisioned, such tension maintenance would be incompatible with a troponin-tropomyosin form of regulation since tropomyosin would in that case block myosin docking at low Ca + concentrations. Hence, the caldesmon-tropomyosin system may be adapted for muscles that enter a latch-state. 

However, a nerve impulse to the muscle triggers a release of Ca2+ from the sarcoplasmic tubular system, where it is ordinarily bound, which increases the intracellular Ca2+ concentration to 10 5-10 6M. This level of Ca2+ allows the actin in the thin filament to accept the energized-ADP-myosin cross-bridge to initiate contraction. As each cross-bridge completes the swivel part of its cycle, it loses the bound ADP and immediately accepts a molecule of ATP that is always supplied to living muscle. The ATP immediately causes a dissociation of the actin-myosin complex, and the myosin catalyzes the hydrolysis of ATP to yield the myosin—ADP energized state again, ready to repeat the cycle. 

There are three types of muscle cells smooth, skeletal, and cardiac. In all types of muscle, contraction occurs via an actin myosin sliding filament system, which is regulated by oscillations in intracellular calcium levels. 

Without ATP, muscle is in a constant state of tension. Its actin and myosin proteins are cross-linked to one another and contracted (Pollack, 2001). ATP breaks actin-myosin crosslinks, and restores the proteins to their extended state. ATP does this despite the fact that its binding sites are located remotely from the sites of cross-linking and contraction. ATP drives the system to its high energy state. 

In its simplest form, actin (A) combines with myosin (M) and ATP to produce force, adenosine diphosphate (ADP) and inorganic phosphate. Scientists now agree that ATP serves at least two functions in skeletal muscle systems first, ATP disconnects actin from... 

Natural muscles are controlled by neurons and network of neurons. We can imagine artificial neurons and network of artificial neurons as well. Artificial muscles with motor proteins are studied and attract attention[79]. One direction is to develop deformable machine with real motor proteins, actins and myosins, and neurons. Another direction is to develop neural network software to control distributed artificial muscles. The author has been developing open brain simulator which can emulate the activities of human nervous system for estimating internal state of human through external observation [231]. Such software is also applicable to control artificial muscle systems, which is implemented on the personal robots and humanoid robots in the future. 

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