Active forces

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. 

The increment to force during activation. Presumably, the structures which maintain the resting force are not those which are activated or even changed following a stimulus. Therefore, activation involves the parallel addition of a nascent force resulting from the emergent interaction of the actin and myosin filaments. Active force is therefore an incremental force added to the resting force. This is a fundamental assumption of almost all analyses of contraction from A.V. Hill onward. 

Just as important as the maximum active force a muscle can exert at various lengths, is the rate at which the muscle shortens as a function of the force load, i.e., the force-velocity curve. Both the length-tension curve and the force-velocity curve vary according to the degree of activation of a muscle. The rate at which crossbridges cycle is an inverse function of the load force (Figure 4). 

The compliance in series with the active force. Force exerted by the activated elements must be transmitted or borne by whatever structural elements are in series with them. In skeletal muscle there is clearly a tendon in series but not so with smooth muscle. In smooth muscle, the total length of contractile apparatus is broken up into individual cells with intercalating extracellular connective structures. In addition, the portions of the crossbridges in series with the pulling site must also be stretched before force can rise to isometric levels. Taken together, the... 

The second of these steps, the rate of hydrolysis at 12 °C, measured by analysis of the ADP content in fibers rapidly frozen at different times after ATP release from caged-ATP, is 40-60 s (Ferenczi, 1986) which is similar to the rate of active force increase at 20°C once the difference in temperature has been accounted for. This rate is similar to that measured in solution however, and is not rate limiting in solution. Therefore, force generation in the caged-ATP experiments could be limited by hydrolysis, or more likely, by a step following hydrolysis such as Pj release. The idea that release of phosphate is linked to force production in muscle... 

Robertson, S.P. Kerrick, W.G.L. (1979). The effects of pH on Ca -activated force in frog skeletal muscle fibers. Pfluegers Arch. 380,41 5. 

Wendt, I. and Stephenson, D., Effects of caffeine on calcium-activated force production in skinned cardiac and skeletal muscle fibers of the rat, European Journal of Physiology, 398, 210, 1983. 

The quantity of water that can be retrieved from a medium is related to size and shape of the connected pore spaces within that medium. The quantity of water that can be freely drained from a unit volume of porous medium is referred to as the specific yield. The volume of water retained in the medium by capillary and surface active forces is called the specific retention. The sum of specific retention and specific yield is equal to the effective porosity (see Table 3.4). Neither term has a time value attached. Drainage can occur over long periods (i.e., weeks or months). 

The stable anion-radical in Scheme 3.63 contains two perchlorotriphenylmethyl radical units linked by an all-trani-p-divinylbenzene bridge. At 200 K, the unpaired electron of the anion-radical is localized (within the ESR timescale) on one stilbenelike moiety only. At 300 K, thermal activation forces the nnpaired electron at one strong electrophilic center to move to another one. Such an electron transfer takes place between two eqnivalent redox sites (Bonvoisin et al. 1994). In contrast to this situation, no electron transfer was observed for the anion-radical that contains two perchlorotriphenylmethyl radical units linked by an all-trani -m-divinylbenzene bridge (Rovira et al. 2001). Such results can be ascribed to the localization of frontier orbitals in the meta-isomeric anion-radical because of the meta connectivity of this non-Kekule structure. 

Stephens, J. R., S. Acharya, E. J. Gntmark, and D. C. Allgood. 1997. Active forcing of air and fuel feed in swirl stabilized spray combustion. ISOABE Proceedings. Chattanooga, TN. 

Although much of the focus has been on the DPC of striated muscle, it is likely that desmin attachments to dense plaques of smooth muscle play critical roles in regulating the transmission of contractile forces in this tissue as well. This is particularly relevant in light of the observed defects in smooth muscle of desmin-deficient mice, in which active force per cross-sectional area was reduced to 40% of controls of smooth muscle tissue (Sjuve et al, 1998). IFAP candidates for serving this linking function are plectin and other components of the actin-rich cortex, including calponin (which also plays a role in the cytoplasm of smooth muscle cell dense bodies see below), and the spectrin/ankyrin complex. 

One of the tasks of structural biologists studying muscle contraction is to determine the organization and shapes of the myosin head in muscle under different physiological conditions. The technique of low-angle X-ray diffraction has unique advantages in this process, particularly since it can be applied to living muscle, which can be stimulated to produce active force or can be studied under a variety of different steady-state conditions. The main problem with X-ray fiber diffraction, as detailed in Squire and... 

Little calorimetric research has been done on smooth muscle. Recently, however, Lonnbro and Hellstrand (1991) showed that chemically skinned muscle from guinea pig taenia coli produced threefold more heat on activation (pCa 4.8) than at rest (pCa 9) (pCa being -log Ca2+). With stepwise increments in [Ca2+] from pCa 9 to 4.8, the energetic cost of force maintenance tended to rise at higher [Ca2+]. Even after Ca2+ activation, force still increased beyond the point at which heat dissipation reached its maximum. 

Survivors of shock treatment have become an increasingly active force. In addition to writing and appearing in the media, many who have undergone ECT continue to protest at national psychiatric conventions and shock symposia and even chain themselves to the gates and doors of so-called shock mills. ... 

Sol-gel transition occurs only if there are no active forces which promote coagulation into aggregates of higher silica concentration than the original sol. Metal cations, particularly polyvalent ones, tend to cause precipitation rather than gelling. 

Boerhaave s presentation of the mechanical method in medicine is very optimistic, for he believed that it is certainly a marvellous science, almost superhuman as to its results which exceed all expectations For its most subtle and complicated discoveries are based on principles which are sure, yet very few in number and generally known. Boerhaave held that the particular nature of all bodies depend on the active force and that as soon as the latter is known, the differences between bodies can be disclosed. In fact, Boerhaave argued that the human body is in its nature the same as the whole of the Universe which is open to our view. ... 

According to Thackray s historical account of Newton s matter theory, Newton s atoms differ from the atoms of the classical authors. They not only have the inherent force of vis inertiae, but also the active forces of gravita-... 

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