Fibre shortening

Although blood pressure control follows Ohm s law and seems to be simple, it underlies a complex circuit of interrelated systems. Hence, numerous physiologic systems that have pleiotropic effects and interact in complex fashion have been found to modulate blood pressure. Because of their number and complexity it is beyond the scope of the current account to cover all mechanisms and feedback circuits involved in blood pressure control. Rather, an overview of the clinically most relevant ones is presented. These systems include the heart, the blood vessels, the extracellular volume, the kidneys, the nervous system, a variety of humoral factors, and molecular events at the cellular level. They are intertwined to maintain adequate tissue perfusion and nutrition. Normal blood pressure control can be related to cardiac output and the total peripheral resistance. The stroke volume and the heart rate determine cardiac output. Each cycle of cardiac contraction propels a bolus of about 70 ml blood into the systemic arterial system. As one example of the interaction of these multiple systems, the stroke volume is dependent in part on intravascular volume regulated by the kidneys as well as on myocardial contractility. The latter is, in turn, a complex function involving sympathetic and parasympathetic control of heart rate intrinsic activity of the cardiac conduction system complex membrane transport and cellular events requiring influx of calcium, which lead to myocardial fibre shortening and relaxation and affects the humoral substances (e.g., catecholamines) in stimulation heart rate and myocardial fibre tension. 

Additional information about regional cardiac contraction, like the dynamic fibre shortening and fibre orientation are measured with help of radiopaque markers which are inserted in the cardiac walls at different places. The spatial positions of the markers throughout the cardiac cycle and thus also cardiac dimensions are determined with help of a biplane X-ray system. 

For the measurement of ventricular dimensions, intramural deformations, fibre shortening and fibre orientation, radiopaque markers were implanted at various... 

From the spatial marker coordinates, ventricular dimensions, and dynamic fibre shortening and fibre orientation were calculated in a way described below. 

Heethaar RM, Pao YC, Ritman EL (1977) Computer aspects of three dimensional finite element analysis of stresses and strains in the intact heart. Comp Biomed Res 10 271-285 Heethaar RM, Mol CR, Elshuraydeh K, Heethaar J, Van Dort JMT, Batianen GW, Sneek JHJ, Borst C, Meijler FL (1982) Cardiac function fibre shortening and dynamic geometry. Mayo Clinic Proc suppl, 57 104-113... 

Class IC antiarrhythmic drugs such as flecainide or propafenone block the Na+ channel (open state propafenone open and inactivated state) with a very long dissociation time constant so that they alter normal action potential propagation. Flecainide increased mortality of patients recovering from myocardial infarction due to its proarrhythmic effects (CAST study). Action potential is shortened in Purkinje fibres but is prolonged in the ventricles. 

A very large number of changes take place within the whole fibre suspension during the refining process and a precise interpretation of the influence of the process upon mechanical and other sheet properties therefore becomes extremely difficult. The more important effects can be summarised as follows. There is a shortening of the fibres arising from a cutting effect and part of the fibre cell wall is... 

Hydrolysis of ATP provides the energy required for each stroke of myosin heads to pull on the actin filament so shortening the muscle fibre (Figure 7.3). In the resting... 

Thin actin filaments are pulled inwards by myosin heads so shortening the fibre. The Z disc is an anchorage point for actin... 

The tension which needs to be generated in cardiac muscle fibres before shortening will occur. 

All living cells can move to some degree, but this ability is highly developed in muscles, which act by fibre contraction the fibres can shorten to two-thirds of their resting length. Muscles vary greatly in structure and function in different organs. 

In the cells of this tissue, which are known as fibres, the two major proteins, actin and myosin, are orgaiused to form myofibrils. These are structural rods that can contract (Figure 1.11). This enables muscle cells to shorten, which provides for movement and locomotion (Figure 1.12). There are three types ... 

Figure 13.5 Electron micrograph of part of a longitudinal section of a myofibril. Identification of components and the mechanism of contraction. When a muscle fibre is stimulated to contract, the actin and myosin filaments react by sliding past each other but with no change in length of either myofilament. The thick myosin strands in the A band are relatively stationary, whereas the thin actin filaments, which are attached to the Z discs, extend further into the A band and may eventually obliterate the H band. Because the thin filaments are attached to Z discs, the discs are drawn toward each other, so that the sarcomeres, the distance between the adjacent Z-discs, are compressed, the myofibril is shortened, and contraction of the muscle occurs. Contraction, therefore, is not due to a shortening of either the actin or the myosin filaments but is due to an increase in the overlap between the filaments. The force is generated by millions of cross-bridges interacting with actin filaments (Fig. 13.6). The electron micrograph was kindly provided by Professor D.S. Smith.

Shukla, T.P., Halpern, G.J. (2005). Emulsified liquid shortening compositions comprising dietary fibre gel, water and lipid. US Patent No. 2005/0064068A1. 

The changes that occur when cellulose is subjected to hydrolysis all follow from the destruction of the 1,4-glucoside bond. Thus the cellulose chains are shortened and a decrease in the average molecular weight takes place and the cellulose fibres... 

Generally, shortening the fibres is carried out in beaters, equipment adapted from the paper industry. The operation is known as pulping. 

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