Regulation of contraction

At any one instant during contraction, only about 50% of the cross-bridges in the filament-overlap region are attached to actin and, therefore, only about 50% of the maximum possible force is produced. 

Myasthenia gravis is an autoimmune disease in which antibodies are present in blood and bind to the acetylcholine receptor on the motor end-plate. This prevents the muscles from contracting so that, due to lack of use, they become weak and fatigue easily. In particular, there is difficulty in speaking, swallowing and chewing food. 

The action of acetylcholine is brought to an end by the enzyme acetylcholinesterase, which catalyses the hydrolysis 

T-system membrane results in an interaction between the T-tubules and the SR at specialised junctions, known as excitation-contraction coupling units. Here the two membranes are finked by a foot process consisting of two Ca ion channels that function in concert. 

Depolarisation of the sarcolemma leads to depolarisation of the T-tubule membrane, which increases the activity of 

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Ca2+-pumps. After entering the cell, Ca2+ is reversibly complexed to specific Ca2+-binding proteins that fiilfil multiple functions, including Ca2+-buffering and transport, activation of enzymes, regulation of contraction,... 

Perez GJ, Bonev AD, Nelson MT 2001 Micromolar Ca2+ from sparks activates Ca2+-sensitive K+ channels in rat cerebral artery smooth muscle. Am J Physiol Cell Physiol 281 C1769-C1775 Rembold CM 1992 Regulation of contraction and relaxation in arterial smooth muscle. Hypertension 20 129-137... 

As mentioned above, the junctional SR is connected to sheets of perpendicular SR (Fig. 4), which extend from the PM through a peripheral cytoplasmic region with lower myofilament density into the myoplasm. It is proposed that during the active state of wave-like [Ca2+]j oscillations, Ca2+ taken up by the junctional SR is released by these perpendicular sheets near the calmodulin, which is tethered to the myosin light chain kinase (MLCK) of the thin filaments (M. Walsh, personal communication, 2001). This process would enhance the specificity and efficiency of Ca2+ regulation of contraction. 

Hypertension is a common and progressive disorder which, if not effectively treated, results in an increased risk of atherosclerosis (see above), haemorrhagic stroke and damage to the kidney. For most cases of hypertension, there is no obvious cause, hence it is known as essential hypertension, so called because it was originally thonght to be essential to maintain tissue perfusion. In order to better nnderstand the regulation of blood pressure, a brief description of the regulation of contraction of smooth mnscle is provided. 

Hrometz SL, Edelmann SE, McCune DF, et al. Expression of multiple a,-adrenoceptors on vascular smooth muscle correlation with the regulation of contraction. J Pharmacol Exp Ther 1999 290 452-463. 

A. M. Gordon, E. Honsher, and M. Regnier Regulation of contraction in striated mu.scle. Physiological Reviews 80,853 (2000). 

The answer is b. (A furray, pp 48-73. Scriver, pp 4571-4636. Sack, pp 3—17. Wilson, pp 101-120.) Calcium ions are the regulators of contraction of skeletal muscle. Calcium is actively sequestered in sarcoplasmic... 

It has been established for several years that the major mechanism for regulation of contraction in smooth muscle is myosin phosphorylation (Hart-shorne, 1987). Phosphorylation of the two 20,000-dalton light chains of myosin (LC20) activates the actin-dependent ATPase activity of myosin and this initiates the contractile response. Dephosphorylated myosin is associated with relaxed muscle. In this scheme there are two key enzymes the myosin light chain kinase (MLCK) and the myosin light chain phosphatase (MLCP). Obviously a balance of these two activities determines the level of myosin phosphorylation. 

Since free magnesium plays an important role in the regulation of contraction of smooth muscle (Chacko and Rosenfeld, 1982), variation in its concentration as a functional of length and/or activation could be important in the regulation of contraction and deserves further study. 

Regulation of contraction through LC20 phosphorylation in intact... 

Regulation of contraction by myosin light chain phosphorylation... 

Tropomyosin Binds actin in thin filament a, 34 kDa P. 36 kDa 4 classes fast a, slow a, P and platelet-like but many more transcripts >4 genes gene duplicahon plus mainly alternative splicing Regulation of contraction... 

Troponin Thin filament T, 30 kDa 1, 30 kDa C, 18 kDa 64 variants of troponin T Mainly alternative splicing Regulation of contraction... 

Gordon AM, Homsher E, Regnier M (2000) Regulation of contraction in striated muscle. Physiol Rev 80 853... 

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