Muscle contraction stretch activation

The phenomena that drive muscle contraction—thermal activation, pH activation, calcium ion activation, stretch activation in insect flight muscle, and dephosphorylation itself—have all been shown to drive contraction by hydrophobic association in the elastic-contractile model proteins discussed in Chapter 5. As concerns a pair of hydrophobic domains, all of these processes surmount the T,-divide (the cusp of insolubility in Figure 7.1) to go from a soluble state to an insoluble state either by raising the... 

Finally, an increase in volume or pressure within a tube or hollow organ causes stretch or distortion of the smooth muscle in the organ wall. This may cause activation of stretch-activated Ca++ channels. The subsequent influx of calcium initiates contraction of the smooth muscle. This process is referred to as myogenic contraction and is common in blood vessels. 

Arteriolar resistance changes that take place in order to maintain a constant blood flow are explained by the myogenic mechanism. According to this mechanism, vascular smooth muscle contracts in response to stretch. For example, consider a situation in which blood pressure is increased. The increase in pressure causes an initial increase in blood flow to the tissue. However, the increased blood flow is associated with increased stretch of the vessel wall, which leads to the opening of stretch-activated calcium channels in the vascular smooth muscle. The ensuing increase in intracellular calcium results in vasoconstriction and a decrease in blood flow to the tissue toward normal. 

Kannan MS, Prakash YS, Brenner T, Mickelson JR, Sieck GC 1997 Role of ryanodine receptor channels in Ca2+ oscillations of porcine tracheal smooth muscle. Am J Physiol 272 L659-L664 Kirber MT, Walsh JVJ, Singer JJ 1988 Stretch-activated ion channels in smooth muscle a mechanism for the initiation of stretch-induced contraction. Pfliig Arch Eur J Physiol 412 339-345... 

Somlyo Can we relate the kinetics of this process to what happens in whole muscle As you know, when a smooth muscle is stretched and there is a myogenic contraction, there is a lag phase of about a second. This is surprisingly long for channel activation. How can you relate these kinetics to the kinetics of a lag of 500 ms Stretch activation has also been suggested to involve PLC activation. 

Ca2+ is the major second messenger in the activation of smooth muscle contraction. Thus smooth muscle Ca2+ handling is of major importance to understanding its function. Ca2+ homeostasis is a balance of Ca2+ influx and extrusion. Influx is generally through channels, such as I, or T type Ca2+ channels or the so-called capacitive entry pathway, through stretch activated channels, leak pathways and reversed mode Na+/Ca2+ exchanger, which may... 

Kirber MT, Walsh JV Jr, Singer JJ. Stretch-activated ion channels in smooth muscle A mechanism for the initiation of stretch-induced contraction. Pflugers Arch. 1988 412 339-345. 

Stretch activation of muscle is a well-described phenomenon it was the subject of The Croonian Lecture (1977) given by Pringle,and it has been extensively researched and reported in the literature over the ensuing decades. For example, the basic description becomes When active insect flight muscle is stretched, its ATPase rate increases.. . This we take as yet another demonstration of a fundamental process whereby a phosphate present in a protein can be activated, energized, as the result of an increase in hydrophobicity. It is an example of the competition for hydration between apolar and polar species, that is, an example of the apolar-polar repulsive free energy of hydration active in muscle contraction. 

At the level of the myofibril, the addition of calcium ion, the lowering of pH, and stretching have each been shown to activate muscle contraction as well as to drive contraction of suitably designed elastic contractile protein-based polymers by hydrophobic association (see more extensive discussion in Chapter 7). 

FIG. 6-4 (A) Neutral position. (B) Stretch of extrafusal and intrafusal alpha motor neuron stimulated to reflexively initiate extrafusal muscle contraction. (C) Gamma motor neuron innervation of muscle spindle. Contraction of ends stretches spindle, activating lb fibers, lb sensory fiber activates alpha motor neuron to cause extrafusal muscle contraction. (D) Compression of whole muscle or contraction of extrafusal muscle results in shortening of muscle spindle and deactivation of sensory fiber firing. 

Stimulation, also used for thousands of years, may be directed toward the muscle or the underlying stiuctures. A flaccid muscle may be stimulated to activity by pressming or kneading. This may activate some of the stretch receptors, such as the muscle spindle apparatus, and the body of the muscle contracts. When used to address visceral concerns, the effect may be stirrmlation of the parasympathetic or sympathetic imderly-ing components. Stimulation in the suboccipital region or along the sacrum may initiate the former, whereas activity in the thoracolumbar spine may involve the latter. 

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