Myogenic contraction

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. 

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. 

Sanders In these volume organs, such as the bladder and the gut, things are different. When the gut is stretched, you don t necessarily get a myogenic contraction. It maintains its resting potential. It has to. 

Phenylethylamines. The properties of phenylethylamines have been investigated in some detail as stimulators of cAMP synthesis in homogenates of the CNS of the American cockroach (54 55), in the crayfish behavioral assay (31) and as inhibitors of the myogenic contraction of the locust ETi muscle (56) as shown in Table III. 

More limited data are available for the interactions of imidazolines with the locust ETi muscle (Table VI), but some compounds show extremely high potencies in the inhibition of the myogenic contractions of the proximal bundle. In some cases the... 

Neuromodulator - peripheral Schistocerca Extensor -tibiae muscle Inhibits myogenic contractions Modulates neuromuscular transmission DUMETi 2 41,42... 

Stimulation of the two identified octopaminergic neurons which project to the lateral oviducts also results in an elevation in cyclic AMP. The major physiological effects of octopamine (reduction in amplitude of neurally-evoked contractions and inhibition of myogenic contractions) are induced by agents which artificially elevate cyclic AMP levels (IBMX, forskolin, dibutyryl cylic AMP). 

An octopamine-sensitive adenylate cyclase is not the only means by which octopamine may exert its effects. Whilst DUMETi and octopamine modulate the amplitude and rate of relaxation of twitch tension in the extensor tibiae muscle of locusts via cyclic AMP, their effects upon the myogenic rhythm are mediated via a different route (86) Thus, elevated cyclic AMP does not induce a decrease in the frequency of myogenic rhythm as does octopamine, and the effects of octopamine are not potentiated by IBMX. In fact the mode of action of octopamine in reducing the frequency of myogenic contractions remains unknown. 

Increases in the baseline tone and spontaneous, rhythmic, myogenic contractions of estrogen-dominated rat uterine muscle strips were observed after treatment with an aqueous extract of devil s claw. Effective concentrations were 10 to 800 pg/ml. The authors noted that the activity was consistent with traditional use for induction or acceleration of labor, or for expulsion of retained placentas (Mahomed and Ojewole 2006). 

Action potentials are generated in single-unit smooth muscle. Simultaneous depolarization of 30 to 40 smooth muscle cells is required to generate a propagated action potential the presence of gap junctions allows this to occur readily. Because single-unit smooth muscle is self-excitable and capable of generating action potentials without input from the autonomic nervous system, it is referred to as myogenic. In this muscle, the function of the autonomic nervous system is to modify contractile activity only. Input is not needed to elicit contraction. 

Skeletal muscle is neurogenic and requires stimulation from the somatic nervous system to initiate contraction. Because no electrical communication takes place between these cells, each muscle fiber is innervated by a branch of an alpha motor neuron. Cardiac muscle, however, is myogenic, or self-excitatory this muscle spontaneously depolarizes to threshold and generates action potentials without external stimulation. The region of the heart with the fastest rate of inherent depolarization initiates the heart beat and determines the heart rhythm. In normal hearts, this "pacemaker region is the sinoatrial node. 

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. 

Myogenic mechanism. As discussed in Chapter 16 on the circulatory system, the myogenic mechanism involves contraction of vascular smooth muscle in response to stretch. For example, an increase in MAP would tend to increase RBF, leading to an increase in pressure within the afferent arteriole and distension, or stretch, of the vessel wall. Consequently, the vascular smooth muscle of the afferent arteriole contracts, increases the resistance of the vessel, and decreases RBF toward normal. 

Stimulation of the motoneuron releases acetylcholine onto the muscle endplate and results in contraction of the muscle fiber. Contraction and associated electrical events can be produced by intra-arterial injection of ACh close to the muscle. Since skeletal muscle does not possess inherent myogenic tone, the tone of apparently resting muscle is maintained by spontaneous and intermittent release of ACh. The consequences of spontaneous release at the motor endplate of skeletal muscle are small depolarizations from the quantized release of ACh, termed miniature endplate potentials (MEPPs) [15] (seeCh. 10). Decay times for the MEPPs range between l and 2 ms, a duration similar to the mean channel open time seen with ACh stimulation of individual receptor molecules. Stimulation of the motoneuron results in the release of several hundred quanta of ACh. The summation of MEPPs gives rise to a postsynaptic excitatory potential (PSEP),... 

In in vitro experiments, MSC developed an early myogenic phenotype (Jaquet et al, 2005), however we did not succeed in generating self-contracting or even twitching (cardio-) myoc54es (not shown). Our results reveal that MSC are able to survive at least 10 weeks within the rat myocardium. When paramagnetic iron oxide nanoparticles were incorporated into rMSC prior to transplantation, cells within the myocardial scar area could be detected in our study. This could be some evidence that rMSC are able to migrate into the scar from the site of injection. 

Vascular spasm. In which the blood vessels contract as a result of neurological reflexes and local myogenic (muscle) spasm. The degree of constriction is directly proportional to the degree of trauma. The spasm may last up to 30 minutes. 

Cardiac muscle cells are surrounded by an endomysi-um like the skeletal muscle cells. But innervation of autonomic nerves to the heart do not form any special junction like that found in skeletal muscle. Instead, the branching structure and extensive interconnectedness of cardiac muscle fibers allows for stimulation of the heart to spread into neighboring myocardial cells this does not require the individual fibers to be stimulated. Although external nervous stimuli can enhance or diminish cardiac muscle contraction, heart muscles can also contract spontaneously making them myogenic. Like skeletal muscle cells, cardiac muscle fibers can increase in size with physical conditioning, but they rarely increase in number. 

The factors involved in regulation of GFR are listed in Table 45-3. Autoregulation of renal blood flow and GFR is widely thought to be explained by the myogenic theory. This theory is based on the principle that an increase in the wall tension of the afferent arterioles, brought about by an increase in perfusion pressure, causes automatic contraction of the arteriolar smooth muscle, thus increasing resistance and beeping the flow constant despite the increase in perfusion pressure. 

Myogenic control (also known as the Bayliss effect) of coronary artery tone occurs when the vessel is stretched secondary to an increase in pressure and contracts to return blood flow to normal. It is thought that the myogenic response to stretching in coronary arteries is a modest one and that metabolic factors such as nitric oxide play a much larger role in autoregulation. 

Caldesmon is a cytoplasmic protein with two isoform classes, one of which is found predominantly in smooth muscle cells and other cell types with partial myogenic differentiation. High-molecular-weight isoforms with molecular weights between 89 and 93 kD are capable of binding to actin, tropomyosin, calmodulin, myosin, and phospholipids, and they function to counteract actin-tropomyosin-activated myosin adenosine triphosphatase (ATPase). As such, they are mediators for the inhibition of calcium-dependent smooth muscle contraction." ... 

Blair, K. L., Bennett, J. L. and Pax, R. A. (1994) Schistosoma mansoni myogenic characteristics of phorbol-ester induced muscle contraction. Exp. Parasitol. 78 302-316. 

In the distal stomach, small intestine, and colon, there are intermittent bursts of rapid electrical oscillations, called the electrical response activity (ERA) or spike bursts. The ERA occurs during the depolarization plateaus of the EGA if a cholinergic stimulus is present and it is associated with muscular contractions (Figure 6.3). Thus, neural and chemical control systems determine whether contractions wiQ occur or not, but when contractions are occurring, the myogenic control system (Figure 6.4) determines the spatial and temporal patterns of contractions. 

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