Molecular Basis of Contraction; 1-Binding of Ach to Muscarinic reseptors. 2-Ca2+ influx from the ECF via Ca2+ channels. 3-Ca2+ binds to calmodulin, and the resulting complex activates calmodulin-dependent myosin light chain kinase. This enzyme catalyzes the phosphorylation of the myosin light chain. 4-The phosphorylation allows the myosin ATPase to be activated, and actin slides on myosin, producing contraction. 5-Myosin is dephosphorylated by myosin light chain phosphatase in the cell. 6-Relaxation of the smooth muscle. Dephosphorylation of myosin light chain kinase does not necessarily lead to relaxation of the smooth muscle. a latch bridge mechanism by which myosin cross-bridges remain attached to actin for some time after the cytoplasmic Ca2+ concentration falls. This produces sustained contraction with little expenditure of energy, which is especially important in vascular smooth muscle.
Stimulation of the smooth muscles:
1- Stretch: It contracts when stretched in the absence of any extrinsic innervations. Stretch is followed by a decline in membrane potential, an increase in the frequency of spikes and a general increase in tone.Chemical mediators:1-epinephrine or norepinephrine :The membrane potential usually becomes larger, the spikes decrease in frequency, and the muscle relaxes.Norepinephrine exerts both α and β actions on the muscle. The β action, reduced muscle tension in response to excitation, is mediated via cyclic AMP and is due to increased intracellular binding of Ca2+. The α action, which is also inhibition of contraction, is associated with increased Ca2+ efflux from the muscle cells. 2- Acetylcholin:Has an effect opposite to that of norepinephrine on the membrane potential acetylcholine causes the membrane potential to decrease and the spikes become more frequent , with an increase in tonic tension and the number of rhythmic contractions. Released by stimulation of cholinergic nerves (similar to cold and stretch in vitro). Other chemicals: like progesterone which decreases the activity and estrogen which increase it (in uterine smooth muscles).-Thermal stimuli : like cold which causes spasm.