The uterine tone is anintegral result of the effect of hormones (oxytocin), biologically active substances (prostaglandins, thrombin) and autonomic innervation, rendering a restorative and relaxing effect on the myometrial smooth muscle tissue. Regulation of myometrial tone involves a large number of endogenous compounds (acetylcholine, noradrenaline, serotonin, histamine, bradykinin, oxytocin, vaso-pressin, prostaglandins, etc.), and numerous uterine smooth muscles receptors. There is evidence that interaction with these receptors can influence the uterine muscles.
Changes in uterine contractility may be caused by a disturbance in various elements involved in its regulation. The mechanism of regulation of labor activity is multicomponent and depends on the state of the mother and fetus. If any element in the regulation mechanism is malfunctioning, the uterine contractile activity may change in different ways showing hypoor hyperactivity.
The muscular layer of the uterus performs important functions throughout almost the entire woman?s life, providing not only sufficient strength of contractions inlabor, but also long-term relaxation during pregnancy and adjusting the uterine tone depending on the menstrual cycle phase.
Neurohumoral regulation of the tone of smooth cells provides for a change in their tonewithin a broad range with varying strength and frequency. The main regulator of this process is the concentration of ionized calcium (Ca2+) in the cytoplasm of myometrial cells. Drugs affecting the myometrium can increase or decrease the rhythmic contractions and/or tone of the myometrium. Autonomic (sympathetic and parasympathetic) innervation and various humoral factors regulate myometrial contractile activity. Stimulation of M-cholinergic receptors and б-adrenergic receptors leads to an increase in the contractile activity of the myometrium, while stimulation of в2-adrenergic receptors provides an inhibitory effect. Besides, the tone and contractile activity of the myome-trium are regulated by oxytocin, sex hormones, and prostaglandins E2 and F2б. Oxytocin, estrogen and prostaglandins have a stimulating effect, while progesterone has an inhibitory effect on the myometrial contractility and tone. Prostaglandins also relax and dilate the cervix.