Neural Control of the Reproductive Tract in the Cow as It Relates to Parturition

Abstract

The female genital tract is innervated by the autonomic nervous system. Moreover, there are sensitive nerve fibres located in the uterine cervix and in the dorsal region of the vagina which regulate the release of oxytocin and the abdominal muscular pressure during labour.

Uterine smooth muscles are innervated by the sympathetic nervous system. The postganglionic sympathetic nerves innervating the myometrium originate from the last thoracic and the first lumbar segments of the spinal cord. They travel via the hypogastric nerve to the pelvic plexus, where the preganglionic nerves synapse with the so-called short postganglionic neurons supplying the uterine muscle. The smooth muscles cells of the uterus contain both a (excitatory) and β(inhibitory) adrenoceptors. Stimulation of the myometrial cells by the postganglionic nerve fibres is mediated by noradrenaline which acts mainly on α-adrenoceptors in the bovine uterus. Stimulation of the β-adrenoceptors is done by adrenaline. However, during pregnancy the noradrenaline storage granules in the postganglionic sympathetic neurons are emptied. This means that the sympathetic neurons can not influence the myometrium at parturition. However, the stimulating ability of the adrenoceptors present at the membrane of the myometrial cells remains unchanged. By this way adrenaline release from the adrenal gland caused, by stimulation of the central sympathetic system may inhibit the myometrial contractions and uterine activity during labour. This mechanism plays an important role during the first stage of parturition. Moroever during the second stage the increased oxytocin release which is initiated by the foetus may be inhibited by this system, thus regulating intensity of labour.