Pregnancy induces degenerative and regenerative changes in the autonomic innervation of the female reproductive tract.

The uterus is supplied with an extensive system of adrenergic nerves. The neurobiological properties of this innervation have been investigated in a series of studies primarily using the guinea-pig as model. The guinea-pig uterus is supplied from three different sources: the paracervical plexus, containing short adrenergic neurons; the inferior mesenteric ganglion; and a cranial source, probably the aorticorenal plexus, via nerves in the uterine suspensory ligaments. The nerve density is higher in the tubal end of the uterine horn and in the cervix than in the main part of the uterine horn. The turnover rate of transmitter is lower in the uterus than in a control organ, such as the heart. Noradrenaline levels in the uterus, but not the heart, are influenced by alterations in the endocrine milieu, e.g. during the oestrous cycle and after treatment with sex steroids. In the uterine tissue surrounding the conceptus during pregnancy, there is an early and drastic decay in various functional parameters related to the adrenergic nerve plexus and primarily reflecting a local, pregnancy-induced axonal degeneration. In the main part of the empty horn, in unilateral pregnancy, there is an extensive decay in various adrenergic functional parameters; these, however, reflect changes in a nerve plexus that has an essentially intact structure. No sign of functional impairment is seen in the adrenergic nerves of the uterine cervix. The increased turnover rate and reduced transmitter content in this region during late pregnancy may reflect increased frequency of firing of the adrenergic nerves. The tubal end of the uterine horn shows no signs of altered sympathetic function. This is the only part of the uterine horn that appears unaffected by pregnancy. The deficient recovery post partum of the changes in the uterine horn that previously contained the fetuses suggests permanent damage to the adrenergic nerve plexus after a pregnancy. The post partum recovery of changes seen in the previously empty horn, however, is more pronounced but still incomplete by comparison with the innervation before pregnancy. Studies on the adult human uterus indicate that similar events to those described for the guinea-pig model occur in human pregnancy.