Oxytocin

Oxytocin is concerned in numerous aspects of reproductive function. Its stimulation of smooth muscle reduction inspires the milk ejection reflex in lactating females and maintenance of uterine contractions during parturition (i.e., birth).

Suckling is the most potent stimulus for milk ejection. The prime afferents from the areolar and nipple skin relay with spinothalamic tract neurons in the dorsal horn of the spinal cord. The spinothalamic input causes oxytocin secretion through an undefined neural pathway from midbrain to the PVN and SON. The Neurons that secrete oxytocin are different from those that secrete AVP.

Oxytocin is not the trigger for parturition. Though, at term, a rise in maternal estradiol or progesterone ratio upregulates oxytocin receptors in uterine smooth muscle that accordingly becomes very sensitive to oxytocin. Once parturition is recognized, pressure of the fetal head on the cervix suggests the secretion of oxytocin from the posterior pituitary through a reflex pathway similar to that of milk ejection. The oxytocin arouses contractions of the uterine smooth muscle, further raising the pressure of the fetus on the cervix. This positive feedback method is the Ferguson reflex. Though, as parturition can be common in spinally transected women, or those lacking in oxytocin, the additional mechanisms should also be significant.