Answer
When mechanoreceptors in the nipples are stimulated by suckling, they send impulses to the hypothalamus. In response the hypothalamus releases prolactin releasing factor/hormone (PRLH) which acts on the anterior pituitary (AP).to stimulate prolactin secretion; this causes milk secretion in suckled as well as in the un-suckled breast. This secretion is not discharged immediately but is saved for the next episode of suckling.
The reflex excitation cause by the mechanical stimuli of suckling also causes oxytocin release from the posterior pituitary ( and hypothalamus) by a positive feedback mechanism. This causes milk let-down in both breasts.
During suckling, also oxytocin stimulates the the muscles of the recently emptied uterus to contract.
Work Step by Step
During childbirth, the uterus is severely traumatized and is tender for several postpartum days . The uterine contractions stimulated by oxytocin may cause "cramping" or varying degrees of pain while suckling is taking place during these days. Nevertheless, there are two beneficial effects of postpartum uterine contractions: (1). The contractions reduce postpartum bleeding, and (2), they hasten the return of the uterus to normal size.
