The brain plays no part in lactation. The breast does it all on its own. When a baby sucks on the nipple, the breast produces a hormone called prolactin. Prolactin then tells the breast to start producing milk (lactogenesis) to feed the baby.
The control of mammary gland development and lactation is similar in the cow and human. It requires the action of prolactin secreted by the pituitary gland in concert with the actions of estrogen and progesterone and those supportive actions of other hormones. Milk is synthesized and secreted by the alveolar cells of the mammary gland, which do not fully develop until pregnancy. Full lactation (lactogenesis) is initiated at parturition when progesterone levels fall; progesterone inhibits milk secretion. In the cow, maintenance of lactation (galactopoiesis) does not require the continued production of prolactin, and it is believed that this function is carried out by the secretion of growth hormone from the pituitary gland.