in mothers who are nursing, an automatic release of milk from sacs within the mammary glands in response to infant suckling or associated stimuli (e.g., the infant’s cry). The milk letdown reflex is regulated by the hormone oxytocin: Suckling stimulates the production of oxytocin, which in turn stimulates contractions of the smooth muscle myoepithelial cells surrounding the mammary glands, leading to the expulsion of milk from the sacs into ducts that converge on the nipples. The actual production of milk by the mammary glands is regulated by the hormone prolactin. Also called milk ejection reflex.