Final answer:
The let-down reflex in breastfeeding is controlled by oxytocin, a hormone that stimulates myoepithelial cells to contract and eject milk from the mammary alveoli, through a positive feedback loop initiated by infant suckling.
Step-by-step explanation:
The let-down reflex in breastfeeding is controlled by the hormone oxytocin. This reflex is a positive feedback loop where suckling by the infant stimulates sensory nerve fibers in the areola. This activates a neuroendocrine reflex that prompts the posterior pituitary to release oxytocin, stimulating the myoepithelial cells to contract and push milk from the alveoli into the lactiferous ducts and sinuses, and finally through the nipple pores to the infant.
Oxytocin's role extends beyond lactation, as it is also crucial during childbirth for stimulating uterine contractions. This hormone's release is stimulated not only by nursing but also by the stretching of tissues during birth, creating a positive feedback loop that leads to contractions until delivery is completed. The interplay between the suckling action, oxytocin release, and milk secretion exemplifies a well-coordinated biological system designed to ensure the continued supply of milk as long as the infant breastfeeds.