Final answer:
Oxytocin is the hormone that triggers the let-down reflex, facilitating the ejection of milk from the mammary glands into the milk ducts when an infant suckles.
Step-by-step explanation:
Oxytocin triggers the "let-down reflex," where the mammary glands are stimulated to eject milk into the milk ducts. This process begins when an infant suckles at the breast, activating sensory nerve fibers in the areola. A neuroendocrine reflex ensues, leading to the secretion of milk from lactocytes into the alveoli. Subsequently, the posterior pituitary releases oxytocin, which then stimulates myoepithelial cells to contract, pushing the milk from the alveoli into the lactiferous ducts, through the lactiferous sinuses, and out the nipple pores. This event takes place within a minute of the infant beginning to suckle, indicating a rapid response time known as the latent period.
The ability for continuous milk production is maintained by a positive feedback loop as part of the let-down reflex, ensuring that as long as the infant continues to breastfeed, milk will be produced. This reflex is also important for the development and growth of the infant, providing essential nutrients and immunoglobulins for immune protection against infections.