The reflex that ejects milk
The let-down, or milk ejection reflex, is what moves milk from where it is made deep in the breast to the nipple. When a baby suckles, sensory nerves signal the brain, prompting the posterior pituitary to release the hormone oxytocin. Oxytocin makes the tiny myoepithelial cells wrapped around the milk-producing alveoli contract, squeezing milk into the ducts so it becomes available to the baby. The reflex can also be triggered by hearing or thinking about the baby, not only by physical suckling.
Because it is partly a conditioned response, stress and discomfort can blunt let-down by interfering with oxytocin release.
How supply is sustained over time
Oxytocin handles the immediate ejection of milk, while a second hormone, prolactin, drives the ongoing production that refills the breast. Prolactin release is stimulated by suckling, which is why frequent and effective milk removal tends to maintain supply, an example of demand shaping production.
Lactation patterns vary widely between individuals. This is general educational information about lactation physiology, not medical advice; breastfeeding concerns should be discussed with a qualified clinician or lactation specialist.