Oxytocin and the milk ejection (let-down) reflex
Suckling stimulates the nipple and releases the hormone oxytocin from the pituitary (a small gland beneath the brain), which makes contractile cells around the milk channels squeeze milk out.
Remember
Prolactin, the hormone that drives milk production, makes the milk, while oxytocin moves it: released by suckling, oxytocin makes the contractile cells around the milk channels contract and eject milk.
Memory aid
the front of the pituitary makes (prolactin, for production) and the back pushes (oxytocin, for push)
Use it to keep the two pituitary hormones of the breast apart: prolactin for making milk, oxytocin for pushing it out.
Making milk and moving milk are two separate jobs. The let-down reflex, also called the milk ejection reflex, is the moving part: milk that has already been made is pushed towards the nipple. A baby does not simply suck milk out.
Suckling stimulates the nipple, and the reflex releases oxytocin. When the baby suckles, nerve signals from the nipple reach the hypothalamus (the part of the brain that controls the pituitary gland). Oxytocin is made there and released from the posterior pituitary (the back lobe of the pituitary, a small gland beneath the brain). It travels in the blood to the breast.
There it acts on the myoepithelial cells. These are contractile cells that form a basket around the alveoli (the tiny sacs where milk is made) and the lactiferous ducts (the channels that carry milk towards the nipple). When they contract, milk is squeezed from the alveoli into the ducts and on towards the nipple. Hormones that do this are called galactokinetic (galacto means milk, kinetic means movement).
Milk production and milk ejection are separate steps. Milk production is controlled mainly by prolactin, released from the anterior pituitary (the front part of the pituitary gland), and suckling also keeps its release going. A simple memory aid: prolactin produces, oxytocin ejects.
The reflex can be conditioned (the cry or sight of the baby may trigger it). Pain, embarrassment or alcohol may block it, so a mother can make plenty of milk yet struggle to deliver it.
Key facts
- Trigger
- Suckling stimulates the nipple, and the reflex releases oxytocin.
- Source
- Oxytocin is made in the hypothalamus and released from the posterior pituitary.
- Target
- Myoepithelial cells around the alveoli and the lactiferous ducts contract and squeeze milk towards the nipple.
- Production and ejection
- Prolactin from the anterior pituitary drives milk production, whereas oxytocin moves milk that has already been made.
- Failed let-down
- Full breasts with a baby who struggles to get milk can point to failed let-down, where milk is made but not ejected, rather than failed production.
- Let-down inhibited
- Pain, embarrassment or alcohol may block let-down, even when milk production is normal.
Exam traps
- Prolactin does drive milk production, but it is not the hormone of let-down, because prolactin controls how much milk is made while oxytocin drives the squeezing that delivers milk during a feed.
- Inhibition of GnRH (gonadotrophin-releasing hormone, the signal from the hypothalamus that drives the menstrual cycle) is not an oxytocin effect, because high prolactin tends to suppress GnRH, which is one reason frequent breastfeeding can suppress ovulation.
- Reduced prolactin causes a low supply, not failed let-down: full breasts with milk that will not flow, especially in an anxious mother, can point to failed let-down rather than a low supply.
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