## Milk Ejection Reflex Physiology **Key Point:** Oxytocin is the hormone responsible for milk ejection (letdown reflex), while prolactin is responsible for milk production. ### Mechanism of Action Oxytocin causes contraction of myoepithelial cells surrounding the alveoli and ducts of the breast, forcing milk out through the nipple. This reflex is triggered by: 1. Infant suckling (afferent sensory stimulus via spinal nerves) 2. Hypothalamic response → posterior pituitary release of oxytocin 3. Myoepithelial cell contraction → milk ejection ### Hormonal Roles in Lactation | Hormone | Source | Function | Timing | | --- | --- | --- | --- | | **Prolactin** | Anterior pituitary | Milk synthesis/production | Sustained throughout lactation | | **Oxytocin** | Posterior pituitary | Milk ejection/letdown | Reflex response to suckling | | **Progesterone** | Placenta/corpus luteum | Inhibits lactation (suppressed postpartum) | Decreases after delivery | | **HPL** | Placenta | Lactogenic effect (minor) | Present only during pregnancy | **High-Yield:** Oxytocin also causes uterine contractions (afterpains) during early lactation, which aids involution. **Clinical Pearl:** Stress, anxiety, or pain can inhibit oxytocin release, causing failure of letdown reflex despite adequate milk production (common in anxious mothers). **Mnemonic:** **POLE** — **P**rolactin = **P**roduction, **O**xytocin = **O**utlet (ejection).
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