During lactation, which hormone is responsible for the milk letdown (milk ejection) reflex?

Explanation

## The Milk Letdown Reflex ### Mechanism of Oxytocin Action **Key Point:** Oxytocin, released from the posterior pituitary in response to nipple stimulation, causes contraction of myoepithelial cells surrounding the alveoli and ducts, forcing milk into the nipple for ejection. **High-Yield:** The milk letdown reflex is a neuroendocrine reflex: sensory input (nipple stimulation) → hypothalamus → posterior pituitary oxytocin release → myoepithelial contraction → milk ejection. ### Prolactin vs. Oxytocin in Lactation | Hormone | Source | Function | Timing | | --- | --- | --- | --- | | **Prolactin** | Anterior pituitary | Stimulates milk *production* in alveolar epithelium | Sustained throughout lactation | | **Oxytocin** | Posterior pituitary | Causes milk *ejection* via myoepithelial contraction | Acute reflex during suckling | ### Neural Reflex Arc ```mermaid flowchart LR A[Nipple stimulation]:::outcome --> B[Sensory afferent via intercostal nerves]:::action B --> C[Hypothalamus]:::action C --> D[Posterior pituitary oxytocin release]:::action D --> E[Myoepithelial cell contraction]:::action E --> F[Milk ejection]:::outcome ``` **Clinical Pearl:** Stress, anxiety, or pain can inhibit the letdown reflex by suppressing oxytocin release, even though prolactin levels may be adequate. This is why a calm, supportive environment is crucial for successful breastfeeding. ### Why Other Options Are Incorrect - **Prolactin:** Maintains milk production but does NOT cause milk ejection; it acts on lactocytes, not myoepithelial cells. - **Thyroid hormone:** Supports overall metabolic rate during lactation but has no direct role in milk ejection. - **Cortisol:** Supports lactation indirectly via metabolic effects but does not trigger the letdown reflex. [cite:Ganong's Review of Medical Physiology 26e Ch 23]