Lactation physiology — Which pituitary or hypothalamic hormone is classically termed the “milk ejection” (let-down) hormone in humans?

Introduction:
Lactation involves both milk production and milk ejection. These processes are hormonally distinct. This question tests whether you can differentiate the hormone that drives the let-down reflex from the one that stimulates milk synthesis.

Given Data / Assumptions:

• Nipple stimulation activates neuroendocrine reflex arcs.
• Posterior pituitary releases neurohypophyseal hormones.
• Anterior pituitary secretes prolactin for milk synthesis.

Concept / Approach:
Oxytocin, released from the posterior pituitary, triggers contraction of myoepithelial cells around alveoli and ducts in the mammary gland, expelling stored milk (let-down). Prolactin from the anterior pituitary promotes milk production by alveolar epithelial cells. Vasopressin regulates water retention and vascular tone, not milk ejection.

Step-by-Step Solution:

Verification / Alternative check:
Clinical use of intranasal oxytocin can aid let-down in specific lactation difficulties, confirming its role. Suckling rapidly elevates plasma oxytocin measured in mothers during breastfeeding.

Why Other Options Are Wrong:

• Prolactin: production, not ejection.
• Vasopressin: water balance; no direct role in let-down.
• All of these: overinclusive and incorrect.
• Somatostatin: inhibitory hormone; unrelated to lactation reflex.

Common Pitfalls:
Conflating milk synthesis with ejection; they are coordinated but hormonally distinct processes.

Final Answer:
Oxytocin.