Suckling at the breast triggers milk ejection within seconds to about a minute. Which statement best describes this pathway?

• A Oxytocin acts on ductal smooth muscle to dilate the lactiferous sinuses and allow passive milk flow
• B Prolactin released from anterior pituitary lactotrophs directly contracts the myoepithelial cells surrounding alveoli
• C Afferent signals from nipple mechanoreceptors reach the supraoptic and paraventricular nuclei, releasing oxytocin from posterior pituitary terminals, which contracts myoepithelial cells ✓
• D Dopamine withdrawal from the median eminence stimulates myoepithelial cell contraction through V1 receptors

Explanation

Milk ejection is a classic neuroendocrine reflex: nipple afferents travel via spinal pathways to the hypothalamus, stimulating oxytocin neurons of the supraoptic and paraventricular nuclei, whose axons release oxytocin from posterior pituitary nerve terminals. Oxytocin then contracts the myoepithelial cells around alveoli, ejecting stored milk. Prolactin governs milk synthesis, not ejection, so option B is the key distractor. Oxytocin targets myoepithelium, not ductal smooth muscle, ruling out A.

Reference: Ganong Review of Medical Physiology, 26th ed.

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