During breastfeeding, an infant's suckling triggers milk ejection within about one minute. The neural pathway mediating this response runs from nipple afferents through the spinal cord to the hypothalamus. Which statement correctly describes the final step of this reflex?

• A Oxytocin released from the posterior pituitary contracts myoepithelial cells around the alveoli ✓
• B Prolactin released from the anterior pituitary contracts the lactiferous duct smooth muscle
• C Dopamine withdrawn from the median eminence allows direct mammary gland contraction
• D Vasopressin released from magnocellular neurons increases intraductal hydrostatic pressure

Explanation

Suckling afferents reach the paraventricular and supraoptic nuclei, causing burst firing of magnocellular neurons and pulsatile oxytocin release from posterior pituitary terminals. Oxytocin binds myoepithelial cells surrounding alveoli, contracting them and ejecting stored milk. Prolactin governs milk synthesis over hours to days, not the immediate let-down, which is why option B fails. This is a classic neuroendocrine reflex, since the afferent limb is neural while the efferent limb is hormonal.

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

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