Lactation • Figure 16.18
A positive feedback mechanism governs lactation.
Posterior
pituitary
Prolactin secretion by the anterior pituitary stimulates milk production in
the mammary glands. Milk ejection or let-down begins when the baby
suckles on the mother’s nipple. The stretch receptors stimulate the
hypothalamus and posterior pituitary to release oxytocin, which
stimulates milk ejection by the mammary gland.
Positive feedback: milk
availability encourages
continued suckling, so
touch sensations on
nipple and oxytocin
release continue
Increased
oxytocin in blood
Contraction of cells in
mammary glands
Milk ejection
Interruption of cycle:
Baby ceases to suckle,
thus breaking the
positive feedback cycle
A non-pregnant woman begins secreting breast milk.
Which of the following might be the most likely cause of this symptom?
a. Over-stimulation of the nipple area by tight clothing
b. Pituitary tumor that causes decreased oxytocin levels
c. Pituitary tumor that causes increased prolactin secretion
d. Trauma to the posterior pituitary gland
Giving Birth Requires a Complex Series of Hormonal Changes
501
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