The Suckling Reflex Maintains Lactation and Inhibits Ovulation

The suckling reflex is central to the maintenance of lactation in that it coordinates the release of PRL and oxytocin and delays the onset of ovulation. Lactation involves two components, milk secretion (synthesis and release) and milk removal, which are regulated independently. Milk secretion is a continuous process, whereas milk removal is intermittent. Milk secretion involves the synthesis of milk constituents by the alveolar cells, their intracellular transport, and the subsequent release of formed milk into the alveolar lumen (see Fig. 39.12). PRL is the major regulator of milk secretion in women and most other mammals. Oxy-tocin is responsible for milk removal by activating milk ejection or letdown.

The stimulation of sensory nerves in the breast by the infant initiates the suckling reflex. Unlike ordinary reflexes with only neural components, the afferent arc of the suck-

Arterial blood

Arterial blood

Milk Alveolus

^¬°GUREISPIH^ The structure of a mammary alveolus. Milk-

producing cells are surrounded by a meshwork of contractile myoepithelial cells.

^¬°GUREISPIH^ The structure of a mammary alveolus. Milk-

producing cells are surrounded by a meshwork of contractile myoepithelial cells.

ling reflex is neural and the efferent arc is hormonal. The suckling reflex increases the release of PRL, oxytocin, and ACTH and inhibits the secretion of gonadotropins (Fig. 39.13). The neuronal component is composed of sensory receptors in the nipple that initiate nerve impulses in response to breast stimulation. These impulses reach the hypothalamus via ascending fibers in the spinal cord and then via the mesencephalon. Eventually, fibers terminating in the supraoptic and paraventricular nuclei trigger the release of oxytocin from the posterior pituitary into the general circulation (see Chapter 32). On reaching the mammary glands, oxytocin induces the contraction of myoepithelial cells, increasing intramammary pressure and forcing the milk into the main collecting ducts. The milk ejection reflex can be conditioned; milk ejection can occur because of anticipation or in response to a baby's cry.

PRL levels, which are elevated by the end of gestation, decline by 50% within the first postpartum week and decrease to near pregestation levels by 6 months. Suckling elicits a rapid and significant rise in plasma PRL. The amount released is determined by the intensity and duration of nipple stimulation. The exact mechanism by which suckling triggers PRL release is unclear, but the suppression of dopamine, the major inhibitor of PRL release, and the stimulation of prolactin-releasing factor(s) have been considered. Lactation can be terminated by dopaminergic agonists that reduce PRL or by the discontinuation of suckling. Swollen alveoli can depress milk production by exerting local pressure, resulting in vascular stasis and alveolar regression.

Lactation is associated with the suppression of cyclic-ity and anovulation. The contraceptive effect of lactation

Hypothalamus

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