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NEUROENDOCRINE CONTROL OF THE MENSTRUAL CYCLE A neuroendocrine cascade involving the hypothalamus, pituitary, and ovaries controls the menstrual cycle (Figure 57-2). A neuronal oscillator or "clock" in the hypothalamus fires at intervals that coincide with bursts of gonadotropin-releasing hormone (GnRH) release into the hypothalamic-pituitary portal vasculature (see Chapter 55). GnRH interacts with its receptor on pituitary gonadotropes to cause release of luteinizing hormone (LH) and follicle-stimulating hormone (FSH). The frequency of the GnRH pulses, which varies in the different phases of the menstrual cycle (see below), controls the relative secretion of FSH and LH.

The gonadotropins (LH and FSH) regulate the growth and maturation of follicles in the ovary and ovarian production of estrogen and progesterone, which then exert feedback regulation on the pituitary and hypothalamus.

Because GnRH release is intermittent, LH and FSH secretion is pulsatile. The pulse frequency is determined by the hypothalamic GnRH pulse generator (Figure 57-2), but the amount of gonadotropin released in each pulse (i.e., the pulse amplitude) is largely controlled by the actions of estrogens and progesterone on the pituitary. The intermittent, pulsatile nature of hormone release is essential for the maintenance of normal ovulatory menstrual cycles; constant infusion of GnRH inhibits gonadotropin release and ovarian steroid production (see Chapter 55).Ovarian steroids, primarily progesterone, regulate the frequency of GnRH release by direct and indirect effects on GnRH neurons.

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