The Glandular System
What controls the sexual organs or how the glandular release system operates. The Gonadotropin Releasing Hormone GnRH. A neuropeptide
(a decapeptide) is produced in the hypothalamic surge and tonic centers.
The target tissue in both males and females is the anterior pituitary gland, specifically Gonadotroph cells. In males and females, secretion of GnRH results in the release of Follicle Stimulating Hormone (FSH) and Leutinising Hormone (LH) from the anterior pituitary gland.
GnRH-producing neurons are stimulated into production in response to spontaneous rhythms and by sensory impulses from sensory inputs derived from the external environment. In other words the bee attracted to the flower is ready to pollinate. Alterations in the internal conditions of the body can also result in altered GnRH production.
Prior to ovulation when estrogen concentration reaches a certain threshold, large quantities (a surge) of GnRH is released. This causes a corresponding peak in LH that stimulates ovulation. This surge centre is often called the preovulatory centre corresponding with the release of eggs from the ovaries. During pregnancy the plasma concentration of progesterone is maintained at an elevated level. Progesterone also inhibits secretion of FSH and LH by inhibiting GnRH therefore preventing the ovulation of follicles during the luteal phase and during pregnancy.
In males there are between 4-12 GnRH peaks per day. Plasma concentrations of LH peak approximately 10mins post GnRH surge. This accounts for the hardening of the penis throughout the day whether the male thinks of sex or is stimulated by the site of it or the appearance of a women.
Although the hypothalamus via GnRH stimulates the secretion of LH and FSH, it cannot regulate LH and FSH independently. Another hormone produced from the developing ovarian follicle in the female and sertoli cells in the male acts as a negative feedback mechanism for FSH. Sex hormones also alter the level of production of GnRH from the hypothalamus via a negative feedback system. High concentrations of progesterone or testosterone will reduce the secretion of GnRH and also the secretion of LH and FSH.