Reproduction One of the most important functions of the HPG axis is to regulate reproduction by controlling the uterine and ovarian cycles. In females, the positive feedback loop between estrogen and luteinizing hormone help to prepare the follicle in the ovary and the uterus for ovulation and implantation. When the egg is released, the empty follicle sac begins to produce progesterone to inhibit the hypothalamus and the anterior pituitary thus stopping the estrogen-LH positive feedback loop. If conception occurs, the placenta will take over the secretion of progesterone; therefore the mother cannot ovulate again. If conception does not occur, decreasing excretion of progesterone will allow the hypothalamus to restart secretion of GnRH. These hormone levels also control the uterine (menstrual) cycle causing the proliferation phase in preparation for ovulation, the secretory phase after ovulation, and menstruation when conception does not occur. The activation of the HPG axis in both males and females during puberty also causes individuals to acquire secondary sex characteristics. In males, the production of GnRH, LH, and FSH are similar, but the effects of these hormones are different. FSH stimulates sustentacular cells to release androgen-binding protein, which promotes testosterone binding. LH binds to the interstitial cells, causing them to secrete testosterone. Testosterone is required for normal spermatogenesis and inhibits the hypothalamus. Inhibin is produced by the spermatogenic cells, which, also through inactivating activin, inhibits the hypothalamus. After puberty these hormones levels remain relatively constant. Life cycle The activation and deactivation of the HPG axis also helps to regulate life cycles. At birth FSH and LH levels are elevated, and females also have a lifetime supply of primary oocytes. These levels decrease and remain low through childhood. During puberty the HPG axis is activated by the secretions of estrogen from the ovaries or testosterone from the testes. This activation of estrogen and testosterone causes physiological and psychological changes. Once activated, the HPG axis continues to function in men for the rest of their life but becomes deregulated in women, leading to menopause. This deregulation is caused mainly by the lack of oocytes that normally produce estrogen to create the positive feedback loop. Over several years, the activity the HPG axis decreases and women are no longer fertile. Although males remain fertile until death, the activity of the HPG axis decreases. As males age, the testes begin to produce less testosterone, leading to a condition known as post-pubertal hypogonadism. The cause of the decreased testosterone is unclear and a current topic of research. Post-pubertal hypogonadism results in progressive muscle mass decrease, increase in visceral fat mass, loss of libido, impotence, decreased attention, increased risk of fractures, and abnormal sperm production. Sexual dimorphism and behavior Sex steroids also affect behavior, because sex steroids affect the brains structure and functioning. During development, hormones help determine how neurons synapse and migrate to result in sexual dimorphisms. These physical differences lead to differences in behavior. While GnRH has not been shown to have any direct influence on regulating brain structure and function, gonadotropins, sex steroids, and activin have been shown to have such effects. It is thought that FSH may have an important role in brain development and differentiation. Testosterone levels have been shown to relate to prosocial behavior. This helps create synaptogenesis by promoting neurite development and migration. Activin promotes neural plasticity throughout the lifespan and regulates the neurotransmitters of peripheral neurons. Environment can also affect hormones and behavior interaction.
Hypothalamic–pituitary–gonadal axis
Hypothalamic–pituitary–prolactin axis
Hypothalamic–pituitary–thyroid axis
Hypothalamic–pituitary–somatotropic axis
Hypothalamic–pituitary–adrenal axis
Sympathoadrenal system
Endocrine system
Ovarian reserve