Sex hormones wiki

An example of positive feedback is the hormone that causes childbirth (when babies are born.) The hormone that causes this is oxytocin . This hormone is made by the pituitary gland . When the baby starts coming out, it stretches the muscle in the cervix (the bottom of the uterus ). Nerves in the cervix send a message to the pituitary. This message makes the pituitary release more oxytocin. The oxytocin then causes the muscles of the uterus to contract, or squeeze. This causes more stretching in the cervix. This stretching then tells the pituitary to make even more oxytocin. So levels of oxytocin keep rising until the squeezing or contractions of the uterus force the baby out.

The male sex hormone is called testosterone and this hormone is required for spermatogenesis . Testosterone is a steroid hormone that is produced in the leydig cells within the testes. A relatively high concentration of testosterone is maintained within the testicular tissue and testosterone is circulated around the body by diffusion of the hormone from the spermatic cord into the testicular veins and arteries. The primary action of testosterone is anabolic growth, spermatogenesis promotion and promotion of secretion from the accessory sex glands.

Male sex hormones are regulated by negative feedback systems that operate at various levels within the male sex hormone system. The starting point for the production of testosterone (and therefore the production of spermatozoa)is the hypothalamus. The hypothalamus contains neuroendocrine cells that are capable of secreting a substance called Gonadotropin-releasing hormone or GnRH. GnRH stimulates basophilic cells in the adenohypophysis, via the "portal system" to secrete two intermediate hormones within the male sex hormone cycle; Luteinizing hormone (LH) and Follicle-Stimulating Hormone (FSH).

The secretion of GnRH is pulsatile and can vary greatly throughout the day and/or year, and therefore the secretion of LH and FSH are also pulsatile (although the plasma concentration of FSH does not fluctuate as much as LH due to the effect of Inhibin, see below). The activity of GnRH neuroendocrine cells is determined by spontaneous rhythms and by sensory impulses. Cycles such as seasonal sexual activity are controlled by this pulsatile system. In male animals there are generally 4 to 12 GnRH pulses per day.

Some medications, such as certain anabolic steroids like mesterolone and danazol and certain progestins like levonorgestrel and norethisterone , have high affinity for SHBG and can bind to it and displace endogenous steroids from it, thereby increasing free concentrations of these endogenous steroids. [32] [33] [34] It has been estimated that therapeutic levels of danazol, methyltestosterone , fluoxymesterone , levonorgestrel, and norethisterone would respectively occupy or displace from testosterone 83–97%, 48–69%, 42–64%, 16–47%, and 4–39% of SHBG binding sites, while others with low affinity for SHBG such as ethinylestradiol , cyproterone acetate , and medroxyprogesterone acetate would occupy or displace from testosterone 1% or fewer SHBG binding sites. [32] [35]

Sex hormones wiki

sex hormones wiki


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