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Oxytocin
Oxytocin:
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Oxytocin |
Physiologic Effects of OxytocinIn years past, oxytocin had the reputation of being an "uncomplicated" hormone, with only a few well-defined activities related to birth and lactation. As has been the case with so many hormones, further research has demonstrated many subtle but profound influences of this little peptide, particularly in regards to its effects in the brain. Oxytocin has been implicated in setting a number of social behaviors in species ranging from mice to humans. For example, secretion or administration of oxytocin in humans appears to enhance trust and cooperation within socially-close groups, while promoting defensive aggression toward unrelated, competing groups.Oxytocin has been best studied in females where it clearly mediates three major effects: Stimulation of milk ejection (milk letdown): Milk is initially secreted into small sacs within the mammary gland called alveoli, from which it must be ejected for consumption or harvesting. Mammary alveoli are surrounded by smooth muscle (myoepithelial) cells which are a prominant target cell for oxytocin. Oxytocin stimulates contraction of myoepithelial cells, causing milk to be ejected into the ducts and cisterns. Both sexes secrete oxytocin - what about its role in males?Males synthesize oxytocin in the same regions of the hypothalamus as in females, and also within the testes and perhaps other reproductive tissues. Pulses of oxytocin can be detected during ejaculation. Current evidence suggests that oxytocin is involved in facilitating sperm transport within the male reproductive system and perhaps also in the female, due to its presence in seminal fluid. It may also have effects on some aspects of male sexual behavior. Control of Oxytocin SecretionThe most important stimulus for release of hypothalamic oxytocin is initiated by physical stimulation of the nipples or teats. The act of nursing or suckling is relayed within a few milliseconds to the brain via a spinal reflex arc. These signals impinge on oxytocin-secreting neurons, leading to release of oxytocin.If you want to obtain anything other than trivial amounts of milk from animals like dairy cattle, you have to stimulate oxytocin release because something like 80% of the milk is available only after ejection, and milk ejection requires oxytocin. Watch someone milk a cow, even with a machine, and what you'll see is that prior to milking, the teats and lower udder are washed gently - this tactile stimulation leads to oxytocin release and milk ejection. A number of factors can inhibit oxytocin release, among them acute stress. For example, oxytocin neurons are repressed by catecholamines, which are released from the adrenal gland in response to many types of stress, including fright. As a practical endocrine tip - don't wear a gorilla costume into a milking parlor full of cows or set off firecrackers around a mother nursing her baby. Both the production of oxytocin and response to oxytocin are modulated by circulating levels of sex steroids. The burst of oxytocin released at birth seems to be triggered in part by cervical and vaginal stimulation by the fetus, but also because of abruptly declining concentrations of progesterone. Another well-studied effect of steroid hormones is the marked increase in synthesis of uterine (myometrial) oxytocin receptors late in gestation, resulting from increasing concentrations of circulating estrogen. |
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