Oxytocin: A hormone secreted by the posterior pituitary gland which targets the smooth muscle of the uterus and mammary glands, stimulating uterine contractions amd the letdown of milk.
During childbirth, the release of oxytocin is triggered by the stretching of the unterine cervix by the baby's head, causing the smooth muscle of the uterine walls to contract, which pushes on the baby and cuauses more stretching of the cervix which, in turn, tirggers the release of more oxytocin, and so on in a positive feedback loop. This loop is ended by the birth of the child, as when the stretching of the cervix ends, so does the oxytocin secretion. If given as a drug, oxytocin helps stimulate labor by the same mechanism. It may also be used to help control bleeding post-labor after delivery of the placenta.
The sensitivity of the uterus to oxytocin is dependent on the ratio of estrogen to progesterone levels. Estrogens increase its activity, making the uterus more sensitive, and progestins make the uterus less sensitive. This explains why oxytocin has more effect at the later stages of pregnancy, when estrogens are increased in larger amounts by the placenta.
The second role of oxytocin begins after birth, when the baby begins to suckle. The suckling sensation on the nipple causes oxytocin to be released and this triggers the nipple to eject milk, which is called milk letdown. This motivates the infant to keep suckling, which in turn release more oxytocin in another positive feedback response. Once the baby is satisfied and the suckling stops, so does the oxytocin release, so that no more milk is ejected.
Oxytocin is also thought to affect the CNS, helping to encourage mother-infant bond. It is also present during sexual intercourse, during the later stages. In males, in facilitates smooth muscle contractions for ejactulation. In females, it triggers uterine contractions which help to move the sperm and seminal fluid higher into the female's reproductive tract so that fertilization is more likely.
Since oxytocin receptors are widely distributed in the brain and in other tissues, there may be various other affects, including anxiolotic effects, satiety, stimulation of insulin and glucagon secretion, natriuresis, memory impairment, influence on reproductive behavior, analgesa, and cardivascular control. These effects may vary in different mammalian species. Research is ongoing into oxytocin's further roles in human (and mammalian) behavior, including its influence on pair bonding, social recognition, anxiety, love, trust, etc.
This page created 25 Oct 2012 20:17
Last updated 28 Feb 2016 23:31