Previously I posted on the general features of oxytocin, what it acts on, and where it basically acts, and what it's mostly known for. But the reality is that oxytocin is a LOT more complicated than that, and has different effects of your body and your behavior, depending on who you are. It varies from person to person (as all biological things do) as well as between men and women. And today, we're going to discuss the ladies. Because if there is anything oxytocin is famous for, it's for its effects on women.
You may have noticed lots of links in the previous post. Those links are to the literature which I searched before posting. There will be lots more links in this one to examples of studies which support what I'm going to tell you about. Of course, all of these are in science-ese, and so if you are puzzling over something and can't make it out, give a shout out in the comments with the particular paper, and Sci will do her best to cover the paper later on. I have a feeling that oxytocin is going to be a recurring topic.
So here we go.
This one's for the ladies
So as we know from the previous post, oxytocin is a hormone (by definition, a hormone is a chemical that is released from one part of the body that has effects on another part of the body, but that doesn't really narrow down the function much, does it?) that is secreted from the posterior pituitary (the nutsack of the brain). In women, it is most known for its extensive roles in reproductive function, particularly in parturition (that's birth), lactation (that's nursing), and in orgasm. So we'll cover those bits first, mostly because Sci LOVES the positive feedback loop in parturition. We'll also talk about the role of oxytocin in maternal bonding, one of the social aspects. I was going to cover the major effects on social bonds as well, but there's quite a bit on that, so I think we're going to have to save that for another post. So we'll stick just to the specifically female stuff for the moment.
Roles of Oxytocin in the body:
In pregnant women, oxytocin has one of it's major roles (you might say it's the most important, but given the importance of the OTHER roles of oxytocin, it's a toss-up). Oxytocin is released at the end of fetal development (and sometimes during), and results in uterine contraction, making it one of the most important hormones at birth (the other one is progesterone, which gets HUGE right before birth).
The coolest thing about oxytocin during parturition is that it's controlled under a positive feedback cycle. Goes like this (in the very, very simplified version):
1) Baby is big and heavy, and presses against the cervix.
2) Cervix feels the burn and stretches a little.
3) The muscle stretching of the cervix triggers receptors which send nerve impulses to the brain.
4) Brain releases oxytocin.
5) Oxytocin further softens and dilates the cervix.
6) Baby weight pushes downward on the softened cervix and stretches it.
7) Lather, rinse, repeat.
"Positive feedback" is the stretching of the cervix which promotes oxytocin, which promotes stretching, which promotes MORE oxytocin. The actions of the hormone trigger the release of more hormone. If you're like Sci, you will think this is really, really cool.
However, oxytocin is not necessarily NEEDED for birth. There are oxytocin knockout mice that do not have oxytocin and can still reproduce (but these are knockout animals, and so growing up without oxytocin, their bodies could compensate). But it's very clear that oxytocin is very important in the normal birth process. In fact, it's SO important that oxytocin analogues (drugs that mimic oxytocin, such as Pitocin) are used to induce labor, and oxytocin receptor antagonists (which block the actions of oxytocin, like the drug Tractocile) are used to stop premature labor and uterine contractions. So, pretty important.
Another one of oxytocin's big roles in women (and in female mammals in general) is in lactation, where it serves a very important role in milk letdown.
To go to HOW this works, we're going to need a little anatomy
You can see up there the important things for oxytocin, the alveoli (those little frond things), the milk sinus (also known as the sac), and the nipple opening, which is surrounded on the outside by the areola, which is the dark area of the nipple. While other hormones (like prolactin) are involved in actually MAKING milk, oxytocin is very important for releasing it, in what's known as the let-down reflex.
Goes like this (again, the simplified version, the complicated version involves dopamine and a host of other hormones):
1) Baby sucks at nipple
2) The sensation of suckling heads to the brain (skipping some detailed steps here).
3) The hypothalamus is stimulated and oxytocin is released from the posterior pituitary.
4) Oxytocin in this case acts to contract the muscles around the alveoli in the breast.
5) The squeezed alveoli deliver milk to the milk sinus via the duct.
6) Baby sucks, milk comes out.
And now we get to the third, best part of oxytocin in women:
Oxytocin is actually important in sexual arousal for both men and women (I'll cover more about the men in the next post). During sexual arousal, oxytocin increases rapidly, with a big burst at orgasm. In fact, in women, the strength of orgasm is directly correlated with the amount of oxytocin. Oxytocin levels correlate to sexual arousal in women, as well as the amount of vaginal lubrication present. Not only that, oxytocin fluctuates along with a woman's menstrual cycle, being highest in the ovulatory phase and follicular phase, and lowest in the luteal phase (The follicular phase and ovulatory phase are the preparation and release of the egg, respectively, and fertility will peak at ovulation for obvious reasons. The luteal phase is the phase after ovulation, as the egg sits around and grows old until the shedding of the uterine lining during menstruation at the end of the month. Sci will probably have to blog about this sometime).
So oxytocin is very important physically, helping us enjoy the procreation of the species and allowing the species to get out of the womb and get fed. Now, we move on to the first of many fuzzier roles for oxytocin:
This is the part where some people get antsy. For it's true, oxytocin DOES influence behavior. Oxytocin DOES promote things like affiliative behavior with one's young.
Examples: rats, once they give birth, will pick up pups and carry them around. They'll build a nest for them (if they haven't already), and lick them and basically associate with them like good rat mommies. However, if you give a rat mom something to block oxytocin, she will not pick up the pups, carry them around, or make a nest. Giving oxytocin to sheep will induce maternal behavior by sheep for lambs that are not their own, even if the sheep has never given birth.
And this pattern appears to hold true in humans as well. Mothers who have secure attachments to their newborns have stronger activation of the pituitary (where oxytocin is released) when they see pictures of their child, than mothers with less secure attachments. Some studies also indicate that women with particular oxytocin gene regulation may show more signs of "sensitive" parenting. It appears that oxytocin release is a significant part of how women react to their babies, and how much they affiliate with them in the first months of life.
And this is where people get squirmy. Because doesn't this mean that women are the TOOLS of their HORMONES?! And if women are the tools of their hormones, doesn't this mean they that BIOLOGY has DECREED that they stay indoors, barefoot and pregnant? And of course there's the even more squirmy question: if some women are better at raising children than others due to their genes, should women be screened for these genes before having children?
In my opinion, heck no. Women (and men) are certainly influenced by their hormones, and certainly influenced by their genes. But hormones and genes do not act in a vacuum. They act within an environment, and very importantly, within a society. A woman is very capable of both affiliating and responding sensitively to her baby and to still maintain a life.
And we are certainly not forced by our hormones to play certain roles. It's important to remember that societal interactions are as important, and sometimes MORE important, than hormones and genes. Oxytocin never prevented the societal norms of sending rich women's children to wet nurses and governesses back in the day. Oxytocin genes might help more sensitive parenting from the start, but someone who lacks them is not necessarily an insensitive parent. We are all capable of learning from our surroundings and learning what works best for our children, and it is obvious that fluctuations in oxytocin levels are not the be all end all of parenting. If they WERE, then all parents would have uniformly high oxytocin levels, evolution having weeded out over time the primate females who had low oxytocin levels, and thus did not care for their young effectively and raise successful offspring. As people with high and low oxytocin are still out there and still raising kids, you can be pretty sure there are other factors at work.
So when it comes down to it, yes, we're influence by our hormones. But we are ALSO under the influence of human society and the environmental influences coming in through our vast prefrontal cortex. And to discount the incredible influences of society and the cortex would be very naive indeed.
So that's basic actions of oxytocin as related to women. For it is late, and Sci is very tired. Next we move onward, to oxytocin in MEN. Oh yes, it's there, boys.