Sometimes nipples are hard, sometimes they are not. Whatever they are, it seems we have absolutely no control over nipples.
It is well-known physiological mechanism that, during chilly weather, our skin would tighten and contract, causing nipples to shrink and harden. It’s to reduce the surface area of the nipple, and therefore decrease the amount of energy required to keep it warm.
And sure, we also appreciate nipples’ function in sexual activity. They react in the same way as other parts of the body. But a new study has found that nipples sometimes go hard for no reason whatsoever. They seem to have a mind of their own.
Researchers at the Karolinska Institutet in Sweden have found that nipples have their very own "special nerve" that causes goose bumps and nipple erection. The study, published in the journal Nature Neuroscience, explains that the sympathetic nerve system comprises different neurons that regulate specific physiological functions.
Basically, even when we’re not aroused, neurons that regulate nipple erection will function involuntarily – and without "conscious control".
Source article: http://iservice.ltn.com.tw/Service/english/english.php?engno=1074188&day=2017-01-26