Getting hard nipples is definitely one of the weirder parts of being turned on. But how does that actually happen? Well, according to a new study, that process relies on some highly specialised cells. In the study, published online this week in Nature Neuroscience, researchers looked at the different kinds of cells in mice that make up the sympathetic nervous system. Yes, mice. But you also have a sympathetic nervous system made up of the same kinds of cells. This complex system of signals starts with your brain and goes on to affect your lungs, heart, and many other things. This network is responsible for controlling your stress-related responses, including revving up your heart rate and giving you a surge of energy. The researchers found that there are many types of cells making up this vast system, and a few of them are dedicated to regulating very specific processes. For example, they found that two types of neurons are involved in the process that leads to goosebumps and erect nipples. When activated by the neurotransmitter noradrenaline, these neurons cause erector muscles connected to your skin to contract. That contraction is what causes your hair to stand on end or your nipples to get erect. So there you have it — just one more of our many complex physiological responses to gettin' it on.