Scientists trying to understand the biological processes behind involuntary human responses say they have been able to determine what makes a person’s nipples become hard.

Swedish researchers made the discovery while mapping out the various nerve cells that ferry messages around the body in times of emotional distress, such as when the “fight or flight” reaction takes hold and prepares us for imminent danger.

Another type of nerve that emerged from the study hooks up with hair follicles. When activated, the nerves contract tiny muscles around the hairs, making them stand up and produce goosebumps.

All together, the team found eight different types of nerve that carry messages for the sympathetic nervous system, the part of the body’s neural wiring that makes the heart race and the palms sweat in anxious moments, but which also produces goosebumps, constricted blood vessels and hard nipples in cold weather.

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What it boils down to is that different nerves are used to wire different physical reactions. And bad nerve wiring could lead to someone responding to danger with erect nipples rather than, say, sweaty palms.

Said one researcher, “It’s the same challenge that an electrician faces when they wire up a house. If the wires all look the same, it’s going to be hard to wire them up correctly.”

[Photo via Instagram]