An investigation into the biological machinery that underpins involuntary responses in the body has led scientists to a distinct type of nerve that makes nipples go 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.



“We are trying to understand how the body is constructed, how we go from an egg to a full individual, and how we are wired up is a fascinating process,” said Sten Linnarsson who worked on the study at the Karolinska Institute in Stockholm. “This is a beautiful example of how it works.”



Until now, the wiring diagram of the sympathetic nervous system has been something of a mystery. But in an unprecedented effort, Linnarsson and his team traced nerves from the spine to the tissues and structures in the body with which they connect.



They began by collecting clumps of nerve cells from the spines of mice and examining them cell by cell for differences in their genetic activity. The team found they could divide the nerves into eight distinct types, each defined by a unique genetic signature.



“We knew there must be multiple types of nerves wired to different tissues and organs, but we didn’t know what they were,” said Linnarsson, whose study appears in the journal Nature Neuroscience.



The findings suggest that cells in the sympathetic nervous system are separated into distinct types to ensure that after leaving the spine and threading their way through the body, they connect to the right tissues. Any mistakes in the wiring could leave a person in the unfortunate position of responding to danger with erect nipples.



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

