Researchers have discovered that the human body has a completely separate sensory system aside from the nerves that give most of us the ability to touch and feel.
This sensory network is located throughout our blood vessels and sweat glands, and is for most people, almost imperceptible.
The research team discovered it by studying two patients with a previously unknown abnormality called congenital insensitivity to pain.
"Although they had a few accidents over their lifetimes, what made these two patients unique was that they led normal lives. Excessive sweating brought them to the clinic, where we discovered their severe lack of pain sensation," said Dr David Bowsher of the University of Liverpool.
"Curiously, our conventional tests with sensitive instruments revealed that all their skin sensation was severely impaired, including their response to different temperatures and mechanical contact. But, for all intents and purposes, they had adequate sensation for daily living and could tell what is warm and cold, what is touching them, and what is rough and smooth."
Bafflingly, skin biopsies showed that the patients lacked all the nerve endings normally associated with skin sensation - implying that there was something else going on.
"For many years, my colleagues and I have detected different types of nerve endings on tiny blood vessels and sweat glands, which we assumed were simply regulating blood flow and sweating. We didn't think they could contribute to conscious sensation," said lead author Dr Frank Rice of Albany Medical College.
"However, while all the other sensory endings were missing in this unusual skin, the blood vessels and sweat glands still had the normal types of nerve endings. Apparently, these unique individuals are able to 'feel things' through these remaining nerve endings."
The researchers believe that problems with these nerve endings may contribute to mysterious pain conditions such as migraine headaches and fibromyalgia.
The research appears in Pain.