Our fingerprints are an indelible marker of our identity. Fully developed by birth, they will remain constant throughout our adult life. But beyond their utility for criminal-catching and spy movie cliches, why exactly do we have fingerprints? A new study has provided an answer: they may help improve our sense of touch.
A team of Canadian and Swedish researchers teased out the labyrinthine connections that constitute the nerve ending fibers in our fingertips to reveal that sensitive touch “hotspots” map to individual papillary ridges. Their findings were published in the Journal of Neuroscience.
A sensitive touch
“It has been known for a very long time that nerves of neurons responsible for discriminatory touch divide into smaller branches before they reach their end receptors,” says Ewa Jarocka, a senior research engineer at Umeå University’s Department of Integrative Medical Biology (IMB).
Whilst it was known that these fast-adapting type 1 (FA-1) and slowly-adapting type 1 (SA-1) neurons had receptive fields in the fingertips, the way in which these fields mapped to our sense of touch hadn’t been fully investigated.
To take a closer look, the team recruited 12 volunteers who had their fingertip neurons’ electrical activity mapped whilst they reclined comfortably in a dentist’s chair. The team then rubbed a polymer ring against the participants’ fingers. This cylinder was embedded with tiny, raised dots, less than a millimetre in diameter that stimulated the volunteers’ fingertips. By recording the resulting electrical signals, the team were able to tease out the spatial acuity – the sensitivity in response to a surface – of these neurons.
Previous studies, conducted mainly in monkeys, had hypothesized that fingertip neurons had a single point of maximum sensitivity. In contrast, Jarocka’s data showed something quite different. “Our receptive fields display this non-uniform layout: namely they have multiple, highly sensitive zones – we call them “hotspots” – which respond to a stimulus with a very high intensity,” summarized Jarocka.
And those fields also showed a far more refined area of acuity, something Jarocka describes as an "exciting" finding. Whilst the monkey models suggested primate fingertips would only be able to detect down to a 1 mm area, Jarocka’s human data suggested a detection area of 0.4 mm for each hotspot – mapping clearly onto individual papillary ridges.
Whilst this finding was surprising enough, what further piqued the team’s interest was that these "hotspot" fields were maintained regardless of the speed at which the raised dots were swept across the volunteers’ fingers.
“We stimulated the skin with a pattern that was sweeping across the skin at different velocities – 15, 30 and 60 mm/s,” explains Jarocka. “No matter how fast we moved the surface across a finger, the same information was always there.”
Resilient fingerprints keep our senses stable
These findings go some way to explaining the incredible sensitivity of our fingertips, Jarocka points out. It might also explain why our fingerprints are so resilient. Fugitives, especially those on screen, have long tried to outwit identification by removing their fingerprints through chemical exposure or even by burning them off. But fingerprints can also disappear for less nefarious reasons; some chemotherapy treatments can result in the loss of fingerprints. Nevertheless, lost fingerprints will usually regenerate after a few months. “If we have these receptors in the fingerprint, which is stable over our lifetime, then the brain has a stable connection to the receptors. This is a very robust design,” concludes Jarocka.