Our super sensitive fingerprints

New research has emerged that suggests fingerprints may be good for more than catching criminals, and may in fact be responsible for the incredible sensitivity that humans have in their fingertips. Eva Higginbotham spoke to Ewa Jarocka, about her work published in the Journal of Neuroscience...

Eva - As burglar Harry Jackson found to his dismay, when he was the first person convicted of a crime in the UK based on fingerprint analysis, your fingerprints are unique to you. Your combination of whorls, arches, and loops arose while you were developing as a foetus in the womb, and your fingerprint - or pattern of papillary ridges - are yours for life. No-one's quite sure why exactly we have fingerprints; though some scientists have suggested that they provide either protection for our fingers, or increased skin sensitivity. But the evidence is still murky. Now, scientists that Umeå University have made a new, remarkable connection involving your fingerprints in your sense of touch.

Ewa - The fact that we can feel objects that we touch, or that we can feel even small vibrations - for example, if the table is vibrating and you put your fingers on it, you can feel it - that is enabled by our receptors that are located in the skin.

Eva - That's Ewa Jarocka, and she's part of a team who recently did an unusual experiment to try and get a deeper understanding of how the nerve cells in our fingertips work. The reason we can feel things at all is down to specialised proteins called receptors on our skin that respond to different kinds of stimuli like heat or pressure, which are then connected to our nerve cells which innervate our fingertips. Ewa was interested in how close together two points on your fingertip can be while still creating individual responses in your nerve cells; in other words, the spatial acuity of your fingertips.

Ewa - We knew that the neurons innervating our fingertips, they branch before they reach their end receptors. So you could imagine a nerve fibre as a trunk of a tree, and then you see multiple branches coming out of it, and each of them reaches a number of end receptors. We wanted to see how precise our touch can be, considering that we have those patchy fields of sensitivity, and if that was related to our fingerprint layout.

Eva - Essentially, you have these areas of higher sensitivity on your fingertips called receptive fields. And these receptive fields each activate a single nerve cell. Ewa and the team used a technique called microneurography to record single electrical impulses going through individual nerve cells.

Ewa - We inserted an electrode in the participant's arm, to a nerve that contains nerve fibres that innervate our fingertips. And when we were stimulating the skin, the impulses were triggered, and we were recording them. And we stimulated the skin with a pattern of raised dots that was wrapped around a drum, and we were spinning this drum with different velocities, and also with two different directions -  towards the body and away from the body. During each drum revolution, we were able to scan one receptive field. And as a result, we created a map of sensitivity, so we could map where those branches were located in the skin.

Eva - What they found was that the spatial sensitivity of the nerve cells was the same as the distance between each of your fingertip ridges -  0.4 millimetres. This level of fine detail is why your fingertips are so sensitive in comparison to, say, the side of your arm. Importantly, they didn't test perception by asking the participants if they could feel a difference between two dots. But now we know that that level of incredible precision and detail is noticed and recorded by our nerve cells.

Ewa - Those neural responses were in the fingerprint ridges, they were anchored to the fingerprint ridges.

Eva - So next time you're enjoying the softness of a knitted blanket, or rummaging in the cutlery drawer for that specific spoon that you prefer, spare a thought for your fingerprints. They may be directly helping you feel your way through the world!