Whether it is feeling the coarse texture of a pair of jeans, the gnarled bark of a tree or the smooth skin of a lover, our sense of touch is a crucial aspect of how we interact with the world around us.
Now scientists say they have shed new light on why human fingertips are so sensitive, revealing that the culprit hides in our fingerprints.
It was already known that tiny receptors sensitive to mechanical stimuli are found in fingertip ridges, and these are interact with two types of neurons in the fingers. The result is a “receptive field”: an area of skin where touch activates a particular neuron.
However, it was previously unclear just how small a structure could be detected by each neuron. “You would expect that a single papillary ridge would play a role, but it hasn’t been shown [before],” said Dr Ewa Jarocka, co-author of the study from Umeå University in Sweden.
To investigate the issue, Jarocka and colleagues asked 12 participants to each sit in a dentist’s chair, with their right arm at an angle and their fingernails glued to a plastic holder.
Writing in the Journal of Neuroscience, the researchers report that they used a robotic drum device to run a surface of raised dots over each participant’s fingertips, with each dot 0.4mm in diameter and the dots separated by 7mm. The team monitored the response of single neurons in the fingertips using electrodes inserted into a nerve in the participant’s arm.
Using these responses, together with the position of the dots, the researchers were able to map the receptive field for each neuron. The results confirmed that each receptive field covered an area spanning several fingertip ridges, and that within each field there were particularly sensitive zones. But it also revealed that these zones were sensitive to a single dot – an object around the same size as the width of a fingertip ridge. In fact, they found stronger neural responses within these zones mapped on to the fingertip ridges.
“It is enough to deflect one ridge to evoke a neural response,” said Jarocka, adding the results held regardless of how fast the drum rotated, or the direction in which it moved.
Chris Miall, emeritus professor of motor neuroscience at the University of Birmingham, who was not involved in the research, said the study increased understanding of how the information received by the brain from myriad neurons represented the object being touched.
“What the authors show is that the fine detail of the ‘receptive fields’ of individual nerve fibres that contact the mechanoreceptors in the fingertips closely match the troughs and ridges of the fingertips. So our very high sensitivity in the fingertips is because there are many, many nerve fibres with very small receptive fields,” he said.
Prof Nathan Lepora, an expert in tactile robotics at the University of Bristol, agreed. “This work is welcome because it shows for the first time that regions of the skin felt by tactile neurons appear to align with the fingerprint ridges, showing these ridges are directly involved in our sense of touch,” he said.
Miall added that while fingerprints also allow better grip, the study highlights their role in helping us detect tiny details on a surface. “Wearing gloves – even thin surgical gloves – has a huge impact. So spare a thought for everyone forced by the Covid pandemic to spend all day wearing clinical gloves,” he said.
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