A single neuron in the human fingertip is sensitive to tactile details as fine as 0.4 millimeters, which is about the width of the ridges in a fingerprint, according to new research in which scientists measured the electrical impulses from nerve cells in people's fingertips.
The study was published March 15 in the Journal of Neuroscience, and the findings demonstrate one way that fingerprints sharpen people's sensitivity to touch.
Fingertips contain several kinds of tactile neurons that respond to vibrations, taps, skin stretching and spatial details such the corners and edges of objects we touch, says Ewa Jarocka, a senior research engineer in the department of integrative medical biology at Umeå University in Sweden and first author of the study. She and her colleagues were interested in two types of neurons that provide the last sort of information.
Just before reaching the skin surface, the information-carrying fibers of these neurons divide into smaller branches. These branches innervate clusters of touch receptors in the ridges of the fingerprints. As a result, the so-called receptive field of each neuron, which encompasses five to 10 ridges, includes a number of highly sensitive zones, or subfields.
"We wanted to study the topography of the receptive field in detail," Jarocka said.
She and her colleagues lightly stimulated the fingertips of 12 adults with a pattern of raised dots on a plastic film wrapped around a rotating drum, and used electrodes to record when the study subjects' nerve cells fired. The plastic dots were 0.4 millimeters wide at the tip and were spaced far enough apart for each neuron to be stimulated by just one dot at a time. The researchers varied moving the dots in opposite directions and at speeds of 15, 30 and 60 millimeters per second, which are representative of how people often move their hands.
"This was not about the perception [of touch]," Jarocka noted. "We were assessing the sensitivity of neurons, but we did not measure what participants felt or did not feel."
She and her team created "maps" of the receptive fields of individual neurons, including areas where the electrical impulses were most densely packed. This allowed them to estimate the size of the particularly sensitive hotspots.
The team found that a neuron's subfields were sensitive to touch on the scale of about a half-millimeter.
"We show that the neural responses are anchored to the ridges, because even though we scanned the receptive fields multiple times, or we scanned with different speeds or in different directions, the maps were approximately the same," Jarocka said.
The findings indicate that touching a single fingerprint ridge is enough to rouse our sensory neurons. The two types of neurons Jarocka and her team observed are densely packed in the fingertip, with an area of about 2.5 square centimeters containing the overlapping receptive fields of around 500 neurons.
"When you touch an edge of, for example, an earring or a coin or some small detail, in reality it's a [number] of neurons that are stimulated and even if the detail is as tiny as 0.4 millimeters, you still get the information about it," Jarocka said.
Human fingerprints don't change over time, and even regenerate after injuries.
"Once we have this mapping between the receptors from the skin and our brain, it is maintained throughout our life, which is kind of cool because we have this very stable infrastructure for receiving information from the outer world," Jarocka said.
The study, "Human touch receptors are sensitive to spatial details on the scale of single fingerprint ridges," published March 15 in the Journal of Neuroscience, was authored by Ewa Jarocka and Roland S. Johansson, Umeå University; and J. Andrew Pruszynski, Umeå University and Western University.