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Discovery

A single finger nerve cell distinguishes between touch stimuli better than the whole person

19.09.2014

In a nutshell: One explanation for this unexpected finding is that signals from single nerve cells are corrupted as they travel from cell to cell to the brain. An alternative explanation is that as the brain pools information from many nerve cells to make sense of the real world, sensitivity to information carried by any single cell is reduced.

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A single finger nerve cell distinguishes between touch stimuli better than the whole person

The Big Picture:

“We are dealing with a very surprising scenario—we didn’t expect that a single cell would perform better than the brain,” says CIBF chief investigator Ehsan Arabzadeh at the John Curtin School of Medicine in Canberra.

Arabzadeh is referring to a study by his team which shows that a single nerve cell in the fingertip can discriminate between two consecutive stimuli of differing strength better than the brain, which gets information from multiple nerve cells experiencing the same stimuli.

In their study, the researchers inserted very fine electrodes through the skin of volunteers into the median nerve at the wrist. The electrodes were fine enough to contact just one of the many nerve cells that send tactile information from fingers to the brain. A quick test established the “receptive field”—the exact area of skin which, when touched, triggered an electrical impulse in that cell. It is about the size of a pencil tip.

Next, the researchers used a device to stimulate the receptive field with pulsed vibrations.

The volunteers were asked to say which of a sequential pair of vibrations was stronger—this tested psychophysical performance, based on the brain interpreting the signals from tens to hundreds of nerve cells. It was compared to the neuronal response — the electrical activity generated by one nerve cell in response to each stimulus. The magnitude of this electrical activity depends on the strength of the stimulus.

Unexpectedly, single nerve cells were better able to discriminate between two subtly different stimuli than the brain.

There are two possible explanations. One is that the signal gets corrupted as it is relayed from the fingertip to the brain via a series of nerve cells. The second possibility is that as the brain pools signals from all nerves from the fingertip, signals from individual cells, which may vary in their response to stimulus, are lost.

In the end, it is probably a combination of both, says Arabzadeh.

Next steps:
Brain researchers are intrigued and will continue to investigate how the brain combines and interprets information from its outlying nerve cells.


Reference:
Arabzadeh, E., Clifford, C. W., Harris, J. A., Mahns, D. A., Macefield, V. G., and Birznieks, I. (2014). Single tactile afferents outperform human subjects in a vibrotactile intensity discrimination task. Journal of neurophysiology, 112(10), 2382-2387.


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