A team of scientists has identified 16 different types of nerve cells in a new study of human touch.
An investigation that brought together experts from Linköping University and Karolinska Institutet, in Sweden, and the University of Pennsylvania, in the United States, recently revealed a new “landscape of the human sense of touch”, in the words of specialist Håkan Olausson, cited for the .
Since humans understand touch, temperature and pain through the somatic sensation system, it was plausible that a specific type of nerve cell existed for each type of sensation, such as pain, pleasant touch or cold. However, new discoveries challenge this paradigm and reveal, for the first time, that bodily sensations are much more complex than we thought.
Most of the knowledge scientists have comes from animal research, but how similar are we to mice, for example? In this study, the researchers set out to build a detailed atlas of the different types of nerve cells involved in human somatosensation and compare it with those of rats and monkeys.
After deep RNA sequencing, nerve cells that had similar gene expression profiles were grouped together as a sensory nerve cell type.
The team thus identified 16 distinct types of nerve cells in humans. However, in the future, as they analyze more cells, they are likely to discover even more distinct types.
The next step was to discover how the gene expression of nerve cells relates to their function. Basically, if a nerve cell produces a protein that detects heat, does that mean the nerve cell responds to heat?
To find out, scientists used a method that allows them to listen to nerve signaling one nerve cell at a time. With the help of this method – known as microneurografia -, scientists subjected nerve cells in the skin to temperature, touch and certain chemicals to “listen” to an individual nerve cell and discover whether it reacted and sent signals to the brain.
During the experiments, the team discovered that a type of nerve cell that responds to pleasant touch also reacts to heating and capsaicinthe substance that gives peppers their heat. This reaction is typical of pain-sensitive nerve cells, which is why the discovery surprised researchers a lot.
The same type of nerve cell also responded to coolingdespite not producing the only protein so far known to signal the perception of cold. This particular discovery suggests that there is another cold-sensing mechanism that has not yet been discovered.
“For ten years, we listened to the nerve signals from these nerve cells, but we had no idea about their molecular characteristics. In this study, we see what type of proteins they express, as well as what type of stimulation they can respond to. It’s a big step”, highlighted Håkan Olausson.
When comparing mice, monkeys and humans, the scientists discovered that many of the 16 types of nerve cells they identified in the study are similar between species.
The biggest difference lies in the very fast driving of pain-sensitive nerve cells that react to stimuli that can cause injury. Compared to a mouse, for example, humans have many more nerve cells of this type that send pain signals to the brain at high speed.
With all the advances described, it was published in Nature Neuroscience.
