A multi-study, involving researchers from Karolinska Institute, Sweden’s Linköping University, and The University of Pennsylvania has been published in the journal Nature neuroscience.
There are few similarities and differences between humans, mice, and macaques. “Thus, our study only offers a bird’s eye view of the human sense of touch. As a next step, we want to make portraits of the different types of nerve cells we have identified,” which are as follows, according to Håkan Olausson, a professor at Linköping University. Touching, perceiving extent of heat and cold and pain are all transmitted by the somatic sensation system.
In the study, researchers from the University of Pennsylvania under the supervision of Assistant Professor Wenqin Luo, went further and conducted comprehensive tests of the genes used by a single nerve cell, so-called deep RNA sequencing. Sensory nerve cells that showed similar performance of the genes in their pathways were then pooled together into one type of sensory nerve cell. Like this, they determined that there are 16 distinct types of nerve cells in human organisms. The researchers have only scrutinized a limited number of cells and as they study additional cells, they will probably define even more subtypes of sensory nerve cells.
Investigators from the
Department of Neuroscience and Physiology at Linköping University, led by Saad Nagi and Håkan Olausson, used a method called microneurography to record nerve cell activity one by one. In this technique, the researchers apply stimuli such as temperature, touch, or certain chemicals, to skin nerve cells in awake participants allowing them to ‘eavesdrop’ on individual nerve cells and determine if specific cells are firing and sending signals to the brain.
During these experiments some discoveries were made that could not have been made otherwise, especially given the new ideas about the cellular equipment of the different types of nerve cells the researchers were given by the project. One of the findings in this area is related to a specific type of nerve known as touch pleasure nerve cells.
Much to the surprise of the researchers, this cell type also responds to heat and capsaicin, the chemical responsible for giving chili its burning sensation. The heat receptors’ reaction to capsaicin is expected since the cells are designed to feel pain, then the synchronisation was a shock to the researchers since touch receptors were acting to such stimulation.
The authors imagine that these nerve cells create an entire sensory route in the brain for pleasurable feelings. “It has been ten years since we started to record the nerve cell activity, nerve signals and until now we did not know their molecular profile. In this study, we learn what kind of proteins these nerve cells operate, what sort of signal they can recognize and the sort of interaction that is feasible, now we can connect it. It is quite a leap,” argues Olausson.
The largest disparity the researchers identified was in very fast firing pain-sensing nerve cells which respond to a stimulation that could result in an injury. These were first identified in humans in 2019 by the same group at Linköping University using microneurography.
It could be that the fact of signaling pain at higher velocity in men as compared to mice is just a matter of size. A mouse doesn’t need as fast communication to its nerves as species in the next category do. But in humans, the distances are greater, and the signals need to be sent to the brain more rapidly; otherwise, you will be injured before you even react and withdraw,” notes Olausson.
Reference:Yu H, Nagi SS, Usoskin D, et al. Leveraging deep single-soma RNA sequencing to explore the neural basis of human somatosensation. Nat Neurosci.2024. doi: 10.1038/s41593-024-01794-1
