The role of sensory neurons is to send and receive messages between the brain and other parts of the body. These neurons are involved in different responses, including mechanical pain. Mechanical pain is pain that results from an action or stimuli like certain movements or an injury or impact.
Pain is difficult to treat because there are different kinds and how individual neurons process pain isn't that well understood. A new study from the National Institutes of Health showed how a new group of neurons works in processing pain that comes from stimuli like pinching, heat or pulling hair. If more is known about pain and what responses the sensory neurons produce, the treatments for it can be more efficient. With opioid addiction at an all time high, finding better ways to treat pain is more important than ever.
Alexander Chesler, Ph.D., lead author of the study and principal investigator with the National Center for Complementary and Integrative Health's (NCCIH) Division of Intramural Research (DIR), explained, "Scientists know that distinct types of neurons detect different types of sensations, such as touch, heat, cold, pain, pressure, and vibration. But they know more about neurons involved with temperature and touch than those underlying mechanical pain, like anatomical pain related to specific postures or activities." The new study is published in the journal Neuron.
Using mice, the team in Chesler's lab conducted functional imaging studies (fMRI scans) which showed the activity of specific neurons in response to different stimuli. The neurons they focused on were those that expressed a gene called Calca. This gene and its expression in sensory neurons are often studied when scientists investigate how the brain processes pain.
The team applied different kinds of mechanical stimuli to the mice. Some were gentle such as a puff of air or stroking and brushing the sensitive whiskers on the cheeks of mice, and some were "high threshold." High threshold mechanical stimuli involves pinching the skin, pulling hair or putting heat or pressure on an area. Neurons that express calca do not respond to gentle stimuli but are active in high threshold stimuli. The neurons they were looking at fell into two categories. One was expected, it was a pain fiber known as a "polymodal nociceptor." What they did not expect to find was a group of neurons that lit up in response to hair pulling as well as heat and pinching. These neurons and their response had not been seen in previous pain research. They are called HTMRs (high threshold mechanoreceptors), but these were just a bit different. The team referred to the fibers of the neurons that wrapped around the follicle when a hair was pulled as "circ-HTMRs" for the circular way they wrapped around the base of the hair.
Using controlled amounts of light and optogenetic techniques, the team was able to see how even tugging on a single hair would produce an electrical response in a large area of these circ-HTMR neurons. They signaled across a wide field of receptors and fired faster than standard pain fibers. NCCIH Director Josephine Briggs, M.D. stated "These findings add insight into how the somatosensory system encodes pain. Learning more about the distinctive features of circ-HTMRs could contribute to rapid, accurate localization of brain regions activated in mechanical pain, and ultimately to the rational design of new approaches to pain therapy."
Check out the video for more information on this newly discovered pain mechanism.