What separates us from our physical environment is our skin. From the minute we wake up, we use our somatosensation to start our tangible interaction with the world. Is the water in the shower warm enough? Ouch, that razor blade hurts! These socks are really soft! All of these are sensations attributed to numerous neurons that innervate our skin. While some sensations are quick and call for immediate response (like a pinprick), others are relatively slow and grow on us (like the warmth of water or the pleasure during a massage).
The fast or slow nature of our response arises from anatomical differences between the nerve fibers conducting them. The fast-conducting fibers, Aβ afferents, are thick and myelinated, whereas the slow-conducting ones, C fibers, are thin and unmyelinated. The figure below depicts the differences between fast- and slow-conducting nerve fibers. Responses from the C fibers that perceive pleasurable touch or caress-like stroking (referred to as C-tactile afferents) are processed at the insular cortex, a region of the human brain responsible for positive feelings (Löken et al., 2009). This explains the feel-good sensation that comes with a gentle touch from a partner or foot massage!
Moving the fascinating science of “feel-good” neurons forward, a group of researchers at Caltech decided to study molecular subtypes of these C fibers in mice. They had previously identified a rare set of neurons in mice whose receptive field resembled that of human C-tactile fibers. These unmyelinated neurons were found to innervate hairy skin of rodents. The scientists hypothesized that these neurons were responsible for detecting massage-like strokes in mice (Vrontou et al., 2013).
The team designed state-of-the-art imaging techniques to monitor the activity of these neurons in real-time in a live mouse! To do this, the mice were genetically altered so that these neurons fluoresced only when they were active. Like all sensory neurons, these neurons project to the spinal cord, so the scientists were able to image these neurons using a microscope at the spinal cord. Then, they provided a stimulus to the hindpaw of the mouse – a brush stroke or a pinch. When the mouse’s hindpaw was gently petted, the special set of neurons lit up! However, these did not show any activity when the hindpaw was pinched. These results confirmed that these neurons uniquely detect only gentle, massage-like touch.
Having shown in vivo that there definitely are a population of “feel-good” neurons that exist in mice, the scientists decided to test its functionality using a behavioral test called the conditioned place preference test. They placed the mouse in a three-chamber arrangement, as shown below. The mouse entered through the center chamber and was able to differentiate between the left and right chambers because of their distinctive feel, smell, and color scheme. The researchers noted which chamber the mouse preferred. Then, the mouse was given a drug that activated the special set of neurons that detect pleasurable touch and was placed in the non-preferred chamber. This was hypothesized to make this chamber more appealing to the mouse. Indeed, the mouse soon preferred the chamber it had previous rejected! This change in place-preference behavior can be attributed to the sensation of pleasure that the mouse experienced with the firing of the “feel-good” neurons.
This study successfully characterized, on a molecular level, the subset of C-fibers that uniquely recognize massage-like strokes in mice. The behavior test that accompanied the finding suggests that gentle touch can also have positively reinforcing effects. Perhaps this explains why your Positron emission tomography (PET) involves injecting a mole... grooms himself, since these gentle stroking trigger pleasant sensations, a behavior wired to probably encourage self-hygiene.
So, the next time you’re getting a relaxing massage, remember to say a quick “thank you” to these “feel-good” neurons!
Löken L.S., Wessberg J., Morrison I., McGlone F. & Olausson H. (2009). Coding of pleasant touch by unmyelinated afferents in humans, Nature Neuroscience, 12 (5) 547-548. DOI: 10.1038/nn.2312
Vrontou S., Wong A.M., Rau K.K., Koerber H.R. & Anderson D.J. (2013). Genetic identification of C fibres that detect massage-like stroking of hairy skin in vivo, Nature, 493 (7434) 669-673. DOI:10.1038/nature11810