Our somatosensory nervous system enables us to consciously feel multiple inputs from our environment in a highly rich manner. The heat of a cup of coffee, the cooling refreshment of a cold drink, the softness of a blanket, or the prickliness of grass are all detected by neurons that have terminals embedded in our skin. This information then undergoes processing by the brain for sensory perception to occur. The cerebral cortex is the key area where such somatosensory processing occurs, but exactly where and how is still unclear. This layered structure, that is characterized by twisting gyri and folds in humans, is organized into distinct regions for different functional purposes and interestingly there are two spatially distinct regions that encode somatosensory information, the primary and secondary somatosensory cortex (S1 & S2). It was always assumed that these two regions work closely together to produce what we feel, from innocuous to noxious stimulation of our body. However, recent work in the mouse has uncovered that S1 only responds to touch and cool temperatures and lacks the ability to detect heat. This suggested to us that there may exist a functional division for different sensory inputs of different intensities.

Artistic interpretation depicting how/the idea that different somatosensory modalities such as touch and temperature are processed by different regions of the cortex. Understanding perception requires figuring out the puzzle of which region is important for a given modality.

To test this, we inhibited the other cortical region, S2, to determine if this alters different aspects of sensation than S1. Testing the reaction to noxious stimuli is relatively straightforward in mice. We can apply different intensities of stimulation to a paw and determine when they withdraw as a metric of pain sensitivity. Surprisingly, when we inhibited S2, we found that mice had drastic increases in their sensitivity to tactile and heat stimuli, but no changes in their reactiveness to cold. This is the opposite with what we and others have found in S1, where inhibition only produces altered tactile and cold sensations. This suggests that the common model, that S1 and S2 work together to generate defined sensations, needs to be revised and instead these regions process different types and intensities of somatosensory information. How this information ultimately gets aggregated to produce an integrated perception (for example the feeling elicited by a cold, rough boulder when climbing) is an exciting direction we are actively pursuing, but it’s clear that different sensory modalities are distributed differently across our cortex and require distinct neural computations by defined circuits for accurate perception.

Daniel G. Taub, Ph.D. is a postdoctoral fellow in Clifford J. Woolf’s laboratory in the F.M. Kirby Neurobiology Center at Boston Children’s Hospital and Harvard Medical School.

Clifford J. Woolf, M.B., B.Ch, Ph.D. is Director of the F.M. Kirby Neurobiology Center at Boston Children’s Hospital and Professor of Neurobiology and Neurology at Harvard Medical School.

Learn more in the original research article:
The secondary somatosensory cortex gates mechanical and heat sensitivity.
Taub DG, Jiang Q, Pietrafesa F, Su J, Carroll A, Greene C, Blanchard MR, Jain A, El-Rifai M, Callen A, Yager K, Chung C, He Z, Chen C, Woolf CJ.  Nat Commun. 2024 Feb 12;15(1):1289. doi: 10.1038/s41467-024-45729-7.