Why don’t we touch things with our head?

by Selina Zhao, Contributing Writer

Have you ever wondered why babies unintentionally touch dangerous things with their hands? Or why people always reach out with their hands first? This is because our hands are our first point of contact with the surrounding world! But how did humans evolve to favour sensitivity in their hands, instead of their legs, head or chest? The answer lies in the glabrous skin that constitutes our palms and the soles of our feet. 

Glabrous skins are sensitive, hairless skins that mammals often use to navigate the world, and they come in different distributions depending on the animal. For example, pigs have glabrous skin on their snouts and humans have it on their hands and soles. What makes glabrous skin so interesting is that our brains devote a disproportionate amount of space to it. At first, scientists believed that this was because glabrous skin contains a higher density of mechanoreceptor neurons. (1) These sensory receptors react to external stimuli such as touch and re-transmit them as intercellular signals. (3) A higher density of mechanoreceptors means more signals that need to be processed, and more brain activation to do the processing. However, a recent study shows that there must be additional factors that contribute to this overrepresentation of glabrous skin.

In a 2021 study, researchers at the Harvard Medical School discovered this discrepancy began to develop during the first 14 postnatal days. Although the study was done on mice, the findings may apply to other mammals. Researchers first examined the representation of glabrous skin regions in mouse brains – specifically in the primary sensory cortex, or “S1 cortex” – during the first 14 postnatal days (P14). The procedure involved labelling the hindlimb and forelimb S1 cortices, where mechanoreceptors innervate signals to the brainstem. Then, scientists captured brain activity while the mice were stroked at both glabrous and non-glabrous regions of their forelimbs and hindlimbs, allowing scientists to gauge how much of the brain was used in each instance. As expected, brain activity in the hindlimb S1 cortex correlated with the density of mechanoreceptors activated by the strokes, in either skin region. However, activity in the forelimb S1 cortex was greater than expected for the glabrous skin regions. Researchers observed this bias at an even greater scale in adult mice, where the representation of glabrous skin in the brain is approximately three times greater than predicted given the density of mechanoreceptors. This disproportionate representation seemingly developed and enhanced as the mice entered adulthood, suggesting that the overrepresentation in the brain is not solely controlled by the density of mechanoreceptors. (1)

Puzzled by this notion, researchers hypothesized that the signals must have been enlarged during their transmission from sensory neurons to complex brain neurons to account for such drastic overrepresentation, when the receptor density remained constant. (2) To find a possible explanation, researchers decided to dive deeper and study the brainstem, a region that transmits signals received from sensory neurons to more complex brain neurons. They analyzed the relationship between mechanoreceptor and their innervation sites (where neurons are being excited) in the brainstem. To do so, researchers studied mechanoreceptor synapses in the brainstem by labelling presynaptic terminals with fluorescent proteins that can be observed under microscopy. Then, they repeated the stroking method to observe mechanoreceptor excitation. As hypothesized, results showed that glabrous-skin-innervating neurons form more and stronger synapses in the brainstem than hairy-skin innervating neurons. These enhanced synapses allow signals to become stronger and more specific, hence the high sensitivity in the glabrous skin regions. Moreover, greater sensitivity in glabrous skins was observed in adult mice than in P14 mice, suggesting that these synaptic refinements develop during the postnatal period to allow greater sensitivity. (1)

With the findings of their study, researchers proposed a mechanism where the amplification of sensitivity in glabrous skin regions is affected by both the density of mechanoreceptors and the enhanced synapses of these mechanoreceptors in the brainstem. With this knowledge in mind, it is easier to understand why babies touch everything with their hands: they are using outside stimuli to boost their mechanoreceptor sensitivity! So next time, don’t be too worried when a baby touches a cactus or the teeth of a dog: it’s their way of exploring the world and developing their senses.

Edited by Laura Reumont

References:

  1. Lehnert, B. P., Santiago, C., Huey, E. L., Emanuel, A. J., Renauld, S., Africawala, N., Alkislar, I., Zheng, Y., Bai, L., Koutsioumpa, C., Hong, J. T., Magee, A. R., Harvey, C. D., & Ginty, D. D. (2021). Mechanoreceptor synapses in the brainstem shape the central representation of touch. Cell, 184(22). https://doi.org/10.1016/j.cell.2021.09.023 
  2. Caruso, C. (2021, October 11). Unraveling the mystery of touch. Unraveling the Mystery of Touch | Harvard Medical School. Retrieved November 24, 2021, from https://hms.harvard.edu/news/unraveling-mystery-touch. 
  3. French, A. S., & Torkkeli, P. H. (2009). Mechanoreceptors. Encyclopedia of Neuroscience, 689–695. https://doi.org/10.1016/b978-008045046-9.01921-5

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