Circulatory, digestive, endocrine, nervous, respiratory systems: they all happen on the inside, behind the curtain of the skin. The skin itself, though? Sure, it has some follicles, hair, pores, but other than that, it is pretty plain as far as anatomy goes.
Not according to David Linden, a neuroscientist at Johns Hopkins University. “Our entire skin is a sensing, guessing, logic-seeking organ of perception, a blanket with a brain in every micro-inch.” Our brain (the big one, in the cranium) is in constant contact with the skin to determine whether something is safe, harmful, fine, coarse, sharp, smooth. The skin plays a vital role in our sense of touch, perhaps the most unsung of all the senses.
A 2-D cortical sensory homunculus created by OpenStax College - Anatomy & Physiology, Connexions Website- https://openstax.org/books/anatomy-and-physiology/pages/1-introduction
Let’s take an example: have you ever felt your thigh vibrate, but upon removal of phone from pocket, you have no new notifications? It’s called phantom cell phone, and yes, it’s a real thing. Measuring no more than one millimeter, the Pacinian corpuscle detects changes in pressure and vibrations on the skin, sometimes up to a few centimeters away. But sometimes the nerve ending gets fooled and sends the brain a faulty message—the end result of which is your confusion. (Next time you are tricked, just blame it on Filippo Pancini, who discovered the corpuscle in 1831.)
Aside from the corpuscles, the skin possesses specific touch systems, each with its own team of neurons. Xinzhong Dong, another neuroscientist at Johns Hopkins, recently found a group of cells that serve specifically as itch receptors. Dong injected an “itchy chemical” into two groups of mice: one normal group, and the other whose gene for the receptor was “shut off.” The latter group pawed mildly at their face, in slight pain. The first group “scratched like crazy” with their hind legs.
The somatosensory cortex is where receptors and nerves go to tell your brain about feeling and texture. It turns out that the cortex is a good organizer, as it separates hand neurons from tongue neurons, arm neurons from hip neurons. But space is in the cortex is not allocated equally: tongue, hands, and finger neurons occupy the most. Think about it—those three body parts are constantly touching, sensing, and sending data from the outside world to the brain.
Perhaps this may be another reason why Igor Spetic, the amputee engineer of our first post, continued to suffer pain despite his right hand no longer existing. The hand was gone, but the pathways to the cortex were not. And there was still a space reserved (a large one, at that!) for hand neurons in Spetic’s brain.
V.S. Ramachandran, in the early 2000s, demonstrated that such neurons can get desperate. Deprived of sensory input, the hand neurons start responding to signals from other areas of the brain… “sensory inputs to the face might actually start ‘invading’ the space around it and activating the neurons in the hand area of the cortex as well.” For an amputee, that means that pouring water on one’s face can feel like giving one’s phantom limb a bath.
We have only scratched the surface of the fascinating anatomy embedded in the skin, our very own smart blanket. A third post will touch on the antennae of the human body: fingers!
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