In 1971, Ian Waterman suddenly collapsed from a severe case of what seemed to be gastric flu. His illness passed after a few days, but a stranger set of symptoms lingered. Although his muscles and joints remained healthy, Waterman was unable to move.
In fact, he was unable to feel anything from the neck down. Eventually, he was diagnosed with a rare and extreme form of deafferentation, a neurological condition in which certain signals from the nervous system are interrupted or impaired. Without his body's constant feedback on how his limbs were moving, Waterman was unable to sit up, stand, or walk.
But over time, he taught himself to use sight to judge the distance of his limbs from other objects. And eventually he regained complete control of his body— so long as he could see it. We often don’t think of touch as being a vital part of movement.
But touch is just one part of the somatosensory system, a network that oversees all the sensations arising from the surface and interior of our bodies. Touch, pain, temperature, and our awareness of our bodies in space— also known as proprioception— are regulated by this system. And when something goes wrong, the effects can be dramatic.
All these sensations are processed by millions of tiny receptor cells embedded in our skin, muscles, tendons, and organs. Every square centimeter of our skin is packed with hundreds of these cells, and their shape, size, and depth determine what kind of stimuli they respond to. Mechanoreceptors sense mechanical deformation of the skin.
This could be triggered by low or high frequency vibrations, a stretch, or simply light, static pressure. Thermoreceptors respond to temperature changes, while nociceptors sense pain. And propriocepters sit deep in your muscles and tendons, continually detecting and relaying information about the position of your body.
Your brain then combines this information with other sensory data to move through space without needing to see your limbs. All of these receptors work by sending electrical signals to the brain through fibers they’re attached to. And the speed of those signals varies with the fiber’s thickness.
For example, some nociceptors are attached to fibers with slightly more conductive, fatty myelin than others. So when you get hurt, the electrical impulses from thicker nociceptors trigger sharp, intense pain, while thin, unmyelinated nociceptors are responsible for the dull, aching pain that follows. And since the fibers carrying tactile information are much thicker than those carrying nociceptive signals, rubbing an injury can produce temporary relief from the pain These receptors generate a constant flood of signals that travel through the nervous system to the brain.
But if this process is disrupted— either by damage to the skin, the nerves, or the brain— the network breaks down. And since it underpins so many bodily functions, damage to the somatosensory system can manifest in a wide variety of ways. In Waterman’s case, an autoimmune reaction attacked a large swath of his nervous system, leaving him with no tactile or proprioceptive sensations from the neck down.
But deafferentation is just one of many somatosensory disorders. Individuals can receive damage to a specific brain area or a section of skin, resulting in the loss of certain sensations in particular locations. And the impact of this loss can be significant.
Losing tactile sensations makes it difficult to gauge how much strength to use in a situation. Without the warning signals provided by thermal and pain stimuli, we don’t react when our bodies are damaged. And, being deprived of social touch can cause a condition known as touch starvation, characterized by anxiety, depression, high blood pressure, and even a weakened immune system.
Many individuals who face these realities have found innovative ways to adapt. But it’s undeniable that all these invisible sensations play a vital role in how we navigate the world— even if they can be difficult to put your finger on.
