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, he was unable to move or feel anything from the neck down. He was eventually 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. However, he taught himself to use sight to judge the distance of his limbs from other objects and eventually 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 it is an important part of the somatosensory system, a network that oversees all the sensations arising from the surface and interior of our bodies. This includes touch, pain, temperature, and our awareness of our bodies in space (known as proprioception). Millions of tiny receptor cells embedded in our skin, muscles, tendons, and organs process these sensations. If something goes wrong with this system, the effects can be dramatic.
Mechanoreceptors sense mechanical deformation of the skin, thermoreceptors respond to temperature changes, nociceptors sense pain, and proprioceptors sit deep in our muscles and tendons, continually detecting and relaying information about the position of our body. Electrical signals from these receptors travel through fibers of varying thickness to the brain, with thicker fibers producing faster signals. For example, thicker nociceptors trigger sharp, intense pain, while thin, unmyelinated nociceptors are responsible for the dull, aching pain that follows.
Damage to the somatosensory system can manifest in a wide variety of ways, including deafferentation. 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. Other somatosensory disorders can involve damage to a specific brain area or a section of skin, resulting in the loss of certain sensations in particular locations. The impact of this loss can be significant, as it can make it difficult to gauge how much strength to use in a situation and can lead to touch starvation, characterized by anxiety, depression, high blood pressure, and even a weakened immune system. However, many individuals have found innovative ways to adapt. It is irrefutable that these imperceptible feelings are essential to how we maneuver our surroundings, even though it may be hard to pinpoint them.