Foreign objects, toxins, air, fluids, and living organisms can seldom penetrate the boundary of the human body’s watertight seal. Even when unconscious, sensory systems are seldom completely offline; instead, they remain poised to capture any vital changes around us, even those which are seemingly minor. But what else can be learned by our highly sensitive skin receptors?
The sense of touch is the composite of three sensory qualities — temperature, pain, and pressure, which can be experienced individually or in various combinations. Characteristically, the broad swatches of the human skin are classified as either hairy or glabrous (hairless). These categories are best represented by the palms and backs of our hands. Together, they put us in instantaneous contact with the outer world.
When this skin is pressed, poked, vibrated, or stroked, there are specialized corpuscles that respond to the four stages of perception: detection, amplification, discrimination (among several stimuli), and adaptation (the reduction in response to a stimulus — for example, we are only consciously aware of our clothes during the moments we put them on). Over five million touch receptors for experiencing light or heavy pressure, warmth or coldness, pain, etc., cover the body, sending essential information to the brain via a massive sensory expressway. However, the distribution of receptor cells is undemocratically concentrated into those parts of the body that are most involved in direct tactile perception, which partially explains why hands-on learning is so incredibly effective as an educational tool. Wherever the hands go, that is where the brain focuses its attention. For decades, these receptor fields were thought to be fixed and unchanging. Instead, cortical representations and sensory projections are rapidly reorganized following injury or surgical alteration to specific areas of the body.
When it comes to sensory acuity, the hand is to the human sense of touch what the fovea is to our sense of vision. Respectively, both house exceptionally sensitive receptive fields that quickly send the brain a wealth of sensory information with optimum levels of detail and discrimination. The corresponding brain areas for touch and sight dedicate a substantial amount of cortical real estate to each of these senses.
As the hands and fingers move across an object, receptor cells respond to the infinitesimal indentations created on the surface of the skin, giving us priceless data disclosing the shape, texture, hardness, and form of that particular object. Interestingly, reading braille does not require abnormally sensitive fingers. On an otherwise completely flat surface, the human fingertip can detect a raised dot 0.04 mm wide and measuring only 0.006 mm high. A typical braille dot is nearly 170 times that height, rendering it an easy read for our fingers.
There are two main layers of the human skin, each of which performs distinctly different functions. The wafer-thin 0.05 to 1.5 mm epidermis, which varies in thickness according to the particular location on the body, is the outermost visible layer of our skin. Its greatest measurement of 1.5 mm is found on the soles of the feet and the palms of the hands. The 0.3 mm to 3.0 mm–thick dermis is the larger, inner-layered counterpart. Comparable to a well-disguised basement-level speakeasy of the Prohibition era, very little in the world of tactile perception transpires on the surface layer. The “happening place” is the lively second layer, where nearly all of the sensory action occurs. Processing in the dermis is quite active, not passive.
The ability to interpret a sensation to our skin rests solely on the number of densely packed mechanoreceptors residing in a given area. Sensitivity to pressure varies considerably throughout the vast exterior of the body. Regions that are highly sensitive correlate directly with a massive number of receptors compressed into a small geographical area. Over 100 mechanoreceptors per cubic centimeter are found in the face and fingertips. By contrast, only 10 to 15 detectors are found beneath the same measure of skin in the back, torso, thigh, or calf. More importantly, these sensory disparities are reflected in the amount of cortical real estate taken up by neurons representing each of these areas in the somatosensory cortex.
The largest receptors are the onion-shaped Pacinian corpuscles, which encode vibration and changes in pressure indicated by skin indentations. The tiny, egg-shaped Meissner’s corpuscles, about one-tenth the size of their Pacinian cousins, are located in the dermis on the ridges of hairless skin — the soles of our feet and the raised portions of our fingertips. Over 9,000 receptors are densely packed into each square inch, where they encode the slightest stimulation and the smallest fluctuation to the skin. These two types of receptors respond instantly if activated, but adapt quickly to that initial change and cease to fire if the stimulus remains continuous.
Hair connects to touch receptors and plays a central role in information gathering. When hair in a thin-skinned area is slightly bent or pulled, we are alerted by sensory receptors lodged at the base of each individual hair. An external object may be closing in on us, possibly in an attack mode. The term hair-trigger response is not a metaphor; rather, it serves as a physiological signal designed to assure our safety and survival.
Brain-Sight
Learning is conventionally described as a sophisticated cognitive responsibility which involves the brain, not the skin. Though most noted for its function as the body’s sentry, the skin also serves the process of learning. The multilayered, sensory-rich membrane evolved over the millennia not only to examine objects, maintain our body temperature and capture valuable data concerning environmental dangers and opportunities, but it also assists in giving meaning to experiences, by means of neural assimilations.
Exteroception (perceiving the outside world) is achieved by interpreting incoming sensory information, including tactile sensations derived by identifying such features as contour, size, pattern, texture, etc., which gives an object perceptual constancy. An object’s full identity is extracted from our memory. Through tactile sensory input, we can perceive the qualia (Latin for “aspects”) of an object. It is the qualia that we use to explain the qualitative or subjective features in objects, events, experiences, etc., which enrich our visualizations, allowing us to “get the picture.” Combined with eyesight, touch informs us of the what and the where of objects within our sight and reach.
