The sense of touch is composed of feedback from several different sensors in the skin, which is linked and evaluated by the brain and is available to us as tactile perception. This can involve a perception of being passively touched or being actively touched. In a broader sense, the sensation of pain and temperature also belong to tactile perception and thus to the sense of touch. The study of the sense of touch and all related topics can be classified under the term haptics. However, some authors use the term haptics only for active touching and the term tactile only in the sense of being touched.
What is the sense of touch?
The sense of touch is composed of feedback from several different sensors in the skin, which is linked and evaluated by the brain and is available to us as tactile perception. The sense of touch involves all tactile perceptions and is composed of feedback from several different mechanoreceptors in the skin and mucous membranes. Among the mechanoreceptors, which respond mainly to pressure and vibration, there are slow adapting and fast adapting sensors. The slow-adapting receptors have the advantage that they can, for example, provide feedback on the sensation of pressure over a longer period of time – as long as the mechanical stimulus continues – while the fast-adapting receptors can only provide feedback at the beginning and at the end of a mechanical load, i.e. always only when the mechanical stimulus changes. In a broader sense, temperature and pain sensation are also part of tactile perception, and so are sensors such as nociceptors for pain sensation and thermoreceptors for temperature sensation. Most mechanoreceptors are equipped with special sensory heads that, with the exception of the Vater-Pacini tactile corpuscles, project into the middle layer of the skin, the dermis or corium. Thermoreceptors and nociceptors do not have special sensor heads, but only branched nerve endings in the dermis. The distribution of the individual sensors on the skin varies greatly. The greatest density of receptors is reached on the fingertips (finger berries), the tip of the tongue, the lips, and under the soles of the feet.
Function and task
The sense of touch is very important for “sensing” the immediate environment. The most important primary task is to warn of direct danger and risk of injury that may exist from thorns and spines or from dangerously hot or cold temperatures. Another important task is to obtain information about the nature of objects. In the interaction of the different mechanoreceptors, a realistic picture of the immediate environment is created. The Vater-Pacinian tactile corpuscles are counted among the fast adapting sensors. They transmit large-scale touch, pressure and vibrations only at the beginning and then only again after a change in touch or pressure, whereas the so-called Merkel cells, for example, work on a smaller scale but emit sustained signals. They are counted among the slowly adapting sensors and are therefore able to continuously report the perceived touch or pressure with a certain repetition frequency as long as the pressure or touch situation does not change. To some extent, mechanoreceptors also serve supportive proprioceptive purposes, i.e., determining the body’s position in space. For example, receptors in the soles of the feet support upright standing by immediately reporting to the brain pressure point migrations in the foot due to shifts in the center of gravity as a result of swaying. The brain can perform corrective movements with unconscious counter-reactions in the form of targeted muscle tension, so that falling over is avoided. Beyond the purely technical component of exploring the nature of certain objects or averting danger, the sense of touch also has an often underestimated function in social interaction. Touching or feeling inanimate objects can already have an influence on the current mood. Taking a “hand-pleasing” object in one’s hand has a positive effect, even though the object does not enter into any direct interaction with the person touching it. The psyche can react all the more strongly when touching another person. In order to take into account the need for mutual touch on the one hand and possible misinterpretations on the other, practically all societies have developed ritual body contacts that are accepted by members of the society.This also includes shaking hands during a personal greeting. The entire communication potential hidden behind interactive physical touch is only revealed in friendly and exclusive-intimate touch. Touch stimuli through caressing can be directed by the brain to the limbic system, which stimulates the synthesis of the “happiness hormone” oxytocin in the hypothalamus, and the concentration of stress hormones such as cortisol decreases. At the same time, an increase in social bonding occurs.
Diseases and ailments
Diseases that directly affect a few of the thousands of receptors of the sense of touch and result in regionally limited impairments or failures of the sense of touch are rare. Far more common are diseases and impairments that result from problems in neuronal transmission or processing of stimuli in specific areas of the brain. Symptoms and complaints that may occur are primarily an impairment of tactile sensations up to a feeling of numbness. Perceptual disturbances may also develop or a tingling sensation or “formication” may be perceived. There are numerous diseases which, among other things, carry disorders of the sense of touch in their baggage. These are almost always secondary damages, which cause an impairment of the affected nerves due to a reduced oxygen supply. In some cases, the reduced supply can also be triggered by mechanical problems such as herniated discs or narrowing of certain bone grooves for the nerves (e.g. carpal tunnel syndrome). Because skin sensors are particularly sensitive to impairments in nerve conduction, symptoms can also serve as early indicators of potentially developing polyneuritis, systemic damage to multiple nerves.
