痛み感覚と皮膚の機械的変形との関係について

抄録

Pain sensations on the skin are generated by mechanical nociceptive stimuli. However, the question of what constitutes a mechanical nociceptive stimulus is still unclear. Therefore, in this study, mechanical factors which cause cutaneous mechanical nociception were investigated by computer simulation of an elastic model of the skin. In order to compare them with the results of the simulation, psycho-physical experiments on awake human subjects were performed. Indentation threshold which is indentation amplitude of the stimulus needle from the skin surface to the depth where the pin prick is first felt, were measured as a parameter of stimulus for various pins and cylindrical needles. The apparatus which measured indentation thresholds was composed of a touch sensor which detects the initial touch of the needle to the skin, a noncontacting type displacement measurement device which measures the amplitude of displacement of the needle, and the button by which the subject indicates the threshold of pain sensation. The results showed that sharp pins had shallower indentation thresholds and cylinders with small radii had shallower thresholds. The elastic model of the skin was based on the three-dimensional elastic theory that assumes that the skin is a homogeneous, isotropic, and semiinfinite elastic medium. The nociceptors, the receptors of pain sensation, were supposed to be distributed pseudo randomly at a certain depth in the skin. Each nociceptor was assumed to respond to a mechanical factor such as vertical strain, horizontal stress, and so on. The indentation threshold was defined as the smallest indentation amplitude of the needle on which the mechanical factor at one of nociceptors reached the predetermined threshold. The value of the threshold was determined to give a fit between the result of simulation and experimental observation for one of the needles. Setting the threshold at this value, indentation thresholds of other needles were examined by computer simulation, and a comparison with the experimental results was made. In all, twenty kinds of mechanical factors, stresses, strains, principal stresses and strains, and differentials of strains were included in the hypothesis for adequate stimulus to the nociceptor. Computer simulation showed that the vertical differential of vertical compressive strain and the vertical differential of averaged horizontal tensile strain explained the experiment most effectively. From this result these two differentials of strain are considered to be the candidates for the mechanical factor of nociception.