Primitivism and plasticity of pain--implication of polymodal receptors.

Bio-warning and defense mechanisms play the most fundamental roles in living organisms. From an evolutionary point of view, nociceptive systems are very primitive and are richly provided with humoral signaling mechanisms of aboriginal humoral defense systems, as reflected in the primitive nature of the polymodal receptor, a poorly differentiated sensory receptor signaling nociceptive information. Recent advances in studies on pain have made it possible to explain neural mechanisms of pain systems under physiological conditions and reveal that there is a large gap between physiological and pathological pains. Protracted nociceptive inputs under pathological conditions induce plastic, either functional or structural, alterations in the nociceptive pathways. These plastic changes lead to crosstalk among the neural networks, including circuits related to motor, autonomic, or psychological functions. These plastic changes, once established, persist even after the original pain sources disappear in a memory-like fashion. Thus, it is revealed that chronic pain cannot be treated by blocking pain pathways, which is effective against acute pain, but require treatment from a multidisciplinary perspective.