[Functional neuroanatomy for pain stimuli. Reception, transmission, and processing]

Abstract

This short overview presents some of the current neuroanatomical knowledge concerning pathways and nuclei mediating pain sensations. The axonal membrane of the nociceptor is equipped with a multitude of receptor molecules that specifically bind pain-producing and sensitizing substances. Recently, adenosine triphosphate and protons have attracted much interest. The different nociceptor types are probably characterized by different sets of receptor molecules in the membrane of the nociceptive ending. Nociceptive cells are present in the superficial laminae and the neck of the dorsal horn. The cells in the former region include nociceptive-specific ones that receive input from nociceptors exclusively, whereas in the neck of the dorsal horn a convergent input from nociceptive and non-nociceptive afferent fibers prevails. At the spinal level, neuroplastic sensitizing processes take place that are assumed to underlie the allodynia and hyperalgesia of pain patients. In addition to the lateral spinothalamic tract, the spinoreticular and spinomesencephalic tracts are involved in pain sensations. The medial and lateral thalamus contains several nociceptive nuclei, the medial ones mediating the affective-emotional component of pain, the lateral ones the sensory-discriminative component. In contrast to other sensory modalities, the modality of pain does not have a specific cortical center. The cortical areas that are activated by painful stimuli are distributed over large parts of the cortex surface. During chronic painful conditions, at all levels massive neuroplastic changes take place that lead to rewiring of connections and structural alterations in the nuclei of the nociceptive pathways. In chronic pain patients the neuroanatomy of pain probably differs from that of healthy people.