Basic Mechanisms of Pain.

Pain itself occurs in the central nervous system (CNS), while the peripheral nervous system reports on tissue damage via a mechanical event, termed nociception, characterized by the discharge of high-threshold primary afferent fibers. Neuronal responses in these peripheral fibers drive spinal cord activity and result in signals ascending to the brainstem and thalamus through spinal cord tracts. It is from the patterned activity of these ascending signals, distributed throughout a cerebral neuromatrix, that the brain assembles a feeling of pain. This may or may not be directly associated with nociceptive events occurring in peripheral tissue. Nociception can occur in peripheral tissue and not be detected as pain by the central nervous system; this typically is due to the activation of powerful inhibitory systems in the spinal cord and brainstem. Conversely, pain can be perceived in the brain in the absence of any peripheral nociception; a situation that can be fostered by a powerful facilitating system in the brainstem. The mechanisms by which these processes occur are outlined in this chapter. [ABSTRACT FROM AUTHOR]