The ascending pain pathway consists of many complex neural structures, including specialized nociceptive receptors and peripheral nociceptive neurons in the dorsal root ganglion, spinal dorsal horn nociceptive and wide-dynamic range neurons, that play a role in the initial central neural processing for nociceptive information, several divergent relay stations (the thalamic nuclei and several nuclei in the brainstem) that properly mediate a variety of reflex reactions to painful stimuli, and are variously distributed among the thalamo-cortical and limbic circuits for sensory perception, learning, memory and other cognitive activity, and emotional responses. In addition to the ascending pain pathway, there are descending pathways that modulate pain by inhibiting nociceptive transmission. The purpose of the present review is to summarize the current knowledge of the structure of the pain system, including both the ascending and the descending components.
