PAIN MECHANISMS - A REVIEW: II. Afferent pain pathways

This paper reviews some major advances in our understanding of the organization of afferent pain pathways, and relates these and other findings to the limited success rate achieved by various surgical interventions used in the treatment of chronic intractable pain. First-order pain afferents, many of which use the transmitter substance P, may enter the spinal cord via both the dorsal and ventral roots. After terminating superficially in the dorsal horn, information may apparently ascend to higher centres via numerous pathways, some of which are contralateral some ipsilateral. The preferred route to the primary somatosensory cortex appears to be the contralateral spinothalamic tract, but alternate pathways may be recruited following anterolateral cordotomy or thalamotomy. In addition to divergence of the central pain pathways and the presence of ventral root afferents, other factors considered that may contribute to surgical failure include denervation supersensitivity, the occurrence of prolonged spontaneous injury discharge, and divergence of the peripheral branches of nociceptive fibres.