Figure 1.

Ascending and descending nociception pathways. A, Nociceptive signals enter the spinal cord through nociceptive neurons that have specialized sensory receptors which lie in the tissue and cell bodies which lie in the dorsal root ganglia. These neurons synapse in the dorsal horn of the spinal cord onto primary projection neurons that travel in the anterolateral fasciculus through the spinal reticular tract (to the NTS and the amygdala) and spinal thalamic tract (to the thalamus). Projections from the thalamus continue to primary sensory cortex. B, The descending pathways begin in the sensory cortex and project to the hypothalamus and amygdala. Projections from the hypothalamus and amygdala synapse in the PAG, NTS, and RVM. Projections from the RVM carried in the reticular spinal tract modulate incoming nociceptive information by synapsing onto inputs to nociceptive neurons at the level of the dorsal horn. NTS indicates nucleus of the tractus solitarius; PAG, periaqueductal gray; RVM, rostral ventral medulla.