Fig. 1. Schematic diagram of the major neurochemicals involved in somatosensory transmission and processing in the spinal dorsal horn. The figure is organized with the pain signaling output neurons of the dorsal horn, the dorsal horn projection neurons, as the central cellular component. These cells are the source of all inputs for pain and temperature to the rostral central nervous system structures, such as the thalamus, brain stem, and hypothalamus, that in turn influence cortical and limbic brain structures necessary for conscious perception and appreciation of pain. The primary afferents that convey input from peripheral tissues to spinal interneurons and projection cells are shown entering at the right of the figure. The local circuit interneurons that influence the processing of sensory inputs to projection cells are represented by the cell profile at the bottom right. Meanwhile, the inputs to the spinal cord that have come from rostral central nervous system sensory modulatory sites are shown in the cellular component at the top of the figure, alongside the departing axon of the projection cell. The chemicals involved as neurotransmitters (transmitters) and neuromodulators (modulators) associated with each compartment are indicated in the boxes associated with each profile. Boxes at the bottom left list the nonspecific and trans-synaptic signals that provide additional sites for intervention.