Integrative functions of spinal interneurons are well recognized but the relative role of different interneuronal populations in this process continues to be investigated. It therefore appeared useful to review the principles of integration of afferent information by the interneurons analyzed so far as these principles should apply also to those remaining to be analyzed. Considering the results of both functional and morphological studies of spinal interneurons and of the morphology and immunochemistry of afferent fibers that provide input to them, the following five basic principles of processing of afferent information by them will be outlined; 1) afferent information of any origin is forwarded to several neuronal populations, 2) information from any sources of input is distributed unevenly, 3) input from several sources is integrated by individual neurons as well as by their populations, 4) specific combinations of input are integrated by different neuronal populations, and 5) afferent input to spinal interneurons is only one of the features distinguishing their functional populations. As the spinal neuronal organization and properties of neurons and afferent fibers in the so far investigated species (cat, rodents, and primates) have been found to resemble, future studies using molecular techniques in the mouse should allow the new data to integrate with those of the preceding studies and the principles outlined earlier as well as any new ones should apply also in humans.
Keywords: afferent fibers; premotoneuronal processing; spinal cord; spinal interneurons.