The dorsal horn of the spinal cord.

Recent advances in techniques, especially the intraneuronal injection of the enzyme horseradish peroxidase, have led to a new ear in our understanding of spinal cord structure and function. Input to the cord is precisely organized: the primary afferent fibres from different types of receptors distribute their anatomically specific collaterals to particular parts of the dorsal horn, afferent fibres from the skin lay down a precise somatotopic map, input to the dorsal horn from descending systems is also distributed in a localized way. The neurones of the dorsal horn are varied in both structure and function, even so some quite specific cell types can be identified and the dendritic trees may respect laminar boundaries as determined cytoarchitectonically (although the majority of neurones have dendrites that cut across these boundaries). The output pathways from the dorsal horn are many and various, but again they arise from cells in definite parts of the dorsal horn. The dorsal horn must be considered as a well-organized, and complex, part of the central nervous system. It cannot be considered as a structural or functional unit but is made up of many interacting parts that process input from the primary afferent fibres, from other levels of the spinal cord and from many descending pathways from the brain.