A study of the substance P innervation of the intermediate zone of the thoracolumbar spinal cord

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Abstract
Immunocytochemical procedures have been used to examine the distributions of substance P (SP)‐positive fibres within the intermediate zone of the thoracolumbar spinal cords of rabbits, cats, and monkeys. In all three species SP fibres were concentrated in areas known to contain sympathetic preganglionic neurones. These included the intermediolateral nucleus and the funiculus just lateral to it, the medial gray matter in the area of the nucleus intercalatus, and the paracentral region. The density of the SP innervation varied in a characteristic way both between these subpopulations of sympathetic neurones and in its overall input to different segmental levels. Generally the greatest accumulations of SP fibres were found in the T3–T5 and L2–L4 regions and these were concentrated in the intermediolateral nucleus (ILN). The highest densities of SP fibres in the lateral funiculus were in the upper thoracic and upper lumbar segments whereas SP fibres forming transverse bands, possibly in association with neurones in the nucleus intercalatus, were most prominent in T5–T8. Substance P fibres adjacent to the midline were more or less equally dense throughout the segments examined. Substance P‐positive cell bodies situated immediately lateral to the central canal were present at a density of 200–300 per segment throughout the cat thoracolumbar cord. These neurones may be the cells of origin of at least some of the SP fibres in the intermediate zone. The close association of sympathetic preganglionic neurones with SP fibres, many of which are thought to be derived from cells in the medulla, suggests a role for SP‐containing fibres in the modulation of sympathetic activity. The variation in input to different segments and classes of sympathetic neurones further suggests a specificity which may be related to the different functions of the neurones innervated.