[Horseradish peroxidase labeled sources of descending propriospinal tracts in the cat]

Propriospinal interneurons with descending axons were labelled after unilateral horseradish peroxidase microinjections to the cat spinal cord at different segmental levels. Main part of long propriospinal units were bilaterally localized in the ventral quadrants (laminae VII, VIII) of cervical segments; some neurons with long ipsilateral projections were also found in the marginal layer (lamina I) and lateral regions of the dorsal horn basis (laminae IV-VI). Propriospinal neurons with short axons were distributed in wider regions of the spinal gray matter. In the lumbar segments a certain trend for concentration of such neurons was found: ipsilaterally--in the lateral part of the dorsal horn basis, intermediate gray and ventral horn and contralaterally--in the ventromedial parts of gray matter. In the cervical segments short neurons with both ipsilateral and contralateral projections are concentrated in lateral parts of gray matter. Some interneurons in the marginal layer, gelatinous substance and intermediolateral sympathetic nuclei were also identified as sources of short descending propriospinal pathways. A certain part of propriospinal neurons has axons of intermediate length; location of such units was to a certain extent similar to the location of long neurons. Correlation of location of propriospinal neuron populations and patterns of synaptic inputs to them as well as correlation of structural and functional characteristics of the neurons are discussed.