A morphological study of cat dorsal spinocerebellar tract neurons after intracellular injection of horseradish peroxidase.

This work represents an attempt to elucidate structural features of electrophysiologically characterized, individual cat dorsal spinocerebellar tract (DSCT) neurons by using intracellular application of horseradish peroxidase (HRP). Intracellular recordings and HRP injections were made in DSCT neurons of the Clarke's column in cat lumbar (L3) spinal cord. The units were identified by antidromic invasion following electrical stimulation of the ipsilateral dorsolateral funiculus at C1. In addition, sensory inputs to the DSCT neurons were determined by natural (adequate) stimuli applied to the hind limb with intact innervation. The morphological analysis is based on data obtained from 19 well-stained electrophysiologically identified neurons located in Clarke's column. Thirteen of these units received excitatory sensory inputs from muscle receptors, two were activated by cutaneous afferents only, and four had a convergent (muscle + cutaneous) input. The DSCT--muscle cells were equivalent to the large Clarke cells (class C of Leowy, '70). Their dendrites were oriented primarily in the rostro--caudal direction (up to 2500 micron) and appeared generally smooth except for some branchlets. In four of these cells, the axon was traced into the lateral funiculus. In light microscopic analysis there was no evidence that axon collaterals arose from these axons during the initial trajectory through the spinal grey matter. The four DSCT--convergent neurons were similar in shape to the DSCT--muscle units although they appeared to have somewhat smaller cell bodies. Of the two DSCT--cutaneous neurons one was found to be of the B type, with the dendritic tree having fewer branches and oriented mainly in the medio--lateral direction. The other cell, however, turned out to be similar in appearance to the C type Clarke neurons.