The morphology of spinocervical tract neurones revealed by intracellular injection of horseradish peroxidase (cat)

Abstract

The morphology of physiologically identified spinocervical tract neurons was studied using the intracellular injection of horseradish peroxidase in anesthetized cats. Spinocervical tract neurons (36) were reconstructed from serial sections of the lumbosacral spinal cord, cut in either the transverse or longitudinal planes. Horseradish peroxidase provided a more complete picture of the dendrites of spinocervical tract neurons than earlier experiments using Procion Yellow injection (Brown et al, 1976). The longitudinal (rostro-caudal) spread of dendrites from an individual cell was much greater in the present material; neurons in the medial parts of the dorsal horn had dendrites extending for about 500 .mu.m from the soma (1 mm total spread) and neurons in the lateral horn had dendrites extending for about 1 mm from the soma (2 mm total spread). Earlier conclusions on the medio-lateral and dorso-ventral extents of dendritic trees, and the shapes of dendritic trees viewed as reconstructions in the transverse plane, were confirmed. Dendrites of spinocervical tract cells barely entered lamina II of Rexed; they often ran in the longitudinal direction along the border between laminae II and III for several hundred microns. Dendritic spines were observed on many spinocervical tract neurones. Horseradish peroxidase reaction product stained up to 2.5 cm of the axon of spinocervical tract neurons. Axons usually pursued a tortuous path through the grey matter close to the cell body, giving off up to 6 collaterals before entering the ipsilateral dorsolateral funiculus. In the funiculus, further collaterals often arose at distances of up to 5.5 mm from the soma; these collaterals ran back into the dorsal horn. Collaterals could be traced sometimes to presumed terminal boutons. The majority of collateral terminal arborizations was between the level of the cell body and 500 .mu.m ventral to it (in laminae IV and V). They were in the same medio-lateral and rostro-caudal region as the dendritic tree of the parent cell. The spinocervical tract has a segmental function in addition to its function of forwarding information towards the cerebral cortex.