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1. The morphology of physiologically identified spinocervical tract neurones was studied using the intracellular injection of horseradish peroxidase in anaesthetized cats.

2. Thirty-six spinocervical tract neurones were reconstructed from serial sections of the lumbosacral spinal cord, cut in either the transverse or longitudinal planes.

3. Horseradish peroxidase provided a more complete picture of the dendrites of spinocervical tract neurones than earlier experiments using Procion Yellow injection (Brown, House, Rose & Snow, 1976a). The longitudinal (rostro-caudal) spread of dendrites from an individual cell was much greater in the present material; neurones in the medial parts of the dorsal horn had dendrites extending for about 500 µm from the soma (1 mm total spread) and neurones in the lateral horn had dendrites extending for about 1 mm from the soma (2 mm total spread). However, the conclusions of the earlier work, on the medio-lateral and dorso-ventral extents of dendritic trees, together with the shapes of dendritic trees viewed as reconstructions in the transverse plane, have been 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 µm. Dendritic spines were observed on many spinocervical tract neurones.

4. Horseradish peroxidase reaction product stained up to 2·5 cm of the axon of spinocervical tract neurones. Axons usually pursued a tortuous path through the grey matter close to the cell body, giving off up to six 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 µm ventral to it (in laminae IV and V). They were, however, in the same medio-lateral and rostro-caudal region as the dendritic tree of the parent cell.

5. It is concluded that the spinocervical tract must now be considered as having a segmental function, in addition to its function of forwarding information towards the cerebral cortex.