The morphology and projections of dorsal horn spinocerebellar tract neurones in the cat.

1. The morphology of dorsal horn neurones located in the mid-lumbar segments of the spinal cord and which have an axonal projection to the cerebellum has been investigated. The neurones were identified by antidromic activation from the cerebellum and by their characteristic input from group II afferents as described in the preceding paper (Edgley & Jankowska, 1988). 2. The cell bodies of the neurones were distributed across the width of the spinal cord in laminae IV and V, but particularly at the border between these laminae. Most were in the caudal half of the fourth lumbar segment (L4), caudal to Clarke's column. However, neurones of this type were encountered as far caudal as the middle of the fifth lumbar segment (L5) and as far rostral as the middle of the third lumbar segment (L3). 3. The morphology of the neurones was investigated following intracellular staining with horseradish peroxidase (HRP). Fourteen well-filled cells were recovered. They had large somata and extensive dendritic arborizations within the dorsal horn which could extend more than 2 mm rostro-caudally. The most dense arborization was in laminae III and IV, just dorsal to the cell bodies. 4. The axons of all fourteen cells could be followed well into the white matter. All of them passed into the dorsal part of the ipsilateral lateral funiculus where they ascended. All followed a similar indirect course through the grey matter. Despite careful inspection, initial axon collaterals were never found. 5. All of the neurones were antidromically activated by low-intensity electrical stimulation of the dorsolateral part of the ipsilateral lateral funiculus in the thoracic region and from the cerebellum. The conduction velocities of the axons ranged from 62 to 112 m s-1 (mean 84.2 (S.D. +/- 10.1) m s-1). 6. The axonal terminations of some neurones were investigated by mapping the most effective locations for antidromic activation from the cerebellar cortex. Most neurones were activated with lowest stimulus intensities from the rostral part of the anterior lobe. A second effective area was found in the posterior lobe, deep to the paramedian lobule. The majority of neurones were activated from both locations, suggesting that their axons branched to terminate in both areas. 7. On the basis of their projection and termination, it is proposed that the axons of these dorsal horn spinocerebellar tract neurones contribute to the dorsal spinocerebellar tract (DSCT).