Axon-collateral activation by dorsal spinocerebellar tract fibres of group I relay cells of nucleus Z in the cat medulla oblongata.

1. The problem whether the group I hind limb cerebral tract and the dorsal spinocerebellar tract (DSCT) have common spinal axons has been investigated in the present study. 2. Thirty-two cells located in the nucleus Z of the cat medulla oblongata and activated by spinal fibres in the dorsolateral fascicle were selected for the study. 3. Extracellular recording from these neurones demonstrated that most of them were monosynaptically linked to spinal fibres excited by ipsilateral hind limb group I muscle afferents. The cells exhibited a restricted spatial convergence and had a limited excitatory convergence from group II muscle and from skin afferents. 4. Antidromic activation from the contralateral thalamus showed that they were bulbothalamic relay cells. 5. Cerebellar surface or depth stimulation activated 88% of the twenty-six cells tested at a short latency. With a collision technique it was demonstrated that twelve out of twenty-three (52%) of these group I relay neurones were activated by axon-collaterals of the DSCT. 6. 43% of the cells activated from the cerebellum, but not proven to be linked to the DSCT, could, nevertheless, have been excited by DSCT axon-collaterals, if it is assumed that different fibres converging with excitation on the group I relay cells were activated in the collision test.