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

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Abstract
Whether the group I hind limb cerebral tract and the dorsal spinocerebellar tract (DSCT) have common spinal axons was investigated. Cells (32) located in the nucleus Z of the cat medulla oblongata and activated by spinal fibers in the dorsolateral fascicle were selected. Extracellular recording from these neurons demonstrated that most of them were monosynaptically linked to spinal fibers 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. Antidromic activation from the contralateral thalamus showed that they were bulbothalamic relay cells. Cerebellar surface or depth stimulation activated 88% of the 26 cells tested at a short latency. With a collision technique it was demonstrated that 12 of 23 (52%) of these group I relay neurons were activated by axon-collaterals of the DSCT. Of the cells activated from the cerebellum, but not proven to be linked to the DSCT 43% could nevertheless have been excited by DSCT axon-collaterals, if it was assumed that different fibers converging with excitation on the group I relay cells were activated in the collision test.