Projections of group II-activated midlumbar spinocerebellar tract neurones to the region of nucleus Z in the cat.

1. The possibility that dorsal horn spinocerebellar tract neurones in the midlumbar segments of the cat spinal cord which convey information from hindlimb group II muscle afferents to the cerebellum send collateral projections to medulla in the region of nucleus Z has been examined. 2. Dorsal horn spinocerebellar tract neurones (n = 25) were identified by antidromic activation from the cerebellum and by synaptic activation following stimulation of hindlimb group II afferents. A high proportion (21/25) were also antidromically activated by stimuli delivered to the region of nucleus Z. 3. The results of collision tests between antidromic spikes evoked from the cerebellum and the medulla and the fact that the latency for antidromic activation from nucleus Z at threshold was greater than from the cerebellum indicates that at least 11/25 (44%) of the neurones had collateral projections to the medulla. 4. Antidromic threshold mapping revealed that some of the neurones could be activated from parts of the dorsal medulla adjacent to, but not directly overlying, nucleus Z. The possible relevance of these data with regard to sensation of lower limb position and motion is discussed.