Stimulation of pre- and postsynaptic elements in the red nucleus

Summary Stimulation of the red nucleus evokes a two-component descending discharge in the contralateral dorsal quadrant of the spinal cord; the first component is caused by direct and the second by synaptic activation of rubrospinal neurones projecting to the lumbosacral cord. Threshold maps for direct and synaptic activation are given. The low threshold focus for direct activation covers not only the hindlimb region of the red nucleus where the antidromic field potential is large but also the medial part of the nucleus where the axons leave. Direct activation can also be evoked from a region extending caudally from the medial part of the red nucleus. Synaptic activation of rubrospinal neurones, evoked at very low threshold from the ventro-medio-caudal border zone of the red nucleus, is due to stimulation of interposito-rubral efferents. The threshold, indicated by unitary EPSPs in rubral cells and antidromic activation of cells in interpositus, is about 1 A, which is 1/10 of the strength required for direct activation of rubrospinal neurones. There is neither evidence that rubral stimuli activate collaterals of corticospinal axons to the lumbosacral cord, nor that antidromic activation of interposito-rubral fibres evokes descending effect via their collaterals to the reticular formation.This work was supported by the Swedish Medical Research Council (Project No. 14X-94-07C).