Messages conveyed by descending tracts during scratching in the cat. II. Activity of rubrospinal neurons

(1) The activity of rubrospinal (RS) neurons giving axons to the lumbosacral spinal cord was recorded during actual and fictitious8 scratching in thalamic cats. (2) During both actual and fictitious scratching, the discharge frequency of many RS neurons was rhythmically modulated. Different neurons were active in different parts of the scratch cycle, but most neurons were active in the flexor phase. (3) The discharge frequency within the bursts during fictitious scratching was, on the average, equal to that during actual scratching. Immobilization usually resulted only in a small displacement of the burst position in the scratch cycle. Therefore, rhythmical modulation of RS neurons is determined mainly by central mechanisms and not by a rhythmical sensory input. (4) In decerebellate cats, the overwhelming majority of RS neurons had no rhythmical modulation. Very weak modulation was found only in a few neurons. (5) Transection of the ventral spinocerebellar tract (VSCT) resulted in considerable reduction or complete cessation of rhythmical modulation in RS neurons during fictitious scratching. On the contrary, transection of the spino-reticulocerebellar pathway (SRCP) resulted in just a small decrease of modulation. Therefore, of the two pathways (the VSCT and SRCP) transmitting messages about intraspinal processes to the cerebellum during scratching2,3, the VSCT is of major importance for modulating RS neurons.