Spatio-temporal organization of the somaesthetic projections in the red nucleus transmitted through the spino-rubral pathway in the cat.

Although it has been known for a long time that in awake cats, natural stimulation of the skin induces short latency responses in rubrospinal cells, the pathway possibly involved has been identified only recently (Padel et al. 1988). This tract, which was described in acute, chloralose anaesthetized cats, ascends in the ventromedial spinal cord and is activated via collaterals of primary afferent fibres running in the dorsal columns of the spinal cord. The present study demonstrates that this newly described spino-rubral tract is able to send detailed somaesthetic information to the red nucleus. After lesions leaving intact only the spino-rubral pathway, excitatory and inhibitory responses to natural peripheral stimulations were recorded in identified rubral efferent cells. The most effective stimuli were touching the skin, passive joint rotation and hair displacement. Each cell was found to possess a particular receptive field. These fields which could be ipsi-, contra-, or bi-lateral were generally located on a single limb, although they could include two or more limbs, or even exceptionally the whole body with or without preferential zones. The topographic organization of receptive fields was arranged somatotopically in the red nucleus and overlapped the motor representation. The somaesthetic inputs transmitted through the spino-rubral pathway to the red nucleus are very similar to those previously observed in the intact cat, which supports the idea that this pathway may play a functional role in motor control. The spino-rubro-spinal loop may provide a fast adaptation of the descending motor command, thus producing a fine and harmonious tuning between the changing surroundings and the animal's movements.