The differential contributions of the parvocellular and the magnocellular subdivisions of the red nucleus to skilled reaching in the rat

- The rat red nucleus is a critical structure for the execution of aiming, arpeggio and supination.
- The two subdivisions of the red nucleus play a differential role in skilled reaching.
- The parvocellular division of the red nucleus abolishes supination.

During the execution of the skilled reaching task, naïve rats bring their elbow to the midline of their body to aim at the food target, perform the arpeggio movement to grasp it and supinate the paw to bring the food to their mouth. Red nucleus lesions in the rat interfere with each of these three movement elements of reaching. On the other hand, lesions to the rubrospinal tract, which originate from the magnocellular subdivision of the red nucleus, only interfere with the arpeggio movement. This latter evidence strongly suggests that impairment in aiming and supinating could be under the control of the parvocellular subdivision of the red nucleus. In order to test this hypothesis, rats were trained on the skilled reaching task and then received either complete lesions of the red nucleus or lesions restricted to its parvo- or magnocellular subdivision. In line with previous data, complete excitotoxic lesions of the red nucleus compromised limb aiming, arpeggio and supination. Lesions restricted to the parvocellular division of the red nucleus abolish supination and interfere with aiming, although the latter result did not reach significance. The results are discussed in terms of the distinct connectivity and functional significance of these two architectonic subdivisions of the red nucleus.