Does the red nucleus provide the tonic support against which fractionated movements occur? A study on forepaw movements used in skilled reaching by the rat

The red nucleus (RN) is a prominent structure in the motor system of mammals and is thought to play a role in the control of limb movement. In the rat, there are contradictory reports concerning its role in controlling movements of the paw and digits in grasping food in skilled reaching tasks. Some studies report severe impairments in the effectiveness of limb movement and impairments in grasping movements, whereas other studies report only mild impairments. This study is designed to resolve this issue by examining limb and paw movements in rats with RN lesions using a new high-speed video filming procedure. It was found that unilateral ibotenic acid lesions of the RN do not impair reaching or grasping success with the limb contralateral to the lesion. Reaching movements are changed, however. Control rats aim their paw by adducting the elbow, pronate their paw by abducting the elbow, and they place their digits over the food with a lateral to medial arpeggio pattern. In rats with RN lesions, aiming and pronating are reduced and the arpeggio movement does not occur. Rather, the food is grasped with a flexion of the digits as the limb is retracted. This change results in a very rapid grasp and an absence of a pause in limb transport during grasping. These impairments and the 'release' of the grasp response suggests that the RN may provide the tonus or supporting framework that momentarily immobilizes the limb so that arpeggio and grasping movements can occur. It is suggested that the function of the RN more generally may be to provide a tonic framework against which the motor cortex can produce more fractionated movements.