Effects of partial inactivation of the supplementary motor area on bipedal locomotion in Japanese monkeys

Abstract

Our PET study has shown that the neural activity of the supplementary motor area (SMA) during bipedal (Bp) locomotion on the treadmill belt was significantly higher than during quadrupedal locomotion in the same monkey (M. fuscata). To elucidate the functional role of the SMA in the control of upright posture and Bp locomotion, we injected muscimol (10μg/l, 1μl/site) into the trunk/hindlimb representations of the SMA. Local injections along a penetration track were made to partially inactivate the SMA regions identified using MRI and intracortical microstimulation. We found that the unilateral SMA injections did not produce any Bp walking defects, whereas bilateral injections impaired the ability of the monkey to maintain upright posture resulting in locomotor defects. During Bp walking on the treadmill belt, step-by-step movements of single limb joints were not replicable and those of neighboring joints (hip-knee and knee-ankle) were no more coordinated, yielding a tottering gait pattern. These postural and locomotor impairments were due to a general reduction of postural muscle tone in the trunk and limbs. Our results suggest that the SMA is involved in the adjustments of posture accompanying Bp walking, and contribute to the elaboration and refinement of ongoing Bp locomotion together with the primary motor cortex. [Jpn J Physiol 54 Suppl:S184 (2004)]