Quantitative analyses of neurons projecting to primary motor cortex zones controlling limb movements in the rat

The objective was to determine if projections of single neurons to primary motor cortex preferentially terminate in several efferent zones that could form synergies for the execution of limb movements. Intracortical microstim ulation was used to identify zones evoking hip flexion (HF), elbow flexion (EF), and both plantarflexion (PF) and dorsiflexion (DF) about the ankle. H istological examination showed that the zones from which some movements wer e evoked extended beyond the agranular cortex into granular cortex. Fluorog old, Fast blue, and propridium iodide or rhodamine-labeled dextran were inj ected into three of these four efferent zones in each rat. There was a virt ual absence of multiple-labeled cells despite having an intermingling of di fferent-colored cells of which 15% in frontal cortex were less than 1.2 mm away from a neighboring neuron that projected to a different efferent zone. This suggests that single neurons projecting to the motor cortex do not ha rd-wire specific synergies but rather project to single efferent zones in o rder to offer the greatest degree of freedom for the generation of movement s. The distribution of ventral posterolateral and ventrolateral thalamic nu cleus labeling depended on whether the injections were in granular or agran ular cortex. Conversely, frontal cortex projections to motor efferent zones were made irrespective of their location in either granular or agranular c ortex and thereby supporting their presumed role in the control of movement s. Hindlimb motor cortex injections yielded retrograde labeling that extend ed into the more localised distribution of frontal cortex neurons retrograd ely labeled from forelimb injections. This may allow hindlimb movements to be synchronized by forelimb movements during walking on challenging terrain . (C) 1999 Elsevier Science B.V. All rights reserved.