CEREBELLOTHALAMOCORTICAL AND PALLIDOTHALAMOCORTICAL PROJECTIONS TO THE PRIMARY AND SUPPLEMENTARY MOTOR CORTICAL AREAS - A MULTIPLE TRACING STUDY IN MACAQUE MONKEYS

The goal of the present study was to clarify whether the primary motor cortex (M1) and the supplementary motor cortex (SMA) both receive, vi a the motor thalamus, input from cerebellar and basal ganglia output n uclei. This is the first investigation that explores the problem by di rect comparison, in the same animal, of thalamic zones that 1) project to M1 and SMA and 2) receive cerebellar-nuclear (CN) and pallidal (GP ) afferents. These four zones were mapped in two monkeys by means of t wo retrograde tracers for M1 and SMA injections and of two anterograde tracers for CN and GP injections. All injections were performed under electrophysiological control (microstimulation and multiunit recordin gs). Injections in cortical areas were restricted to the hand/arm repr esentation; in the SMA, the tracer deposit was within the ''SMA-proper '' (or ''area F3'') and did not include its rostral extension (''pre-S MA'' or ''area F6''). It was found that zones of all four types formed a number of highly complex patches of labeling that were usually not confined to one cytoarchitectonically defined thalamic nucleus. The ov erlap of clusters of labeled terminals and perikarya was evaluated mor phometrically (area measurements) on a number of coronal sections alon g the anteroposterior extent of the motor thalamus. In line with previ ous studies, the thalamic territories innervated by CN and GP afferent s rarely overlapped. However, zones projecting to M1 and/or to SMA inc luded thalamic regions receiving CN as well as GP projections, providi ng the first evidence of such overlap from individual animals. The pre sent observations support the previous conclusion from this laboratory (based on transsynaptic labeling) that the SMA receives, apart from i ts strong pallidal transthalamic input, a CN transthalamic input. Thes e present findings that both M1 and SMA are recipients of transthalami c inputs from GP and CN thus support the concept that a mixed subcorti cal input consisting of weighted contributions from cerebellum, basal ganglia, substantia nigra, and spinothalamic tract is directed to each functional component of the sensorimotor cortex. (C) 1994 Wiley-Liss, Inc.