DUAL MORPHOLOGY AND TOPOGRAPHY OF THE CORTICOTHALAMIC TERMINALS ORIGINATING FROM THE PRIMARY, SUPPLEMENTARY MOTOR, AND DORSAL PREMOTOR CORTICAL AREAS IN MACAQUE MONKEYS

In the motor, somatosensory, and auditory systems of rodents and cats, the corticothalamic connection is composed of a main projection forme d by small endings and a minor projection terminating with giant endin gs. To establish whether the corticothalamic projection originating fr om motor cortical areas in primates exhibits the same duality, the ant erograde tracer biotinylated dextran amine was injected in eight macaq ue monkeys in the primary motor (M1; n = 3), the supplementary motor ( SMA; n = 3) and the dorsal premotor (PMd; n = 2) cortical areas to lab el corticothalamic axons. The corticothalamic projection originating f rom these three motor cortical areas was characterized by the presence of axon terminals constituting the same two types of endings, observe d both as boutons en passant and terminaux. The population of small en dings exhibited a mean cross-sectional maximum diameter of 0.95 mu m ( S.D. = 0.23), a range of diameters not overlapping that of giant endin gs (mean diameter = 3.46 mu m, S.D. = 0.74 mu m). Topographically, the giant endings originating from M1 were located in the same thalamic n ucleus (ventroposterolateral nucleus, oral part) in which the small en dings were found. In contrast, the giant endings originating from SMA and PMd were located in a thalamic nucleus (mediodorsal nucleus) disti nct from the main termination zone formed by small endings. Along the rostrocaudal axis, the giant endings were distributed in a restricted zone, irrespective of the origin of the projection (M1, SMA, PMd). The dual morphology of corticothalamic endings, previously found in roden ts and cats, is present in the motor system of subhuman primates for b oth primary and nonprimary motor cortical areas. (C) 1998 Wiley-Liss, Inc.