The primate motor thalamus.

Abstract

The functional parcellation of the motor thalamus of primates has suffered from serious historical and technical drawbacks, which have led to extreme confusion. This is a problem when thalamic stereotaxy is again being use clinically. The cause usually imputed is the historical conflict between two main schools, the Vogt and the 'Anglo-American' (Michigan), which used different nomenclatures. In fact, the reasons are more profound and serious. A combination of them led to: an archaic, rigid conception of the 'thalamic nucleus'; overexploitation of cytoarchitectonic technique, comparative anatomy and cortical connections; underexploitation of subcortical afferent territories; recent misuse of these territories; hesitations in the use of the VA-VL system; and opposition between ventral ('relay') and dorsal ('associative') 'nuclei'. Previous and current parcellations and nomenclatures for the lateral region finally appeared inappropriate. Before presenting a new parcellation and nomenclature for the lateral region, we explain why we did not adopt one of most common or of recently proposed nomenclatures, and were led to make our own. This is established according to rational and historically grounded rules. Precise definition of thalamic elements is provided. A thalamic 'region' is a gross topographic division corresponding to the former nuclei. A 'territory' is defined as the cerebral space filled by afferent endings from one source. When having a distinct topography in a region, a given territory makes a 'subregion'. For each of the studied 'motor' territories a review was made of its known cortical projections. The thalamic space where neurons project to a given cortical target constitutes a 'source space'. Topographical comparison of the sources spaces with territories reveals that there is often no coincidence between different (afferent or efferent) neuronal set spaces. It appears that source spaces are coincident in the pallidal and nigral territories but not in the cerebellar territory where two topographically distinct source spaces could be distinguished. A 'thalamic nucleus' is defined as the intersection of a thalamocortical source space with one territory. A rapid review of the general anatomy of the diencephalon is made. The ('dorsal') thalamus is divided into 'allo-' and 'isothalamus', the latter with 'bushy' and 'microneurons'. The lateral region is isothalamic. The 'motor thalamus' makes the anterior part of the lateral region. The present work aims to analyse the functional anatomy of the 'motor thalamus' by using precise topography and three-dimensional analyses of the subcortical territories receiving from the cerebellar nuclei (part II), the medial nucleus of the pallidum (part III) and the pars reticulata and mixta of the substantia nigra (part IV). Large injections were used to obtain the maximal extent of each territory. A major deficiency of previous studies was inadequate catography. Reliance on ventricular (CA-CP) landmarks observed by use of orthogonal teleradiography is mandatory. A study was made of intra- and interspecific variations and their effect on stereotactic and cartographic precision in macaques. All three subcortical motor afferent territories to the motor thalamus of macaques are examined in precise cartography with three dimensional reconstructions, rotations and 'reslicing'. The motor thalamus is made up of three topographically distinct and separate territories: cerebellar, pallidal territory and nigral. They cover the entire anterior part of the lateral region. There is no polar subdivision without lower afferents in front of the pallidal and nigral territories and thus no reason for isolating a nucleus lateralis polaris or a polar VA. The cerebellar territory is continuous and dense, in front of the somesthetic nucleus and everywhere separate from it. It has a complex three-dimensional shape, strongly convex anteriorly. Its caudal portion is dorsal to the somesthetic nucleus.(ABSTRACT