THE PRIMATE MOTOR THALAMUS

The functional parcellation of the motor thalamus of primates has suff ered from serious historical and technical drawbacks, which have led t o extreme confusion. This is a problem when thalamic stereotaxy is aga in being use clinically. The cause usually imputed is the historical c onflict between two main schools, the Vogt and the 'Anglo-American' (M ichigan), 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 cytoar chitectonic technique, comparative anatomy and cortical connections; u nder exploitation of subcortical afferent territories; recent misuse o f these territories, hesitations in the use of the VA-VL system; and o pposition between ventral ('relay') and dorsal ('associative') 'nuclei '. Previous and current parcellations and nomenclatures for the latera l region finally appeared inappropriate. Before presenting a new parce llation and nomenclature for the lateral region, we explain why we did not adopt one of most common or of recently proposed nomenclatures, a nd were led to make our own. This is established according to rational and historically grounded rules. Precise definition of thalamic eleme nts is provided. A thalamic 'region' is a gross topographic division c orresponding to the former nuclei. A 'territory' is defined as the cer ebral 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) neuro nal set spaces. It appears that source spaces are coincident in the pa llidal and nigral territories but not in the cerebellar territory wher e two topographically distinct source spaces could be distinguished. A 'thalamic nucleus' is defined as the intersection of a thalamocortica l source space with one territory. A rapid review of the general anato my of the diencephalon is made. The ('dorsal') thalamus is divided int o 'allo-' and 'isothalamus', the latter with 'bushy' and 'microneurons '. The lateral region is isothalamic. The 'motor thalamus' makes the a nterior part of the lateral region. The present work aims to analyse t he functional anatomy of the 'motor thalamus' by using precise topogra phy and three-dimensional analysis of the subcortical territories rece iving from the cerebellar nuclei (part II), the medial nucleus of the pallidum (part III) and the pars reticulata and mixta of the substanti a nigra (part IV). Large injections were used to obtain the maximal ex tent of each territory. A major deficiency of previous studies was ina dequate cartography. Reliance on ventricular (CA-CP) landmarks observe d by use of orthogonal teleradiography is mandatory. A study was made of intra- and interspecific variations and their effect on stereotacti c 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 topographical ly distinct and separate territories: cerebellar, pallidal territory a nd 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 nuc leus lateralis polaris or a polar VA. The cerebellar territory is cont inuous and dense, in front of the somesthetic nucleus and everywhere s eparate from it. It has a complex three-dimensional shape, strongly co nvex anteriorly. Its caudal portion is dorsal to the somesthetic nucle us. In the lateral dimension it extends from the lateral region to the paralaminar region of the medial complex. The cerebellar territory, p rimarily dentate, also contains islands of axons from the interposed a nd fastigial nuclei, the vestibular nuclei and the spinothalamic tract . It corresponds to at least five previously isolated 'cytoarchitecton ic nuclei'. It cannot be seen as a single entity but contains distinct ventrolateral and a mediodorsal source spaces. Its complex three-dime nsional shape, its particular intersection with different cytoarchitec tonic nuclei and source spaces, can be simplified into two nuclei: nuc leus lateralis intermedius lateralis (LIL) and nucleus intermedius med iodorsalis (LIM). The stereotacticians' ventralis intermedius (Vim) is discussed at length. Tremorosynchronous and kinesthetic neurons are e ssentially located ventrally and laterally. The intrathalamic topograp hy observed in human patients indicates that the stereotacticians's ta rget (where tremorosynchronous neurons are found) is within lateral ce rebellar territory receiving cerebellar afferents and projecting to th e motor cortex. The hypothesis that the nucleus interpositus could be particularly involved is presented. The nucleus lateralis intermedius lateralis (LIL) of the present study is equivalent in macaques to the stereotacticians' 'V.im.e', object to interspecific differences. The p allidal territory is anterior to and separate from the cerebellar terr itory, in the oral part of the lateral region. It covers the whole ven trodorsal extent of the thalamus. It is crescent shaped, strongly curv ed in the anteroposterior dimension, and ends dorsally over the cerebe llar territory. It is also curved in the mediolateral dimension follow ing the lateral border of the thalamus. The territorial border does no t coincide with cytoarchitectonic borders. The pallidal territory incl udes various cytoarchitectonic nuclei. Only the cloud-like ventral ora l part is easily recognizable on purely cytoarchitectonic criteria. Th e others are cytoarchitectonically ill defined. The pallidal territory may be delineated using calbindin immunostaining. Other targets of th e medial pallidum are the central (centre-median-parafascicular) compl ex, the pedunculo-pontine complex and the lateral habenula. In relatio n to its afferent and efferent connections, the pallidal thalamus seem s to be made up of a single piece and could be considered as the later al oral subregion or nucleus (LO). The nigrothalamic connect