Spatial relationships between the terminations of somatic sensory motor pathways in the rostral brainstem of cats and monkeys. II. Cerebellar projections compared with those of the ascending somatic sensory pathways in lateral diencephalon

Abstract

Previous studies have shown that ascending somatic sensory pathways arising from the dorsal column nuclei, lateral cervical nucleus and spinotha‐lamic tract terminate in parts of the thalamus adjacent to those which receive cerebellar terminations. This termination pattern creates a border between the ventroposterolateral nucleus (VPL) and the ventrolateral nucleus (VL) in the cat and between the caudal and oral parts of VPL (VPLc and VPLo, respectively) in the monkey. Since it is not clear how sharp these borders are, a double orthograde labeling strategy was used in the present study to make direct comparisons of the projections to the thalamus from these sources of input.It was found that there was a change in the sources of afferent input to the different target areas that paralleled changes in cytoarchitecture. Moving caudally to rostrally, VPL in the cat and VPLc in the monkey received projections predominantly from the middle, dorsal (clusters) portion of the dorsal column nuclei. These projections were gradually replaced near the VPL‐VL border in the cat and VPLc‐VPLo border in the monkey first by input from the lateral cervical nucleus (cat only) and the rostral and ventral portions of the dorsal column nuclei and then by spinothalamic projections. Towards VL in the cat and the rostral parts of VPLo in the monkey (referred to as Vim by Hassler, '59 and Mehler, 71), these projections were in turn replaced by those from the cerebellum. This sequence resulted in a complex pattern (summarized in Fig. 10) where some thalamic territories received input predominantly trom one source and others received converging input from several sources. The major region receiving converging ascending somatic sensory and cerebellar terminations was located at the border between VPL and VL in the cat and in the caudal parts of Olszewski's ('52) VPLo in the monkey (that is, between VPLc and Vim).In general, the results in the cat were similar to those in the monkey. One notable difference was that the domain containing terminals from the cerebellum and the rostral‐ventral parts of the dorsal column nuclei was located medially between VPLc and Vim in the monkey, whereas it extended across the entire mediolateral border between VPL and VL in the cat.In both species, thalamic neurons received input predominantly from one afferent source and only minor input, if any, from other sources. This dominance of a single source of afferent input occurred even for neurons located in domains that received converging input from several sources.When considered together with the results of others, the results presented here indicate that the “core” and “shell(s)” regions of VPL noted by a number of authors (e.g., Bowsher, '66; Andersson et al., '66; Whitsel et al., '78; Berkley, '80; Friedman and Jones, '81; Spreafico et al., '81) cannot be sharply separated from each other or from more rostral regions by their pattern of afferent input, their efferent output or their cytoarchitecture. Such a situation permits flexibility in the processing of different types of somatic information and indicates that regions subserving different functions in the realms of somatic sensation and kinesthesia are only roughly separated from one another. In addition, in agreement with others, the results suggest that, in both species, one possible function of the domain containing the converging cerebellar and ascending somatic sensory terminations involves the processing of somatic information related to the activation of receptors located in muscles, joints, tendons, vestibular apparatus and other deep tissues.