An analysis of the posterior group of thalamic nuclei on the basis of its afferent connections

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Abstract
The distribution of the axonal and terminal degeneration in the posterior group and principal nuclei of the thalamus of the cat has been studied with the Nauta technique following lesions of the major ascending sensory pathways and of most areas of the cerebral cortex. The cellular changes occurring in the same parts of the thalamus after damage of the cortex of the somatic, auditory and visual sensory areas have also been investigated.It has been found that the posterior group receives fibers from all parts of the cortex on the lateral surface of the hemisphere caudal to area 4, with the exception of areas 17, 18 and 19 of the visual cortex, and that cellular degeneration occurs in the posterior group after damage of the same areas of cortex. Both fiber and cellular degeneration can be found in the posterior group after a lesion confined to either the somatic sensory or auditory cortex.Experiments with smaller lesions have shown that all areas of the cortex which have been studied send fibers to parts of three distinct types of thalamic nuclei — to the relevant principal nucleus, to a part of the intralaminar group of nuclei or the posterior group, and to a part of the reticular nucleus. After larger lesions of the cortex cellular degeneration is always found in parts of these three types of nuclei. Ascending fibers in the somatic and auditory sensory pathways also terminate in the principal nuclei and in one or other part of the intralaminar nuclei‐posterior group system but not in the reticular nucleus.There is a definite interrelationship in the somatic and auditory sensory systems between the site of termination of the ascending fibers of these pathways, the principal nuclei and parts of the posterior group in which these two pathways end, and the area of cortex to which these principal nuclei project and which, in turn, sends fibers back to them and to the posterior group. It is suggested that there is a similar relationship between the site of termination in the intralaminar nuclei of afferent fibers from subcortical structures, the principal nuclei in which the same afferent pathways end, and the area of cortex with which these principal nuclei are related. This appears to be true at least for the pathways from the cerebellum and globus pallidus to the ventrolateral nucleus and the relevant components of the intralaminar nuclei.On the basis of the experimental findings it is suggested that the posterior group should be included with the intralaminar group of nuclei, and that the region equivalent to the intralaminar nuclei‐posterior group system for the visual cortex is the pretectum (either the whole or a part) or the nucleus of the optic tract. If this hypothesis is correct there is a continuous, well‐organized projection of the neocortex upon the entire extent of the intralaminar nuclei‐posterior group‐pretectal area complex: prefrontal and limbic cortex being related to rostral intralaminar nuclei, areas 6 and 4 to the parafascicular, centre median and central lateral nuclei, the somatic sensory cortex to the medial division of the posterior group, the auditory cortex to the lateral division of the posterior group, the supra‐sylvian cortex to the suprageniculate nucleus and intermediate division of the posterior group and the visual cortex to the pretectum and adjoining regions.