The connexions of the arcuate nucleus of the thalamus

Abstract

As the result of detailed anatomical study, the ventral nucleus of the mammalian thalamus has been shown to be a composite structure, consisting of several nuclear elements which are to be distinguished by their cell and fibre architecture, and also by their fibre connexions. It has been demonstrated that medial fillet fibres (as well as ascending fibres from the spinal nucleus of the trigeminal nerve) terminate in the pars externa of the ventral nucleus (Vogt 1909; Clark 1936 b ; Walker 1937), and that this element projects on to the gyrus postcentralis of the cortex. Brachium conjunctivum fibres, on the other hand, terminate in the rostral part of the ventral nucleus (extending forwards from approximately the plane of the anterior margin of the centre median nucleus) which, in turn, is connected by thalamo-cortical fibres with the motor cortex of the precentral gyrus (Clark and Boggon 1935). It appears, therefore, that, although the motor cortex certainly receives afferent fibres from the thalamus, these have not the same functional implications as the afferent fibres which reach the postcentral gyrus. Of all the elements of the ventral nucleus of the thalamus, the best defined in higher mammals is the pars arcuata or arcuate nucleus (semilunar nucleus of Fleschig, nucleus vb of von Monakow and Friedemann, nucleus ventralis postero-medialis of other authors). The contour of this element can readily be determined by a naked-eye inspection of the freshly cut human or macaque brain. It is sharply circumscribed by a thin medullary capsule, it is darker in colour than the adjacent elements of the ventral nucleus, and it occupies a position in the postero-medial part of the thalamus immediately ventral to the centre median nucleus. The cells of the arcuate nucleus stain rather more deeply with methylene blue than those of the pars externa of the ventral nucleus, and they tend to be arranged in characteristic small clumps. In Weigert sections (fig. 5, Plate 4) the nucleus is not so richly permeated with fasciculi of medullated fibres, and these in general are distinguished by their fineness. The main part of the medial fillet can be seen to skirt its ventral and lateral margins to penetrate the pars externa of the ventral nucleus. Fine fibres run into the arcuate nucleus from the ventral medullary lamina of the thalamus and form within it a delicate interlacing network. The origin of these fibres is uncertain. Marchi studies following lesions in the spinal cord and medulla indicate that they do not arise from the spinothalamic tracts, the spinal nucleus of the fifth nerve, or the posterior column nuclei (Clark 1936 b ). Ranson and Ingram (1932) reported that, in the cat, fibres of the brachium conjunctivum terminate in the “ arcuate nucleus”, and this conclusion seemed to be in accord with the observation (Clark and Boggon 1935; Clark 1936 a ) that, in the monkey, the arcuate nucleus probably projects entirely on to the precentral gyrus. However, as has been previously pointed out, the “arcuate nucleus” as defined in the cat’s thalamus is not wholly homologous with the arcuate nucleus of the macaque (or human) thalamus, and Marchi studies show no evidence that the thalamic fibres of the brachium conjunctivum terminate in the arcuate nucleus of the monkey’s brain. Since, also, it has been shown that in the macaque brain the sensory cortex of the postcentral gyrus extends across the central sulcus to occupy the lower end of the precentral gyrus (Bucy 1935), it remains possible that the arcuate nucleus of the monkey is really related by ascending fibres to the granular area postcentralis (Brodmann’s areas 1, 2 and 3) and not to the true motor cortex (Brodmann’s area 4) as had earlier been assumed. The experiments recorded in this paper allow of a precise definition of the cortical connexions of the arcuate nucleus.