Thalamic Microneurones

Abstract

IN view of the recent deduction by experimental neurophysiologists¹ of the presence of ‘interneurones’ in the region of the somato-sensory relay (ventralis posterior) nucleus of the thalamus, it would seem timely to recollect that such neurones are almost certainly demonstrable under the light microscope (and generous to recall that the presence of such Schaltzellen was deduced sixty-seven years earlier, from brilliant experimental and pathoanatomical observations, by von Monakow²). It is rather surprising that the first histological report of thalamic ‘microneurones’ (to the 1949 International Neurological Congress³) has not been confirmed by anatomists outside Germany^4,5. The explanation probably rests largely in the fact that in most of the commoner mammals used in neuro-anatomical research the somata of the microneurones are too similar in size and shape to those of astrocytes to be distinguished with confidence. From extensive surveys in the macaque (in celloidin-embedded, well-differentiated cresyl-violet preparations) I have ascertained that the microneurones tend to be proportional in size to the macroneurones in most of the sub-nuclei of the thalamus, and hence largest in nucleus ventralis posterior pars gigantocellularis. Unfortunately, again, the most commonly studied sub-primates do not possess a conspicuous pars gigantocellularis. In the dog, however, it is well developed, and its microneurones are as prominent as in the macaque. In both the dog and the rabbit, but not so far in the macaque, I have obtained occasional thalamic sections, impregnated by the silver sulphide method and counterstained by methylene blue⁶, in which the display of heavy-metal granules is distinctly more condensed, and hence blacker, in the cytoplasm of the astroglia than in both orders of neurones.