The specificity of the neuronal structure and topography of the reticular formation in the brain and spinal cord of carnivora

The great importance currently attributed to the reticular formation in the total activity of the brain requires precise knowledge of its fine structural peculizrities and an exact definition of this anatomical concept. However, at present there is no agreement even on the criteria for determining which brain structures are to be considered as part of this system. It is customary in the morphological literature to include in the reticular formation the central parts of the medulla oblongata, of the pons and the mesencephalon. However, some authors consider as reticular structures a number of formations of the spinal cord and the diencephalon; the points of view of different investigators are very contradictory. Thus the reticular process of the spinal cord is most frequently assigned to reticular formation, although some authors emphasize the reticular structure of the spinal cord along the whole boundary between its gray and white substance (Stilling, 1843; SOCOlov, '57). Others point out the identity between the cells of the reticular process and those of the lateral horn (Waldeyer, 1889; Ziehen, 1899; Sukhetskaya, '54, '59). A third group finds common features not only in the cells of the reticular process and those of the lateral horn but also in those of the intermediate zone of the spinal cord (Jacobson, '08; Sukhetskaya, '54, '59). In the diencephalon different authors assign various structures to the reticular formation: zona incerta and the scattered cells of the hypothalamus which are not concentrated into nuclei (Malone, 'lo), the n. reticularis thalami (Rose and Woolsey, '49), the zona incerta (Papez, '29). Roussy and Mosinger ('35) pointed out the wide representation of the reticular formation in the diencephalon; they include the n. subthalamicus Luysi, the nuclei of Forel's field, the n. reticularis thalami, the parafascicular nuclear complex, the zona incerta, the hypothalamus and the ventral part of the lateral geniculate body. At the same time some morphologists definitely deny the existence of the reticular formation at the diencephalic level (Xuhlenbeck, '54). It is only in the last 15 to 20 years that morphologists, under the influence of the electrophysiological investigations of Jasper ('49) and others began to refer to a considerable number of diencephalic formations as reticular structures. These authors did not, however, try to solve the problem from the morphological point of view, but simply described structural peculiarities and connections of the nuclei, which electrophysiologists had considered as diencephalic parts of the reticular formation (Nauta and Whitlock, '54; Papez, '56). As to the problem of the existence of reticular formation structures at subcortical levels oral of the diencephalon, we have not succeeded in finding any such indications in the morphological literature. Obviously, the reason for the contradictory points of view found in the morphological literature is the absence of a precise histological criterion for assigning various structures to the reticular formation. Thus the currently and generally accepted definition of the reticular formation as a net of nerve cells and intersecting fibers formulated in the classical literature on neurmorphology is based on purely anatomical features whereas the fine structural peculiarities of its elements are not taken into considera tion. Numerous cytoarchitectonic studies of the reticular formation, beginning with the works of Bechterew (1885, 1896-98),