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The principal auditory leading to the cerebral cortex and therewith to conscious perception passes from the cochlea, via the cochlear nuclei, the inferior colliculus and the medial geniculate body to the contralateral auditory cortex in the temporal lobe. All components of this pathway are cochleotopically organized. The fibre stream leading from the cochlear nuclei to the contralateral inferior colliculus consists of both direct and multisynaptic components. The latter are interrupted one or more times in the superior olivary complex and/or the nuclei of the lateral lemniscus, cell assemblies that lie embedded into the main ascending fibre stream. The auditory cortex receives not only impulses from the contralateral cochlea, but also from the ipsilateral cochlea. The ipsilateral projection reaches the ipsilateral inferior colliculus after a synaptic interruption in the ipsilateral superior olivary complex. Fibres conveying impulses from the contralateral side back to the ipsilateral side are probably also involved. The auditory system of the brain stem is characterized by the presence of several well-developed commissures. Most of these contain, in addition to decussating components, true commissural fibres. The various auditory centres in the brain stem are not only way stations in the ascending auditory pathways, but also serve as relays in descending auditory projections. Most important among the latter is the cortico-cochlear projection which is synaptically interrupted in the inferior colliculus and the periolivary nuclei. The final link in this descending projection is formed by the bundle of Rasmussen, the fibres of which leave the central nervous system and innervate the inner and outer hair cells in the organ of Corti. The pathways for the auditory startle response, the auditory orientation reflex and the stapedius and tensor tympani reflexes are briefly described.

Bergquist and Källén as well as Puelles and collaborators have presented models of the developing vertebrate brain, the basic units of which are formed by intersection of transversely oriented neuromeres and longitudinally arranged zones. These units represent initially discrete, developmentally independent compartments, but during later development some (many?) are invaded by sizable numbers of neuroblasts generated in adjacent units. No consensus exists with regard to the number and arrangement of the units involved in the formation of the telencephalon and hypothalamus.

In order to determine the time and site of origin and the final location of various cell groups in the spinal cord, tadpoles of Xenopus laevis, ranging from stage 48 to stage 56 were treated with tritiated thymidine and sacrificed at various stages from 49 to 66 (stages according to Nieuwkoop and Faber (1967). From the poorly developed matrix at stage 48-49 not only ventral horn cells, but also neuroblasts of the intermediate zone and the dorsal horn arise. Both the matrix and the ventricle expand in a dorsal direction. From the well-developed matrix at stage 54, in which the mitotic activity is almost exclusively confined to its dorsal part, mainly cells of the dorsal horn develop. However, this later-stage matrix also gives rise to a considerable number of neuroblasts, which become located in the central parts of the intermediate zone and the ventral horn. Generally the later-born cells come to lie dorsomedially to the older ones. The neuroblasts of the lateral motor column, however, migrate through and settle ventrolaterally to their predecessors. Our observations do not support the basal plate-alar plate concept of His (1893).

Publisher Summary This chapter reviews the comparative anatomy of the spinal cord with particular attention to the lower forms. The spinal cord is generally considered as the lowest level and the most simply organized part of the central nervous system. Of all the parts of the central nervous system, the spinal cord or medulla spinalis preserves the early embryonic tube-like shape most clearly, and although secondary form changes occur in some groups, it may be said that this organ is generally of a cylindric appearance. The length of the spinal cord varies considerably. During the course of evolution, a relatively simply spinal nervous mechanism has been gradually overshadowed and superseded by a more complex secondary system. The gray matter of the spinal cord shows a progressive elaboration and an increasing segregation of separate cell masses in the series of vertebrates, but the primitive configuration: a central core of gray surrounded by an outer zone of white matter is principally maintained throughout the subphylum. The chapter gives some general remarks on the embryonic cord.

Contains fulltext : mmubn000001_207145261.pdf (Publisher’s version ) (Open Access) Promotores : R. Nieuwenhuys en A. Gribnau 93 p.