Chapter 138. Phylogenesis of The Nervous System - Review of Medical Embryology Book

Introduction: vertebrates are distinguished from other groups of animals by certain characteristics, principally related to their spinal cords and nervous systems, namely, the presence of a dorsal notochord surrounded by mesodermal tissue, which gives rise to a vertebral column in adults of most species, and a tubular nervous system lying dorsal to the cord, concentrated in the midline, and very well developed at its cephalic end (cephalization). The extent and pathways of nervous system development differs between invertebrates and vertebrates, even though, in all animals that have a nervous system, it is derived predominantly from ectoderm
Comparative embryology and anatomy have demonstrated various phases of the nervous system evolution. Specific aspects of evolutionary differences are

Invertebrate nervous system development: before the evolutionary appearance of the spinal cord, neural organization apparently depends on sensory surface ectoderm

IN COELENTERATES (sea anemones, hydra, jellyfish) the cells are diffuse and the nervous system consists of a diffuse neural network
IN PRIMITIVE WORMS (Convoluta, Platyhelminthes, or flatworms) the sensory cells are concentrated in certain regions and the nervous system has a parallel development. It approaches a central nervous system organization, but it is ventral and without a cavity

At its anterior end, sensory concentration is clear, and there is the beginning of cephalization with cerebral ganglia
Nervous concentration is greatest anterior, and strict parallelism is seen between the anterior concentration of sensory and nervous cells

IN LOWER AND LARVAL FORMS OF ANNELIDS (Serula, Dinophilus) concentration has increased. Segmentation (metamerization) appears. The central nervous system is ventral (bilateral ventral cords), has no cavity and localization of sensory cells seems to be responsible for the segmental concentration which forms the metameric ganglia
THE SUPERIOR AND ADULT ANNELIDS (nereis, Lumbricus): the central nervous system is still ventral (single cord and ganglia) without a cavity, and there is transverse concentration. The cerebral ganglia are specialized for vision and olfaction
THE HEMICHORDATES (enteropneusts): a dorsal tubular and a solid ventral nervous system. There is no cord, but there is a cephalodorsal diverticulum of the intestine corresponding to a dorsal nervous tube. The diverticulum is incompletely separated from the overlying ectoderm from which it is derived
THE PROCHORDATES (Chephalochordata, Amphioxus): a dorsal cord and a dorsal tubular central nervous system entirely developed from dorsal ectoderm. A ventral system is no longer seen. The anterior vesicle represents a true rudimentary brain

Vertebrate nervous system (Chordata): mesoderm determines the development of the nervous system and sense organs. The appearance of mesoderm groups together the inductive capabilities and marks an important stage in the genesis of development

THE NERVOUS SYSTEM is relatively independent of the sensory organization. However, the eye and general sensory receptors, are directly derived from the nervous system

During evolution, relationships between the nervous system and the sense organs are modifie In invertebrates, the nervous system appears to be only an accessory structure to the sensory system. In vertebrates, the importance of the nervous system becomes so great that everything else is organized for it and around it

IN VERTEBRATES SUCH AS FISH, the 5 neural vesicles presage the cephalization of the higher vertebrates. During the slow progression of evolution, the role of the prochordal plate becomes essential since, in humans, it induces brain formation