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This chapter provides a discussion on brain development. It specifically determines the general principles that function in limiting the growth of functionality at the cellular level. It also considers that even at the cellular level, two important factors, activity dependence and context-dependent interactions, shape the morphology and function of individual neurons. It explores the very beginnings of the central nervous system. In its early stages, from the generation of neural precursor cells to neurite outgrowth, the development of the nervous system is both activity-dependent and activity-independent. Moreover, some early aspects of synapse formation are also activity-dependent. Specifically discussed are the influences of activity on the subcellular level, neurite outgrowth and morphology, and connections between cells. The result that individual cells have their function shaped in response to that of their neighbours (their functional context) has implications for interpreting cellular activity in terms of representations.

This chapter concentrates on the characteristics of Nogo-A and its signaling pathway components in neurite growth inhibition and describes mechanisms of central nervous system (CNS) repair after Nogo-A blockade and its implications for clinical application. This chapter shows that targeting the signaling components by appropriate blockers can prevent growth cone collapse and growth inhibition induced by Nogo-A and several other inhibitors. It also suggests that neutralization of the most potent of inhibitory molecules, Nogo-A, in animal models of spinal cord injury (SCI) and stroke has been shown to be effective in triggering axonal repair mechanisms and behavioral improvements.

The nervous system architecture of the Acoelomorpha is very diverse, ranging from a simple basiepidermal nerve plexus over basiepidermal condensations into neurite bundles and—in more derived lineages—an internalization of brain and cords below the muscle sheet. Characteristic for this animal taxon is the lack of a stomatogastric nervous system. The chapter describes details in the different organizations of nemertodermatid and acoel nervous systems and compares them with those of other taxa. It describes a step-wise internalization of the nervous system during evolution.

Gnathostomulida is a taxon with about 100 described species to date. Species live in the marine interstitial system and have a worldwide distribution. The general appearance is more or less flatworm-like, but it is their conspicuous jaw apparatus that gives the taxon its name. There is a brain in the anterior body region, it has a central neuropil and a covering of peripheral somata. Up to six longitudinal neurite bundles run into the body, while in some species they all appear equal, in other species they are of different length and thickness, with the ventrolateral pair being dominant. Fine transverse neurites may be present, connecting the longitudinal bundles. One further element is a buccal ganglion that is positioned posterior of the jaw apparatus. Gnathostomulids contain a number of different sensory structures, in particular in the epidermis. These range from sensory ‘bristles’ to intraepidermal spiral ciliary receptors. Only two types of neurotransmitters have been successfully identified in gnathostomulids, using immunohistochemistry: the peptide family FMRFamides and the monoamine 5-HT (serotonin).