Editing Nervous system (section)

==Development==
{{Main|Development of the nervous system|Development of the nervous system in humans}}
In vertebrates, landmarks of embryonic [[neural development]] include the [[neurogenesis|birth]] and [[cellular differentiation|differentiation]] of [[neuron]]s from [[stem cells|stem cell]] precursors, the [[cellular migration|migration]] of immature neurons from their birthplaces in the embryo to their final positions, outgrowth of [[axon]]s from neurons and [[axon guidance|guidance]] of the motile [[growth cone]] through the embryo towards postsynaptic partners, the generation of [[synapse]]s between these axons and their postsynaptic partners, and finally the lifelong [[synaptic plasticity|changes]] in synapses which are thought to underlie learning and memory.<ref name=KandelCh52/>

All bilaterian animals at an early stage of development form a [[gastrula]], which is polarized, with one end called the [[animal pole]] and the other the [[vegetal pole]]. The gastrula has the shape of a disk with three layers of cells, an inner layer called the [[endoderm]], which gives rise to the lining of most internal organs, a middle layer called the [[mesoderm]], which gives rise to the bones and muscles, and an outer layer called the [[ectoderm]], which gives rise to the skin and nervous system.<ref name=SanesCh1/>
{| align=center
| [[File:Gray17.png|thumb|center|250px|Human embryo, showing neural groove]]
| [[File:Development of the neural tube.png|thumb|center|425px|Four stages in the development of the neural tube in the human embryo]]
|}

In vertebrates, the first sign of the nervous system is the appearance of a thin strip of cells along the center of the back, called the [[neural plate]]. The inner portion of the neural plate (along the midline) is destined to become the [[central nervous system]] (CNS), the outer portion the [[peripheral nervous system]] (PNS). As development proceeds, a fold called the [[neural groove]] appears along the midline. This fold deepens, and then closes up at the top. At this point the future CNS appears as a cylindrical structure called the [[neural tube]], whereas the future PNS appears as two strips of tissue called the [[neural crest]], running lengthwise above the neural tube. The sequence of stages from neural plate to neural tube and neural crest is known as [[neurulation]].

In the early 20th century, a set of famous experiments by Hans Spemann and Hilde Mangold showed that the formation of nervous tissue is "induced" by signals from a group of mesodermal cells called the ''organizer region''.<ref name=KandelCh52/> For decades, though, the nature of [[neural induction]] defeated every attempt to figure it out, until finally it was resolved by genetic approaches in the 1990s. Induction of neural tissue requires inhibition of the gene for a so-called [[bone morphogenetic protein]], or BMP. Specifically the protein [[BMP4]] appears to be involved. Two proteins called [[Noggin (protein)|Noggin]] and [[Chordin]], both secreted by the mesoderm, are capable of inhibiting BMP4 and thereby inducing ectoderm to turn into neural tissue. It appears that a similar molecular mechanism is involved for widely disparate types of animals, including arthropods as well as vertebrates. In some animals, however, another type of molecule called [[Fibroblast Growth Factor]] or FGF may also play an important role in induction.

Induction of neural tissues causes formation of neural precursor cells, called [[neuroblast]]s.<ref name=KandelCh53/> In ''[[Drosophila]]'', neuroblasts divide asymmetrically, so that one product is a "ganglion mother cell" (GMC), and the other is a neuroblast. A GMC divides once, to give rise to either a pair of neurons or a pair of glial cells. In all, a neuroblast is capable of generating an indefinite number of neurons or glia.

As shown in a 2008 study, one factor common to all [[symmetry (biology)|bilateral]] organisms (including humans) is a family of secreted [[Signalling molecule|signaling molecules]] called [[neurotrophin]]s which regulate the growth and survival of [[neuron]]s.<ref name=Zhu/> Zhu et al. identified DNT1, the first neurotrophin found in [[fly|flies]]. DNT1 shares structural similarity with all known neurotrophins and is a key factor in the fate of neurons in ''Drosophila''. Because neurotrophins have now been identified in both vertebrate and invertebrates, this evidence suggests that neurotrophins were present in an ancestor common to bilateral organisms and may represent a common mechanism for nervous system formation.