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== Introduction ==

== Introduction ==

== Neuralation ==

== Neuralation ==

=== Primary Neuralation ===

=== Primary Neurulation ===

Cells of the neural plate proliferate, causing the neural plate to thicken. Cells then converge at the midline and become wedge shaped, which drives the neural plate to become long and narrow. Cells along the midline descend ventrally and contact the notochord, forming a "hinge". This forms a depression which is called the '''neural groove'''. Either side of the neural groove, ectoderm converges towards the midline. This causes elevations either side of the neural groove, called '''neural folds'''. Hinges form at the foot of each neural fold, allowing the folds to be brought together, forming a tube. The tube sinks, as non - neural ectoderm fuses above the neural tube, "zipping" the neural tube up. The non - neural ectoderm will form epidermis. Failure of the neural tube to close causes '''spina bifida''' (spinal cord protrusion) and '''exencephaly''' (brain located out of the skull).  

Cells of the neural plate proliferate, causing the neural plate to thicken. Cells then converge at the midline and become wedge shaped, which drives the neural plate to become long and narrow. Cells along the midline descend ventrally and contact the notochord, forming a "hinge". This forms a depression which is called the '''neural groove'''. Either side of the neural groove, ectoderm converges towards the midline. This causes elevations either side of the neural groove, called '''neural folds'''. Hinges form at the foot of each neural fold, allowing the folds to be brought together, forming a tube. The tube sinks, as non - neural ectoderm fuses above the neural tube, "zipping" the neural tube up. The non - neural ectoderm will form epidermis. Failure of the neural tube to close causes '''spina bifida''' (spinal cord protrusion) and '''exencephaly''' (brain located out of the skull).

=== Secondary Neuralation ===

=== Secondary Neurulation ===

Secondary neuralation occurs posteriorly, from the lumbar level onwards. The neural plate descends ventrally and medially. Cells of the neural plate condense and form a solid rod called the '''medullary cord'''. It is then made hollow by cavitations which join up to make a lumen. Tubes formed by primary and secondary neuralation fuse together to make one continuous tube called the '''neural tube'''.  

Secondary neuralation occurs posteriorly, from the lumbar level onwards. The neural plate descends ventrally and medially. Cells of the neural plate condense and form a solid rod called the '''medullary cord'''. It is then made hollow by cavitations which join up to make a lumen. Tubes formed by primary and secondary neuralation fuse together to make one continuous tube called the '''neural tube'''.

== Neural Tube Development ==

== Neural Tube Development ==

#'''Glia''' - for insulation of electrical signals.

#'''Glia''' - for insulation of electrical signals.

Glia form oligodendrocytes in the CNS, and [[PNS Structure - Anatomy & Physiology#The_Schwann_Cell|schwann cells]] in the PNS. Glial cells surround the axons furthest away form the lumen of the neural tube. Because this part is myelinated, it appears ''white'' and is called the '''marginal layer'''. Axons closer to the lumen of the neural tube, which the glial cells do not surround appear ''grey''. This layer is called the '''intermediate layer'''. The ventricular layer persists, so that the spinal cord develops with these three layers.  

Glia form oligodendrocytes in the CNS, and [[PNS Structure - Anatomy & Physiology#Schwann Cell|schwann cells]] in the PNS. Glial cells surround the axons furthest away form the lumen of the neural tube. Because this part is myelinated, it appears ''white'' and is called the '''marginal layer'''. Axons closer to the lumen of the neural tube, which the glial cells do not surround appear ''grey''. This layer is called the '''intermediate layer'''. The ventricular layer persists, so that the spinal cord develops with these three layers.  

In the brain, further layers are added to increase complexity. This is possible because in the anterior of the embryo, neuroblasts (cells of the ventricular layer) retain their ability to undergo mitosis after leaving the ventricular layer. This difference is achieved by increased hydrostatic forces acting on the neuroblasts.  

In the brain, further layers are added to increase complexity. This is possible because in the anterior of the embryo, neuroblasts (cells of the ventricular layer) retain their ability to undergo mitosis after leaving the ventricular layer. This difference is achieved by increased hydrostatic forces acting on the neuroblasts.  

Sensory neurones also enter the CNS through the dorsal root, with their nuclei in DRG. In contrast to the somatic nervous system, the motor component is in two parts; a preganglionic and postganglionic neurone. The preganglionic neurone has its cell body in the CNS. It exits via the ventral root and synapses with the postganglionic neurone. Cell bodies of the postganglionic neurone lie outside the CNS in ganglia. Ganglia of the ''sympathetic'' nervous system are near the '''spinal cord'''. Ganglia of the ''parasympathetic'' nervous system are near the '''target organ'''.  

Sensory neurones also enter the CNS through the dorsal root, with their nuclei in DRG. In contrast to the somatic nervous system, the motor component is in two parts; a preganglionic and postganglionic neurone. The preganglionic neurone has its cell body in the CNS. It exits via the ventral root and synapses with the postganglionic neurone. Cell bodies of the postganglionic neurone lie outside the CNS in ganglia. Ganglia of the ''sympathetic'' nervous system are near the '''spinal cord'''. Ganglia of the ''parasympathetic'' nervous system are near the '''target organ'''.  

== Neural Crest ==

== Neural Crest ==

[[Image:Neual crest.png|thumb|right|250px|Mithril 2010  Cross section scheme of vertebrate embryo. 1 – neural crest, 2 – neural tube, 3 – dorsal somite, 4 – notochord. Both paths of the neural crest cells migation marked by arrows: red one is the dorsolateral path, blue one signs the ventromedial path.]]  

[[Image:Neual crest.png|thumb|right|250px|Mithril 2010  Cross section scheme of vertebrate embryo. 1 – neural crest, 2 – neural tube, 3 – dorsal somite, 4 – notochord. Both paths of the neural crest cells migation marked by arrows: red one is the dorsolateral path, blue one signs the ventromedial path.]]  

The neuroectoderm is the region at the junction of epidermis and neural plate. When the neural tube closes, the epidermis is brought into contact with the crest neuroectoderm. This contact causes cells of the crest neuroectoderm to undergo an epithelial to mesenchymal transition. Cells that undergo this transition break away from the neuroectoderm and are called '''neural crest cells'''. The peripheral nervous system (PNS) develops from the neural crest. The undifferentiated neural crest cells migrate away from the neural tube along "pathways". These pathways are governed by repulsive cues from the cells, as well as proliferation and survival signals. They differentiate once they reach their destination. Other non - neural tissues also develop from the neural crest. The type of tissue that neural crest forms is dependant on the origination of the cell along the anterior - posterior axis:  

The dorsal neural fold is the site where the neural ectoderm meets the epidermal ectoderm. Signaling at the border between these two tissues leads to the induction of the Neural Crest Cells, which then delaminate (epithelial to mesenchymal transition), migrate to their target sites and differentiate based on the local environmental cues (thus, their fate is plastic). The delamination and migration of neural crest cells occurs during the embryonic stage of neurulation, but is species specific and may occur while the neural tube is still open (mouse and opossum) or closed (chick). In several frog species analyzed, this migration occurs at all phases of neurulation so is not standard for any particular group of vertebrates.  Migratory pathways are highly conserved across all vertebrate groups, with streams going toward the frontonasal region and branchial regions in the cranial region (with the regions lateral to the negatively numbered rhombomeres of the hindbrain free of neural crest streams). These streams are directed based on inhibitory signals in the mesenchyme (mesodermal in origin) lateral to the neural tube (rhombomere 3) or are physically blocked, such as lateral to rhombomere 5 with the otic vesicle. Apoptotic signals also play a role in the migration of these cells. The peripheral nervous system (PNS) develops from the neural crest. The cells differentiate once they reach their destination. Other non - neural tissues also develop from the neural crest. Though previously thought that only the cranial neural crest streams can form skeletal (mesodermal types) of derivatives and the trunk only neural types, recent work over the last decade has shown the extreme plasticity in cell fates of these cells along the anterior-posterior axis. Generally:  

'''Anterior Neural Crest''' forms:  

'''Anterior Neural Crest''' forms:  

:parasympathetic motor neurones of the [[Alimentary System Overview - Anatomy & Physiology#Small_Intestine|gut]], which control peristalsis.

:parasympathetic motor neurones of the [[Alimentary System Overview - Anatomy & Physiology#Small_Intestine|gut]], which control peristalsis.

== Ectodermal Placodes ==

== Ectodermal Placodes ==

The ectoderm thickens in places, with cells changing from cuboidal to columnar. These regions are called '''placodes'''. Placodes may be neurogenic or non - neurogenic. Neurogenic placodes are only found in the head, forming only sensory cells. Morphogenic changes produce the following derivatives:  

The ectoderm thickens in places, with cells changing from cuboidal to columnar. These regions are called '''placodes'''. Placodes may be neurogenic or non - neurogenic. Neurogenic placodes are only found in the head, forming only sensory cells. Morphogenic changes produce the following derivatives:  

'''Sensory Ganglion of Cranial Nerves''' - epithelial to mesenchymal transition.  

'''Sensory Ganglion of Cranial Nerves''' - epithelial to mesenchymal transition.  

[[Category:Developmental_Biology]] [[Category:Nervous_System_-_Anatomy_&_Physiology]] [[Category:A&P_Done]] [[Category:David_Hogg_reviewed]]

[[Category:Developmental_Biology]] [[Category:Nervous_System_-_Anatomy_&_Physiology]] [[Category:A&P_Done]] [[Category:David_Hogg_reviewing]]