Changes

===Neural Tube Regionalisation===

===Neural Tube Regionalisation===

The neural tube undergoes regionalisation to define the anterior as the brain, and the posterior as the [[Spinal Cord - Anatomy & Physiology|spinal cord]]. In the anterior, the neural tube swells into the three ventricles of the brain; the forebrain; midbrain; and hindbrain. This occurs as fluid is pumped into the lumen of the neual tube. Pressure is regionalised to the anterior by occlusion of the neural tube at the base of the hindbrain. The occlusion reopens after enlargement of the ventricles.

The neural tube undergoes regionalisation to define the anterior as the brain, and the posterior as the [[Spinal Cord - Anatomy & Physiology|spinal cord]]. In the anterior, the neural tube swells into the three ventricles of the brain: the forebrain, midbrain, and hindbrain. This occurs as fluid is pumped into the lumen of the neural tube. Pressure is regionalised to the anterior by occlusion of the neural tube at the base of the hindbrain. The occlusion reopens after enlargement of the ventricles.

===Formation of Cell Layers===

===Formation of Cell Layers===

The neural tube begins with the germinal layer which is a single cell layer thick. All cells have the capacity to undergo mitosis.

The neural tube begins with the germinal layer which is a single cell layer thick. All cells have the capacity to undergo mitosis.

Some cells then lose their ability to undergo mitosis, they detach from the germinal layer and move away from the lumen, to exist behind the germinal layer. This produces new layers called the '''mantle and intermediate layer'''. The germinal layer is now known as the '''ventricular layer'''. Cells of the intermediate layer differentiate into one of two cells:

Some cells then lose their ability to undergo mitosis, they detach from the germinal layer and move away from the lumen to exist behind the germinal layer. This produces new layers called the '''mantle and intermediate layer'''. The germinal layer is now known as the '''ventricular layer'''. Cells of the intermediate layer differentiate into one of two cells:

1. '''[[PNS Structure - Anatomy & Physiology#Nerve Fibre|Neurones]]''' - for conductance of nerve impluses.

# '''[[PNS Structure - Anatomy & Physiology#Nerve Fibre|Neurones]]''' - for conductance of nerve impulses.

 

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

2. '''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#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.

In the brain, futher 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.

==Organisation of the Nervous System==

==Organisation of the Nervous System==

===Autonomic Nervous System===

===Autonomic Nervous System===

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 it's 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|right|thumb|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|right|thumb|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 crest 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 ther 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 crest 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:

'''Anterior Neural Crest''' forms; sensory neurones and [[PNS Structure - Anatomy & Physiology#Schwann Cells|schwann cells]]; [[Bones - Anatomy & Physiology|bones]] of the head; dermis of the face; and [[Cartilage - Anatomy & Physiology|cartilage]].

'''Anterior Neural Crest''' forms:

:sensory neurones and [[PNS Structure - Anatomy & Physiology#Schwann Cells|schwann cells]]

:[[Bones - Anatomy & Physiology|bones]] of the head

:dermis of the face

:[[Cartilage - Anatomy & Physiology|cartilage]]

'''Trunk Neural Crest''' forms; sensory neurones of the somatic and autonomic nervous systems (ANS); sympathetic motor neurones of the ANS and melanocytes.

'''Trunk Neural Crest''' forms:

:sensory neurones of the somatic and autonomic nervous systems (ANS)

:sympathetic motor neurones of the ANS

:melanocytes

'''Posterior Neural Crest''' forms; parasympathetic motor neurones of the [[Alimentary System Overview - Anatomy & Physiology#Small Intestine|gut]], which control peristalsis.

'''Posterior Neural Crest''' forms:

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

==Ectodermal Placodes==

==Ectodermal Placodes==

[[Category:Developmental Biology]][[Category:Nervous System - Anatomy & Physiology]]

[[Category:Developmental Biology]][[Category:Nervous System - Anatomy & Physiology]]

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