The development of the nervous system

• Ectoderm gives rise to entire nervous system
• During the 3rd week ectoderm forms the neural plate
• A longitudinal groove is developed on the neural plate called neural groove & as neural groove deepens, either side of it is called the neural fold
• Neural folds fuse, converting the neural groove into a neural tube
• Neural tube has 2 openings : Anterior & Posterior neuropore 
• Anterior(cranial) neuropore closes first & 2 days later posterior(caudal) neuropore is closed (Neural tube closure is complete in 28 days)
• A strip of ectoderm is formed from the lateral margin of the neural plate, called neural crest 

• Neural crest differentiate into the cells of:

1. Posterior root ganglia

2. Sensory ganglia of the cranial nerves

3. Autonomic ganglia

4. Cells of supra renal medulla

5. Melanocytes

• The cephalic end of the neural tube dilates to form 3 primary brain vesicles (Forebrain, Midbrain vesicle & Hindbrain vesicle) 
• Rest of the tube elongates to form the spinal cord

Development of the spinal cord

• The wall of the neural tube consists of a single layer of pesudostratified columnar epithelial cells, called matrix cells
• Matrix cells rapidly divide & produce neuroblasts  
• Neuroblasts migrate peripherally to form the intermediate zone; it will form the grey matter of the spinal cord 
• Intermediate zone forms 2 thickened areas, separated by sulcus limitans

1. Anteriorly : Basal plate (Produce motor cells of the anterior column)
2. Posteriorly : Alar plate – Small (Produce sensory cells of the posterior column)

• Neuroblasts will give rise to nerve fibers that grow peripherally & form a layer external to the intermediate zone, forming the marginal zone; it will form the white matter of the spinal cord 
• Matrix cells also produce

1. Astrocytes
2. Oligodendrocytes
3. Microglial cells
4. Ependymal cells (Line the neural tube)

• The continuous growth of basal plates on either side will form anterior median fissure
• The continuous growth of alar plates on either side will form posterior median septum
• The lumen of the neural tube becomes the central canal

 

Development of spinal nerves

 
   

Anterior end

• The medial group of motor neurons will leave the anterior surface of the spinal cord as the motor outflow
• The lateral group of neurons will leave the anterior surface of the spinal cord as the autonomic Preganglionic fibers; they form the lateral grey column (T1 – L2) – That is the sympathetic outflow
• The 2 outflows leave the anterior surface of the spinal cord & form the Ventral root of the spinal nerves

Posterior end

• Neural crest cells migrate to a posterolateral position on either side of the developing spinal cord 
• They differentiate into neuroblasts & satellite (Capsular) cells 
• Each neuroblast develops 2 processes
• Central process
• Grow into the posterior part of the developing spinal cord
• These central processes are collectively called as posterior root of the spinal nerve
• Peripheral process
• Grow out laterally & form the axon of sensory nerve fibers
• Peripheral processes join the anterior root to form the spinal nerve

Development of the spinal cord in relation to the vertebral column

• Developing vertebral column grows faster than the spinal cord
• At birth the coccygeal end of the cord lies at the level of the L₃ vertebra
• When a LP is performed in a small child, need to use a level below L₃ up to S₂ (SAS ends there)
• As a result of the development of limb buds during the 4^th month, additional sensory & motor neurons are formed (Produces cervical & lumbar enlargements of the spinal cord)
• In adult spinal cord itself is shorter than vertebral column and it usually end at L1-2