Development of the Nervous System

Question 1

What germ layer is the nervous system derived from

Answer 1

The ectoderm specifically the neural plate
- The neural tube → CNS
- The neural crest → PNS and ANS (cranial, spinal and autonomic ganglia)

Question 2

What are the 3 main parts of the nervous system

Answer 2

• The central nervous system (brain and spinal cord)
• The peripheral nervous system (neurons outside the CNS, nerves connecting CNS to peripheral structures)
• The autonomic nervous system (parts in both CNS and PNS, neurons that innervate the smooth and cardiac muscles and glandular epithelium)

Question 3

What structures induce the ectoderm to differentiate

Answer 3

• The notochord and the paraxial mesenchyme to the neural plate
• with the members of the TGF-ß family, Shh and BMPs

Question 4

What regions are established by the signalling molecules?

Answer 4

• BMPs establishes the sensory regions (info from brain → sensory nerves, afferent)
• Shh establishes the motor regions (info from the motor nerves → brain, efferent)

Question 5

Explain the gradient between Shh and BMP on the dorsoventral place of the neural tube

Answer 5

• The notochord and the floor plate of the neural tube secretes high levels of shh
• The dorsoventral concentration of Shh is higher on the floor plate and lower on the roof plate (ventral-to-dorsal)
• a member of the TGF-ß family (BMPs) is highly secreted on the roof plate in the opposing gradient of Shh (dorsal-to-ventral)
• Together, they determine dorsoventral cell fates

Question 6

Explain the development of the nervous system

Answer 6

• The neural tube is formed in the 3rd week & completed by the end of the 4th week (neurulation)
• The anterior neuropore closes on day 25 and the posterior neuropore closes on day 27
• The walls of the neural tube forms the brain and spinal cord
• The neural canal forms the ventricular system of the brain and the central canal of the spinal cord
• The neuroepithelial cells grow and the lumen decreases

Question 7

What 3 types of cells do the neuroepithelial cells produce

Answer 7

• Neuroblasts which become neurons
• Glioblasts which become macroglia (oligodendrocytes and astrocytes)
• cells next to the neural canal become the ependymal cells

Question 8

Explain the differentiation of the neuroepithelial cells

Answer 8

• the walls of the neural tube differentiates into the ventricular, intermediate and the outer marginal layer
• The ependymal cells stay in the ventricular layer
• The neuroblasts and glioblasts differentiate in the intermediate layer
• The marginal zone is the outer parts of the neurons and glial cells

Question 9

Explain the development of the spinal cord

Answer 9

• The neural tube caudal to the 4th pair of somites becomes the spinal cord
• The lateral walls of the neural tube thickens which reduces the size of the canal to be really small at 9-10 weeks
• The roof and floor plates are thin with the sulcus limitans separating the lateral wall wall to a alar (dorsal) and basal (ventral) plate
• Alar plate becomes the dorsal horn of grey matter, neuron cell bodies in the intermediate layer, marginal layer = white matter and it has sensory neurons growing into it (afferent fxn)
• Basal plate becomes the lateral and ventral horns of the grey matter, the neurons grow out of the ventral horn → roots of spinal nerve, it also has motor neurons growing out of it (efferent fxn)

Question 10

Explain the development of the spinal ganglia

Answer 10

• The neural crest cells form the unipolar neurons in the spinal ganglia and the dorsal root ganglia
• The axons of cells in the ganglia are first bipolar then they unite with the unipolar neurons in a T-shaped fashion

Question 11

What is the dorsal root ganglia

Answer 11

• They are the cell bodies of sensory neurons
• They are peripheral processes in the spinal nerves
• This is the unit of affarent fxn as the central processes enter here to get to the dorsal horn of grey matter

Question 12

Explain the differentiation of the spinal nerve

Answer 12

The spinal nerve divides into the:
- Dorsal primary ramy which innervates the limbs, vertebral joints & skin of the back
- Ventral primary ramy which in innervates the limbs and ventral body wall

Question 13

Explain the development of the spinal meninges

Answer 13

• The mesenchyme surrounding the neural tube condenses → primordial meninx (meninges)
• The external layer of the meninges forms the dura mater
• The internal layer of the meninges (derived from neural crest cells)forms the pia-arachnoid mater also known as the (leptomeninges)
• The cerebrospinal fluid (CSF) beings to form in the 5th week

Question 14

Explain the positional changes of the spinal cord

Answer 14

• The vertebral column grows faster than the spinal cord making its position in it change
• The spinal cord lies at higher levels in the vertebral column making it end in the L1 position in adults

Question 15

myelination of nerve fibers

Answer 15

• It begins in the late fetal period and continues during the 1st yr after birth
• in the PNS, myelin is formed by the Schwann cells but in the CNS, myelin is formed by the oligodendrocytes
• tracks become fxnal when myelinated