Learning aims

After finishing this module students should be able to:

• describe typical features of the central and peripheral nervous systems
• distinguish between primary and secondary neurulation
• summarize the molecular mechanisms that underlie the development of the nervous system
• correlate the formation of the brain vesicle with the structures of the completely developed brain
• name the main functional divisions of the brain and the peripheral nervous system
• explain the histological and functional differentiation of nerve tissue cells (neurons and glial cells)
• describe and interpret the importance of the basic phenomena that occur during brain development (apoptosis, cell migration, splicing)
• explain the structural equivalents between embryonic development of the spinal cord and supraspinal centers
• sketch out blood circulation in the brain

What you should already know

• Embryonic disk
• Embryonic period
• Fetal period

Delving deeper

• The organization of the central nervous system further mirrors the anatomic organization of the body. How do the brain sections that correspond to the various parts of the body differentiate along the neural tube?
• The neuronal circuits arise in steps: the cell bodies of the neurons are formed in various regions of the brain, whereupon they enter into precise functional connections with neurons in other brain regions. Which mechanisms make the setting up of these precise connections possible?
• How are missing or imprecise connections corrected and adapted?
• Which somatic functions are steered by brain activity?
• What information about the body reaches the brain?
• Why are pathologic skin alterations so often associated with pathologies of the nervous system?
• What are the peculiarities of nervous tissue?
• Brain perfusion is based on a supply by end arteries. What dangers for the brain are involved with this system?

Introduction

The formation of the central nervous system from the neural tube takes place via several developmental stadia. The caudal part, out of which the spinal cord arises, serves the innervations of trunk and appendages while the rostral part, which leads to the formation of the brain, innervates the head. The spinal cord exhibits a segmental structure from elements with basically comparable organization while the brain arises from 5 vesicles with fundamentally different structures:

• the three caudal brain vesicles form the brain stem
• the two anterior develop into the hemispheres and to the diencephalon

Over the course of embryonic development the cells of the central nervous system are formed in the inner (ventricular) layer of the neural tube. They gradually leave the division cycle, differentiate into neurons (or glial cells) and migrate into the wall of the neural tube where they collect while forming core areas and cerebral cortex (gray matter).

Numerous cellular mechanisms guide cell migration and axonal growth and thus assure the generation of functionally precise connections between various brain sections. The cerebral nervous tissue exhibits a very high cellular metabolism and for this reason reacts very sensitively to alterations of the extracellular milieu. With vessel lesions or perfusion disorders (thromboembolisms) the extent of the affected brain areas and the functional failures that result from them depend on the supply regions of the corresponding vessels.