The human brain consists of 100 billion
neurons and over 100 trillion synapses. The ability of the neurons
to form precise connections with one another is critical for the
proper functioning of the nervous system. Malformation or dysfunction
affecting a subset of cells and/or connections in the nervous
system can manifest in diverse ways through different neurologic
symptoms. Examining a patient who presents with a neurologic problem should
lead to the understanding of two main aspects of the dysfunction:
What part(s) of the nervous system is affected? What is the nature
of the dysfunction? This approach requires an understanding of the
localization of function within the nervous system. From a pediatric neurology perspective,
this may be best achieved by reviewing the development of nervous
system in early life.
Work over the last few decades has shed light on the remarkable cellular
and molecular bases of neural development. While much more needs
to be elucidated, this chapter will highlight some of the major
advances in our understanding of normal human brain development.
The chapter is organized into 3 parts. The first part will review
the process of neural tube formation and regional patterning that
determines the major subdivisions of the human central nervous system.
The second part will provide an overview of the specific steps required
for proper wiring of neurons in the cerebral cortex, the part of
the brain responsible for most of human behavior. The final part
will review the major subdivisions of the human central nervous system
and their most prominent functions.
Normal neural development can be considered in several steps.
In chronological order, these include: neurulation, regionalization,
neurogenesis, migration, differentiation, apoptosis, axon guidance, synapse
formation/pruning, and myelination. In these 9 steps, cells go
from unspecified ectodermal constituents to being fine-tuned at the
The nervous system develops from a region of the ectoderm called
the neural plate. Underlying notochord and adjacent mesoderm induce
the overlying ectoderm to differentiate into the neural plate during
the third week of gestation. Induction is followed by invagination
of the neural plate along its central axis to form a neural groove
that has neural folds on each side. By the end of the third week,
the neural folds start to fuse at the hindbrain/cervical
boundary and form the neural tube, which gives rise to the brain
and the spinal cord.1 Fusion then proceeds from
this level in both rostral and caudal directions. Closure of the
neural tube is finally completed by the closure of the neuropore
at the sacral region at day 26 to 28. As the neural folds fuse,
some neuroectodermal cells detach from the folds and migrate ventrally,
forming the neural crest. The neural crest gives rise to the dorsal
root ganglia, autonomic nervous system ganglia, Schwann cells, meninges,
adrenal medulla, and several key skeletal and muscular components
of the face.