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Summary NEUROLOGY 1- USMLERX BRICKS
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neurology for the step 1 USMLE exams
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Development of theNervous System
By ScholarRx
Updated May 26, 2021
access_time14 min
Learning Objectives (6)
After completing this brick, you will be able to:
● Describe the development of the brain, including the five brain
vesicles, the brainstem, and the cerebral cortex.
● 1
● Describe how the spinal cord develops from the neural tube, and
describe the contributions of each to these layers to the structure of
the mature spinal cord, with emphasis on the alar and basal plates.
● 2
● Explain how the neural tube is organized into three layers.
● 3
● Describe the origin, migration, and fate of neural crest cells,
particularly those that form the peripheral nervous system.
● 4
● Define neurulation and explain how it occurs, including closure and
the dates of neuropore closure.
● 5
● List the derivatives of ectoderm.
● 6
cableCASE CONNECTION
,In a GI specialty clinic at a university pediatric hospital, you are seeing
CG, a 5-year-old girl with severe constipation. CG was recently
diagnosed with Hirschsprung disease, a condition in which there is
lack of innervation of regions of the intestine due to defects in neural
crest cell development and migration. The attending says, “I know this
is tough, but I want you to explain to CG’s parents how this
developed. It’s good practice to explain things to family members in
ways they can understand.”
How will you explain to CG’s parents the underlying etiology of her
condition? Consider your answer as you read, and we’ll revisit at the
end of the brick.
GO TO CONCLUSION arrow_downward
How Does the Nervous System
Develop?
Early in fetal development, the precursors of the major systems in the
body are outlined. The three germ layers (endoderm, mesoderm,
ectoderm) are formed during the third week of development. We’ll
focus on the ectoderm, from which the entire nervous system (central
and peripheral) forms.
But first, let’s back up to review the anatomy of the central nervous
system (CNS) and peripheral nervous system (PNS) so that we know
what the end products of their development are. The CNS comprises
the brain and spinal cord; the PNS is composed of the 31 pairs of
spinal and 12 pairs of cranial nerves and all the ganglia (Figure 1).
,What Nervous System Structures
Does Ectoderm Give Rise to?
The nervous system is one of many structures formed from ectoderm
(Figure 2).
Let’s review derivatives of the surface ectoderm and neuroectoderm
to see how the nervous system fits into the picture. Two mnemonics
will help keep this straight.
What Is Neurulation?
Once the ectoderm is formed, the nervous system gets right to work
and starts to develop that same week.
Neurulation is the process of formation of the earliest neurologic
structures, the neural plate, followed by neural tube and neural crest
(Figure 3). Neurulation starts with induction signals from the
underlying notochord and paraxial mesoderm, which cause the
midline ectoderm to thicken into a neural plate (step 1). Further
induction signals cause the neural plate to invaginate (step 2), while
its edges converge and fuse (close) dorsally to form the neural tube.
As this is taking place, the ectoderm next to the lateral edges of the
neural plate becomes the neural crest, which is pinched off from both
the ectoderm and neural tube by the end of this process (step 3). After
neural tube closure, neural crest cells migrate peripherally to form
different structures, such as the spinal ganglion (step 4).
QUIZ
, Figure 3
Credit: ©ScholarRx
The end result is that the wall of the neural tube forms the substance
of the CNS (the spinal cord and brain). Its lumen forms the central
canal of the spinal cord and the ventricular system in the brain. The
neural crest forms most of the PNS (spinal and cranial nerves and
ganglia).
Neuropore Closure
Let’s examine closure of the neural tube more closely, because
abnormal closure can lead to a variety of congenital anomalies.
Closure does not move continuously from cranial to caudal or vice
versa. Instead it begins at the halfway point on the craniocaudal axis
of the neural plate (Figure 4). From this midpoint, the tube
sequentially closes caudally and cranially, with the most cranial and
caudal aspects—the cranial and caudal neuropores—remaining open
until last. They will ultimately close around day 25 and day 27 (week
4), respectively. In all, the neural tube is created in about 1 month.
QUIZ
Figure 4
Credit: ©Schol
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