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U N I T 1 0
Nervous System
Developmental Organization
of the Nervous System
The organization of the nervous system can be under-
stood in terms of its embryonic development, in which
newer functions and greater complexity result from
the modification and enlargement of earlier developed
structures. Thus, the rostral or front end of the CNS,
which is the last to develop, is more specialized in its
functions than the caudal or tail end structures, namely
the brain stem and spinal cord, which are the first to
develop. The dominance of the rostral end of the CNS
is reflected in what has been termed a hierarchy of con-
trol, with the forebrain having control over the brain
stem and the brain stem having control over the spi-
nal cord. In the developmental process, newer functions
were added to the surface of earlier developed systems.
As newer functions were added to the surface of earlier
developed systems and became concentrated at the ros-
tral end of the CNS, they also became more vulnerable
to injury. Nothing exemplifies this principle better than
the persistent vegetative state (discussed in Chapter 37)
that occurs when severe brain injury causes irreversible
damage to higher cortical centers, while lower brain
stem centers such as those that control breathing remain
functional.
Embryonic Development
The nervous system appears very early in embryonic
development (22 to 23 days). This early development
is essential because it influences the development and
organization of many other body systems, including the
axial skeleton, skeletal muscles, and sensory organs such
as the eyes and ears. Throughout life, the organization
of the nervous system retains many patterns that were
established during embryonic life.
During the 2nd week of development, embry-
onic tissue consists of two layers, the endoderm and
the ectoderm. At the beginning of week 3, the ecto-
derm begins to invaginate and migrates between the
two layers, forming a third layer called the
mesoderm
(Fig. 34-5). Mesoderm along the entire midline of the
embryo forms a specialized rod of embryonic tissue
called the
notochord
. The notochord and adjacent
mesoderm provide the necessary induction signal for
the overlying neuroectoderm to differentiate and form
a thickened structure called the
neural plate
. Within
the neural plate a groove develops and sinks into the
underlying mesoderm. Its walls fuse across the top,
forming an ectodermal tube called the
neural tube
.
The neuroectoderm of the neural plate gives rise to the
brain and spinal cord of the CNS, while the notochord
becomes the foundation around which the vertebral
column develops. The surface ectoderm separates from
the neural tube and fuses over the top to become the
outer layer of skin.
This process involved in the formation of the neural
plate and neural folds and closure of the folds begins at
the cervical and high thoracic levels and zippers both
rostrally and caudally. Complete closure occurs at the
rostral-most end of the brain around day 25 and at
about day 27 in the lumbosacral region. Most congeni-
tal defects, such as spina bifida, result from failure of
fusion of one or more neural arches of the developing
vertebral column during the fourth week of embryonic
development.
As the neural tube closes, ectodermal cells called
neural crest cells
migrate away from the dorsal surface
of the neural tube to become the progenitors or parent
cells of the neurons and neuroglial cells of the PNS.
Some of these cells gather into clusters to form the
dor-
sal root ganglia
at the sides of each spinal cord segment
and the
cranial ganglia
that are present in most brain
segments. Neurons of these ganglia become the affer-
ent or sensory neurons of the PNS. Other neural crest
cells become the pigment cells of the skin or contribute
to the formation of many structures of the face, cer-
tain cells of the autonomic nervous system, and other
structures.
During development, the more rostral (toward the
head) portions of the embryonic neural tube—approxi-
mately 10 segments—undergo extensive modifica-
tion and enlargement to form the brain (Fig. 34-6). In
the early embryo, 3 swellings, or primary vesicles,
develop, subdividing these 10 segments into the fore-
brain, containing the first 2 segments; the midbrain,
which develops from segment 3; and the hindbrain,
which develops from segments 4 to 10. In the prosen-
cephalon or forebrain, two pairs of lateral outpouchings
develop: the optic cup, which becomes the optic nerve
and retina, and the telencephalic vesicles, which become
the cerebral hemispheres. Within the prosencephalon,
the hollow central canal expands to become enlarged
■■
Neurotransmitters can produce either excitatory
or inhibitory effects and can be broadly
categorized into three groups based on their
chemical structure: (1) amino acids (e.g.,
glutamic acid, glycine, and GABA) that serve
as neurotransmitters at most CNS synapses;
(2) peptide neurotransmitters (e.g., endorphins
and enkephalins) that are involved in pain
perception and sensation; and (3) monoamines
(e.g., epinephrine and norepinephrine) that serve
as neurotransmitters for the ANS.
■■
Neuromodulators bring about long-term changes
that subtly enhance or depress the action of
target receptors. Neurotrophic factors are
polypeptides that influence the proliferation,
differentiation, and survival of neuronal and
nonneuronal cells.They are secreted by axon
terminals.
SUMMARY CONCEPTS
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