Chapter 12
The Central Nervous System
475
12
expand dorsally and ventrally to produce the H-shaped central
mass of gray matter in the adult spinal cord.
Neural crest cells that come to lie alongside the cord form the
dorsal root ganglia
containing sensory neuron cell bodies. Tese
sensory neurons send their axons into the dorsal aspect of the cord.
As the brain and spinal cord grow and mature throughout the
prenatal period, gender-specific areas appear. For example, cer-
tain hypothalamic nuclei concerned with regulating typical male
sexual behavior and clusters of neurons in the spinal cord that
serve the external genitals are much larger in males. Females have
a larger corpus callosum, and gender differences are seen in the
language and auditory areas of the cerebral cortex. Te key to CNS
gender-specific development is whether or not the fetus is secret-
ing testosterone. If it is, the male pattern develops.
±o visualize the location of the clot and the catheter, dye is in-
jected to make arteries stand out in an X ray, a procedure called
cerebral angiography
. Cerebral angiography can also help pa-
tients who have had a warning stroke, or ±IA.
Another test to assess risk of a CVA uses ultrasound. Te
carotid arteries of the neck, which feed most of the cerebral ves-
sels, o²en narrow with age, which can lead to strokes. Cheaper
and less invasive than angiography, ultrasound can be used to
quickly examine the carotid arteries and even measure blood
flow through them.
Check Your Understanding
25.
Roy was tackled while playing football. After hitting the
ground, he was unable to move his lower limbs. What is
a loss of motor function called? What level of his spinal
cord do you think was injured (cervical, thoracic, lumbar,
or sacral)? Is this a permanent injury? What diagnostic
procedures might be helpful?
For answers, see Appendix H.
Developmental Aspects
of the Central Nervous System
Describe the development of the brain and spinal cord.
Indicate several maternal factors that can impair
development of the nervous system in an embryo.
Explain the effects of aging on the brain.
Figure 12.33
shows the earliest phase of CNS development.
Starting in the three-week-old embryo:
1
Te ectoderm (cell layer at the dorsal surface) thickens
along the dorsal midline axis of the embryo to form the
neural plate
. Te neural plate invaginates, forming a
neu-
ral groove
flanked by
neural folds
.
2
Small groups of neural fold cells migrate laterally from be-
tween the surface ectoderm and the neural groove, forming
the
neural crest
. Neural crest cells give rise (among other
things) to some neurons destined to reside in ganglia.
3
As the neural groove deepens, the superior edges of the neu-
ral folds fuse, forming the
neural tube
, which soon detaches
from the surface ectoderm and sinks to a deeper position.
Te neural tube, formed by the fourth week of pregnancy,
differentiates rapidly into the CNS. Te brain forms rostrally, as
we described on p. 429, and the spinal cord develops from the
caudal portion of the neural tube.
By the sixth week, each side of the developing spinal cord has
two recognizable clusters of neuroblasts that have migrated out-
ward from the original neural tube: a dorsal
alar plate
(a
9
lar) and a
ventral
basal plate
(Figure 12.34)
. Alar plate neuroblasts become
interneurons. Basal plate neuroblasts develop into motor neurons
and sprout axons that grow out to the effector organs. Axons that
emerge from alar plate cells (and some basal plate cells) form the
white matter of the cord. As development progresses, these plates
1
The neural plate forms from surface ectoderm. It then invaginates,
forming the neural groove flanked by neural folds.
2
Neural fold cells migrate to form the neural crest, which will form
much of the PNS and many other structures.
Head
Tail
Surface
ectoderm
Neural fold
forming
Surface
ectoderm
Neural plate
Neural crest
Head
Tail
Neural
tube
Neural
groove
3
The neural groove becomes the neural tube, which will form CNS
structures.
Figure 12.33
Development of the neural tube from
embryonic ectoderm.
Left: Dorsal surface views of the embryo.
Right: Transverse sections at days 17, 20, and 22.
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