476
UNIT 3
Regulation and Integration of the Body
12
Spina bifida
(spi
9
nah bif
9
ĭ-dah; “forked spine”) results from
incomplete formation of the vertebral arches and typically in-
volves the lumbosacral region. Te technical definition is that
laminae and spinous processes are missing on at least one verte-
bra. If the condition is severe, neural deficits occur as well.
Spina bifida occulta
, the least serious type, involves one or
only a few vertebrae and causes no neural problems. Other than
a small dimple or tuF of hair over the site of nonfusion, it has no
external manifestations.
In
spina bifida cystica
, the more common and severe form,
a saclike cyst protrudes dorsally from the child’s spine. Te
cyst may contain meninges and cerebrospinal fluid [a
menin-
gocele
(mĕ-ning
9
go-sēl)], or even portions of the spinal cord
and spinal nerve roots (a
myelomeningocele
, “spinal cord in
a meningeal sac,”
Figure 12.35
). Te larger the cyst and the
more neural structures it contains, the greater the neurological
impairment. In the worst case, where the inferior spinal cord
is functionless, the infant experiences bowel incontinence,
bladder muscle paralysis (which promotes urinary tract infec-
tion and kidney failure), and lower limb paralysis. Infections
occur continually because the cyst wall is thin and porous and
tends to rupture or leak. Hydrocephalus accompanies spina
bifida cystica in 90% of cases.
In the past, up to 70% of cases of spina bifida were caused by
inadequate amounts of the B vitamin folic acid in the maternal
diet. Te incidence of spina bifida has dropped significantly in
countries (such as the U.S.) that have introduced mandatory sup-
plementation of folic acid in bread, flour, and pasta products.
One of the last CNS areas to mature is the hypothalamus.
Since the hypothalamus contains body temperature regulatory
centers, premature babies have problems controlling their loss
of body heat and must be kept in temperature-controlled envi-
ronments. PE± scans reveal that the thalamus and somatosen-
sory cortex are active in a 5-day-old baby, but the visual cortex
is not. Tis explains why infants of this age respond to touch but
have poor vision. By 11 weeks, more of the cortex is active, and
the baby can reach for a rattle. By 8 months, the cortex is very
active and the child can think about what he or she sees.
Maternal exposure to radiation, various drugs (alcohol, opi-
ates, and others), and infections can harm a developing infant’s
nervous system, particularly during the initial formative stages.
²or example, rubella (German measles) oFen leads to deafness
and other types of CNS damage in the newborn. Smoking de-
creases the amount of oxygen in the blood, and lack of oxygen
for even a few minutes destroys neurons. A mother who smokes
may sentence her infant to some degree of brain damage.
Homeostatic Imbalance
12.11
Cerebral palsy
is a neuromuscular disability in which the vol-
untary muscles are poorly controlled or paralyzed as a result of
brain damage. It may be caused by temporary lack of oxygen
during a difficult delivery, or by any of the factors listed above.
In addition to spasticity, speech difficulties, and other motor
impairments, about half of cerebral palsy victims have seizures,
half are mentally retarded, and about a third have some degree
of deafness. Visual impairments are also common. Cerebral
palsy does not get worse over time, but its deficits are irreversi-
ble. It is the largest single cause of physical disability in children,
affecting three out of every 1000 births.
Te CNS is plagued by a number of other congenital malfor-
mations triggered by genetic or environmental factors during
early development. Te most serious are congenital hydroceph-
alus (discussed previously), anencephaly, and spina bifida.
In
anencephaly
(an
0
en-sef
9
ah-le; “without brain”), the cer-
ebrum and part of the brain stem never develop because the
neural folds fail to fuse rostrally. Te child is totally vegetative,
unable to see, hear, or process sensory inputs. Muscles are flac-
cid, and no voluntary movement is possible. Mental life as we
know it does not exist. Mercifully, death occurs soon aFer birth.
White
matter
Neural tube
cells
Central
cavity
Alar plate:
interneurons
Dorsal root ganglion:
sensory
neurons from neural crest
Basal plate:
motor neurons
Figure 12.34
Structure of the embryonic spinal cord.
At six
weeks of development, aggregations of gray matter called the alar
plates (future interneurons) and basal plates (future motor neurons)
have formed. Dorsal root ganglia (future sensory neurons) have
arisen from neural crest cells.
Figure 12.35
Newborn with a lumbar myelomeningocele.
previous page 510 Human Anatomy and Physiology (9th ed ) 2012 read online next page 512 Human Anatomy and Physiology (9th ed ) 2012 read online Home Toggle text on/off