Clinical Note

Spina Bifida, Epilepsy, and Hydrocephalus

Abnormalities during early developmental stages of the nervous system can cause profound neuronal birth defects. For example, alterations in the production and differentiation rates of neural plate neurons may result in abnormalities in the closure of the neural pores. The cranial and caudal neuropores usually close at 25 and 27 days of gestation, respectively. Failure of the caudal neuropore to close in a timely fashion disrupts the functional development of the lower parts of the spinal cord that normally control movement of the muscles of the legs as well as smooth muscles and glands of the lower body cavity. This severely crippling condition is called spina bifida. Even more devastating is the condition called anencephaly, which results from failure of the rostral or cranial neuropore to close. Gross abnormalities in brain structures can result, leading to severe mental retardation or death of the infant.

Abnormalities in neuronal migration during embryogenesis can also result in clinical disorders, many of which may be so severe that they are incompatible with life. Even if the abnormality is relatively minor, neurologic symptoms may persist through adulthood. The most common condition in these cases is epilepsy, caused by abnormal, synchronous activity in large groups of neurons that results in convulsive seizures. Even small patches of abnormally placed cells in the cerebral cortex can sufficiently disrupt normal physiology to result in epilepsy. Although other types of epilepsy may be responsive to drug therapy, seizures associated with neuronal migration disorders characteristically are not.

Occasionally, infants are born with an obstruction that blocks the flow of CSF through the ventricles. Fluid collects in the ventricles, causing increased pressure and swelling. Because the skull is soft and incompletely formed, the head itself will expand, sparing the brain from damage. This condition, called hydrocephalus, is treated by inserting a drainage tube or ventricular shunt into the swollen ventricle. In adults, hydrocepha-lus is more problematic. Because the adult skull does not expand, increased pressure caused by a ventricular blockage from intracranial bleeding, tumor, and so on will compress and damage brain tissue if not treated quickly.

A. Cross sections of the spinal cord

B. Coronal view of the nervous system

C. Lateral view of the nervous system

left cerebral cortex basal ganglia brain stem nuceli red nucleus tectum reticular formation vestibular nuclei cerebellum cortical spinal tract cervical spinal cord lumbar spinal cord

FIGURE 6 Motor axis of the nervous system. Blue indicates the many areas of the brain contributing to motor control. The corticospinal tract from the cerebral cortex to the spinal cord is shown in black. In addition to commands delivered directly to motor neurons in the brain stem and spinal cord, the cerebral cortex communicates to other motor areas, calling forth the movements specialized by each. For example, the reticular formation and cerebellum program the static and dynamic motor adjustments in space, respectively. Note that, except for the cerebellum, the right brain controls the left hand and leg.

particularly in the extremities (e.g., the digits) (Fig. 6). In contrast, autonomic innervation is more diffuse and movements are highly stereotyped. Many common lesions of the nervous system are relatively localized. Stroke, tumor, or infection many affect only a small portion of the motor or sensory axis. Because of the somatotopic organization of somatic sensory and motor systems, resulting deficits may also be highly specific. For example, a patient experiencing a stroke within a small area of the left cerebral cortex may lose only the ability to move the right hand because of the loss of appropriate input through the corticospinal tract.

Suggested Readings

Conn PM. Neuroscience in medicine. Philadelphia: JB Lippincott, 1995. Cowan WM. The development of the brain. Sci Am 1979; 241(3): 112-133.

Martin JH. Neuroanatomy. Stamford: Appleton and Lang, 1996. Purves D, Lichtman JW. Principles of neural development. Sunderland, MA: Sinuaer, 1985.

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