Brain Diseases

The nervous system is composed of central, peripheral, and autonomic nervous systems and is integral to the functioning of many other organ systems in the body. The brain is part of the central nervous system and is derived from neuroectoderm. Approximately 20 percent of people in the United States suffer from some sort of damage to the nervous system, and a vast number of disorders affect the nervous system. These disease states may begin in utero or develop through the course of a lifetime. Research is currently being conducted in centers across the world to find cures for these diseases. In the United States, Congress developed the National Institute of Neurological Disorders and Stroke in 1950 in an attempt to help address these problems through funding and research. Many other private and public institutions are also involved in understanding and developing treatments for these diseases.

A neurogenetic disease includes any disease process that is due to a defect in one or more genes that affect the differentiation and/or function of the neuroectoderm and its derivatives. The disease may be autosomal dominant, autosomal recessive, sex-linked recessive, or of mitochondrial inheritance. Autosomal dominant disorders include Huntington's disease, spinobulbar [Page 293]muscular atrophy, and the autosomal dominant cerebellar ataxias. Autosomal recessive disorders of the nervous system include diseases such as Fried-reich's ataxia and Refsum disease. Sex-linked recessive disease of the nervous system include disorders such as Duchenne muscular dystrophy and adrenoleukodystrophy. Mitochondrial diseases include disorders such as neuropathy, ataxia and retinitis pigmentosa (NARP), and Leber hereditary optic neuropathy (LHON). The presentation of these diseases varies greatly depending on the gene targets, ranging from motor and sensory deficits to dementia and psychiatric symptoms.

Developmental disorders affect the central nervous system as it develops. The resulting injury may be due to a disease, injury, or genetic defect and affect mental or motor disability. The developing brain in utero is subject to injury at various stages, including the following:

• primary neurulation: three to four weeks gestation;
• prosencephalic development: two to three months gestation;
• neuronal proliferation: three to four months gestation;
• neuronal migration: three to five months gestation;
• organization: five months gestation to years postnatal;
• myelination: birth to years postnatal.

The Diagnostic and Statistical Manual of Mental Disorders, 4th edition (DSM-IV), has established three criteria for identifying an individual with a developmental disability: an IQ below 70, significant limitation in two or more areas of adaptive behavior, and evidence of the limitation occurring in childhood. The causes of developmental disorders range from genetic conditions, such as fragile X syndrome, Down syndrome, and phenylketonuria to problems during pregnancy or exposure to infectious diseases. These disorders may also be caused by environmental and social deficiencies or exposures. Developmental disorders, such as cerebral palsy, may also affect the motor systems. In the case of cerebral palsy, the individual has motor or postural abnormalities without a deficit in intelligence.

Neurodegenerative diseases cause the deterioration of neurons over time. Specifically, they may affect movement and create conditions such as dementia. There are many causes and types of neurodegenerative diseases, ranging from deposition of elements and metabolic disorders to autoimmune disorders and diseases of unknown etiologies. The most common neurodegenerative diseases are Parkinson's disease and Alzheimer's disease. Parkinson's disease is a progressive condition in which cognitive and motor abilities can be affected. It is associated with dopamine deficiency due to the degeneration of nigrostriatal neurons. Alzheimer's disease is associated with cognitive deterioration and is caused by neuronal atrophy and the deposition of amyloid plaques and neurofibrillary tangles. Multiple sclerosis is another neurodegenerative disease.

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