Given the enormous complexity of the brain, it should not be surprising that sometimes it gets broken. Mental disorders range from those with a clear genetic basis, such as Down and Fragile X syndromes, to disorders with high but not complete heritability, such as schizophrenia and autism, to conditions that may be almost completely attributed to life events, like some types of depression.
Some mental disorders are also associated with aging, such as Alzheimer's and Parkinson's diseases. These diseases have no clear genetic basis, although increasing evidence points to associations between some genetic constituencies and risk for these diseases. Huntington's disease is genetic, but its symptoms typically don't appear until adulthood.
What can go wrong with the brain can occur at multiple levels. The following is just a sampling of mental and neurological illnesses that can occur:
- Developmental errors in gross structure: Genetic mutations or environmental toxins can lead to defects in gross brain structure. Defects can include missing or abnormally small brain areas, such as the cerebellum, or missing axon tracts connecting brain areas.
- Developmental errors in specific local circuits: Some recent theories for autism suggest that, in people with autism, the balance between short and long range neural connections is skewed towards an excess in the short range. This is hypothesized to lead to over-attention to details and inability to respond well to the big picture.
- Dysfunctional neural pathways: Mutations in genes that specify neurotransmitter receptors may lead to brain-wide processing deficits. While some brain areas may compensate with other neuronal receptors, other areas may not. Excitatory/inhibitory receptor balance may be implicated in epilepsy and some forms of depression.
- Environmentally caused organic dysfunctions: The brain can be damaged by overt injury, such as by a blow to the head. It can also be damaged by toxins such as lead and mercury that produce developmental delays and other mental incapacities without overt signs of brain damage.
- Environmentally caused psychological dysfunctions: Sometimes mental illnesses, such as some types of depression, occur after environmental triggers in people who have had no previous indications of mental problems. A crucial question in mental illnesses such as depression is whether non-organic causes, such as loss of a loved one, produce depression primarily by changing brain neurochemistry.
