The brain is the most complex and least understood organ in the body. It is the focal point of the central nervous system (CNS), which also includes the nerves of the spine. The CNS receives, processes, and sends billions of signals every day by way of chemicals and electrical impulses. Neurologists (physicians who specialize in studying and treating brain diseases and disorders) are only starting to identify how these chemicals and power surges work, and what we know right now is woefully inadequate for helping when these processes go awry.

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Note: Most of the medical information presented here is derived from studies of people with autism rather than people diagnosed with PDD-NOS or atypical PDD. The findings are believed to apply across the entire spectrum of PDDs, although there may be differences according to the severity of each individual's symptoms. There may also be some special, as yet unknown, factors involved in various PDDs.

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Brain structure 101

Click here for an image of: Parts of the human brain.

Although they can't be used as to diagnose PDDs just yet, brain scans can show where abnormal activity is occurring or whether the brain is structured differently than usual. Tests of cerebrospinal fluid, blood, and urine are also producing hard data.

The brain is mostly made up of two kinds of cells. The first kind, neurons, do the hard work of transmitting all that information. The second kind, glial cells, are twice as numerous as neurons. Glial cells have the less glamorous jobs of making sure the neurons have enough nutrients and other chemicals, repairing the brain if it is injured, and confronting bacteria that try to attack the brain. Obviously, problems with either type of cell could be involved in PDDs, and if one type of cell is affected, that would change how the other type functions as well.

Brain differences in PDDs

One 1997 autopsy study did find brain-stem damage, which would also explain the higher-than-normal number of people with autism who have minor ear malformations. This study was spurred by the observation that, when the dangerous drug Thalidomide was in use, one-third of the children who had been exposed to it between the twentieth and twenty-fourth day in the womb were later diagnosed as autistic. This is the period in which both the brain stem and the ears form.

Researchers are careful to say that certain types of brain damage, such as lesions on the temporal lobe or damage due to infection, can cause "autistic-like" symptoms. One current theory is that the development of some brain cells or structures is slower than normal or never completed in people with PDDs.

Of course, there haven't been very many autopsy studies, since PDDs do not cause death. Very recently, researchers using MRI machines and other high-tech tools have been finding very subtle differences in the brains of living people with autism. The parietal, temporal, and occipital lobes are all reported as being larger in volume than expected, for instance, and there are signs of unusual activity patterns. There may also be differences in the size of the cerebellum.

Although research continues into differences in brain structure, most doctors are more concerned with how the brain actually functions. It's a pretty adaptable organ, after all--with proper medical care, people can recover from strokes, accidents, or illnesses that cause brain damage, because the brain is built to route around problems whenever possible. It seems that where PDDs are concerned, relatively minor structural or chemical differences may be influencing how brain cells communicate with each other, with the CNS, and with the body as a whole.

Neurotransmitters: the brain's telephone system

Much has been learned about the role of neurotransmitters in PDDs by accident. For example, autistic symptoms improved in some patients when they were taking medicines for something else, such as depression. Now, targeted studies of medications indicate that several kinds of neurotransmitters have something to do with PDDs.

The structure of a neuron

Click here for an image of: Neurotransmitters crossing the synaptic cleft.

There are many different neurotransmitters running around in the human brain, the CNS, and the gastrointestinal system. They're all site-specific chemicals that can be absorbed only by certain cells and at certain spots. This ensures that the right kinds of messages get through. They are also used and absorbed differently in various areas of the body, and sometimes turned into other kinds of chemicals.

Several neurotransmitters appear to be involved in pervasive developmental disorders, including:

• Serotonin. Serotonin, also called 5-hydroxytryptamine or 5-HT, controls sleep, mood, some types of sensory perception, body-temperature regulation, and appetite. It affects the rate at which hormones are released and has something to do with inflammation. Studies have shown that autistic people tend to have increased amounts of serotonin in their blood. They may produce, absorb, or metabolize serotonin differently, although studies are still trying to determine how.

• Dopamine. Dopamine, sometimes abbreviated as DA, helps control body movements and thought patterns, and also regulates how hormones are released. Although medications that block dopamine have been useful to some people with autism, researchers are not sure why. Perhaps there are differences in how these people create or absorb dopamine.

• Norepinephrine. Norepinephrine is used by both the CNS and the peripheral sympathetic nervous system (the nerves that communicate with the rest of the body). It governs arousal, the "fight or flight" response, anxiety, and memory. There seem to be small differences in how autistic people make and process norepinephrine.

Physical activity, exercise, diet, vitamins, and herbal supplements can also affect these neurotransmitters. That's one of the reasons that parents and professionals need to be as careful about choosing alternative treatments as they would be about prescription drugs.

Electrical miswiring

Many more people with PDDs may experience other types of seizures, some of which are hard to recognize, even with sophisticated monitoring equipment. For example, some doctors believe that the inexplicable temper tantrums some people with PDDs have may be related to seizures occurring deep within the frontal lobe.