Biology 202
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Understanding Behavior and the Brain through Parkinson's Disease

Sangeeta Iyer

In 1817, James Parkinson, a British scientist, became the first individual to describe a condition, he coined as "shaking palsy" in a paper he set forth to the scientific community. He described within it the major symptoms of this disease.(1) His work sparked a domino of research amongst the community in which scientists began trying to determine the cause and possible treatments for this condition.

Named for the scientist who first described it, Parkinson's disease has affected more than a million people and affects approximately 50,000 each year.(2) The symptoms may appear at any time but most patients are over 50 and rarely under 30.(3) Diagnosis of the disease is difficult because the symptoms are similar to those of other disorders and often, only in later stages, are patients diagnosed as Parkinson's patients. There is no social, economic, or geographic boundary to this disease. Women and men are affected equally. However, studies do present that Asians and African Americans are not as affected as white people. Because no one has been able to explain this phenomenon, scientists presume that all people have the same probability in developing the condition.(1)

There are four primary symptoms for the disease but each of them can take on different levels of severity depending on the individual. First, the patient experiences tremors, beginning typically in the arm. In the beginning, the tremors affect only one part of the body and then, generalize to affect the entire body. The second category of symptoms results in rigidity through out the body because the concept of muscle opposing muscle is disturbed. All the muscles remain constantly contracted such that the person begins to feel weak and stiff. The third type of symptom the patient may experience is known as Bradykinesia. It refers to the slowing down or complete loss of spontaneous or automatic movements. This is considered to be the most distressing symptom as the person can no longer perform routine functions as before. The last major symptom in most Parkinson patients is the inability to maintain their posture. Most patients develop a backward or forward lean and tend to fall easily.(1) There are many other symptoms that follow these four. Most can prove fatal, such as the inability to swallow, which can lead to choking, and others can prove to just be bothersome, such as trouble urinating and constipation.

Parkinson's disease is caused by the lack of the neurotransmitter dopamine in the brain. This neurotransmitter affects the brain processes that "control movement, emotional response, and ability to experience pleasure and pain."(4) Dopamine is produced in the special region of the midbrain known as the substantia nigra - named for its dark pigmentation - by dopaminergic neurons. Parkinson's disease is caused by the loss of these neurons, leading to a deficiency in the brain of dopamine. The loss of dopamine results in the nerve cells firing out of control, leaving patients unable to control their movements. In normal individuals, approximately 2,400 nigral cells die annually. However, in a Parkinson's patient, much more die each year. Approximately 240,000 nerve cells, about 60% of the substantia nigra, is dead in a Parkinson's patient who is displaying all the symptoms of the disease.(5)

What causes the degradation of the substantia nigra is still unknown. However, scientists have come up with multiple theories. One proposes that there are numerous free radicals (or instable molecules lacking an electron) in the brain that react with neighboring molecules especially metals, resulting in the oxidation of the metal. Oxidation is a violent process and can result in damaged tissues, like neurons. In normal patients, antioxidants act as chemicals providing protection to the cells. Scientists have found increased levels of iron in the brain, especially the substantia nigra, which provides evidence that the oxidative mechanism may be one cause of the disease.(1) Another theory is that environment risk factors such as pesticides or food toxins act as an external trigger to an internal trigger, which causes the substantia nigra to slowly degrade. A toxin known as MPTP is known to induce Parkinson-like symptoms in a patient. In studies on animals, MPTP affects mitochondrial DNA. The last theory provides a genetic basis for the disease.(1)

In the past decades, researchers have developed new treatments for the disease. Each does not present any hope as a cure but as a symptom suppressor. The first, and most common is the use of levadopa, which is a chemical that nerve cells can use to generate dopamine. Dopamine cannot be directly injected into the brain because it is unable to cross the complex mesh of blood vessels. Levadopa's success is immeasurable and has been able to extend the lives of many Parkinson's patients but not all the symptoms are alleviated equally. Bradykinesia and rigidity are easily ameliorated but posture instability is overlooked. Another problem is that after the first levadopa therapy, patients need to increase dosage gradually. Eventually, the effectiveness of the drug wanes and other replacement drugs need to be taken.(1) The second most common treatment for Parkinsons is surgical and is known as a pallidotomy. It is a procedure in which a small part of the brain known as the globus pallidus is removed. In animals, the loss of dopaminergic neurons causes the increased activity of the globus pallidus. This excitability is responsible for much of the rigidness and slowness associated with Parkinsons. However, the problem with the pallidotomy is that only a patient who has had Parkinsons for 5-10 years and is still responsive to L-Dopa can have the surgery. Also, symptoms like freezing, difficulties swallowing and falling are not improved.(6)

Parkinson's disease demonstrates a point that was argued in class of the brain being the center of behavior. Patients with this disease undergo a massive change in their behavior as demonstrated by their inability to perform daily routines. This is due not to any change in the environment, although toxins may trigger Parkinsons, but due to the degradation of the substantia nigra, a "box" within the brain. By observing the effects of Parkinson's disease on the brain and subsequently a person's behavior, it can be noted that brain does equal behavior. In addition, the types of treatments that researchers have developed for this condition have to affect the brain in order to ameliorate any difficulties with movement, behavior. The environmental toxins, which can serve as an external trigger to cause Parkinsons, may be considered an external cause for the disease. However, these need to cause an internal trigger in order to cause Parkinsons. Therefore, the environment is accepted as affecting Parkinsons but the degradation of a region in the brain is believed to be the true cause of the disease. What is more over interesting is that in pallidotomy, another region of the brain is removed. This can serve as evidence to the interconnectedness of all regions of the brain. With the lack of the substantia nigra, another part of the brain is affected.

WWW Sources

1) Parkinson's Disease: Hope Through Research

2)Parkinson's and Dopamine

3)Bachmann-Strauss Dystonia & Parkinson Foundation, Inc.

4)Dopamine - A Sample Neurotransmitter.

5)(5) Autoradiogram of Parkinson patient's brain.

6)Pallidotomy for Parkinson's Disease.

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