Parkinson's disease is a devastating and complex disease that interferes with movement more and more as time goes on. It also produces a wide range of other problems for patients. But what goes wrong in the body and the brain that causes this debiliating condition?
How the Brain Controls Movement
When a person initiates a movement, information from the senses, from parts of the brain that control planning, and from other brain regions travels to a region called the striatum. The striatum then interacts with other areas of the brain — the substantia nigra, globus pallidus, and thalamus — to send out signals that control balance and coordination. These signals travel to the cerebellum, which controls muscle coordination, and then finally down the spinal cord to peripheral nerves in the limbs, head, and torso, where they control the muscles.
The molecules that carry information through the brain and spinal cord are called neurotransmitters. Neurotransmitters are special chemicals produced by neurons that accumulate in tiny sacs at the end of nerve fibers. When stimulated, these sacs release neurotransmitters into the gap between neurons, called a synapse. The neurotransmitters cross the synapse and attach to proteins called receptors on the neighboring cell. These signals change the properties of the receiving cell. If the receiving cell is also a neuron, it will carry the signal on to the next cell. If the receiving cell is a muscle fiber, it will react to the stimulation by contracting, which creates movement.
When Neurons Go Wrong
The lives of some neurons can take abnormal turns. In Parkinson’s disease, neurons that produce the neurotransmitter dopamine die off in the basal ganglia, an area of the brain that controls body movements. The brain can no longer control the body and people shake and jerk in spasms.
The primary area of the brain that is affected by Parkinson's disease (PD) is the substantia nigra. It contains a specialized set of neurons that send signals in the form of a neurotransmitter called dopamine. The signals travel to the striatum via long fibers called axons. The activity of this pathway controls normal movements of the body.
When neurons in the substantia nigra degenerate, the resulting loss of dopamine causes the nerve cells of the striatum to fire excessively. This makes it impossible for people to control their movements, leading to the primary motor symptoms of PD. Many Parkinson's patients eventually lose 80 percent or more of their dopamine-producing cells.
Studying Neuron Degeneration
While the neurons' underlying cause of death remains uncertain, researchers have identified several cellular characteristics that are common in this disease and which appear to play a role in the neuronal degeneration. Chief among these characteristics is the presence of Lewy bodies in neurons of the substantia nigra, the brainstem, and other parts of the brain. Lewy bodies are dense clumps, or aggregates, of proteins.
Another cellular characteristic of PD is the presence of Lewy neurites – swollen nerve fibers containing alpha-synuclein and other proteins. The accumulation of alpha-synuclein in these nerve fibers may interfere with transmission of nerve signals or other important neuronal functions.