4.5. Parkinson's Disease
Defining Parkinson's Disease
Biological Processes Underlying Parkinson's Disease
There are a number of parts to the basal ganglia:
- Substantia nigra (which is part of the midbrain);
- Putamen;
- Caudate nucleus;
- Globus pallidus;
- Thalamus and cerebral cortex (not strictly basal ganglia structures but important all the same).
You can see the location of some of these structures in Figure 4.5.1. The neurons that connect the substantia nigra to the putamen are dopaminergic (i.e. they use dopamine as their neurotransmitter). In Parkinson’s disease, these neurons die gradually, meaning the amount of dopamine decreases over time. This has a profound effect on the functioning of the basal ganglia.
A deeper dive
In fact, there are two pathways in the basal ganglia that affect the working of the motor areas of the cerebral cortex; these are called the direct pathway and the indirect pathway. The direct pathway causes an overall increase in activity in the motor areas whereas the indirect pathway causes an overall decrease in the motor areas. In the healthy brain, these are balanced, so movements are smooth and coordinated. But in Parkinson's disease, the indirect pathway gradually dominates because of the lack of input from the substantia nigra; remember, the cells in this tract - the so-called 'nigrostriatal tract' - are dying. Again, the indirect pathway means less excitation of the cortex so patient's with Parkinson's disease develop symptoms that are described as 'poverty of movement', for example, cog-wheel rigidity and slow movements (when walking this is sometimes referred to as a ‘festinating gate’ or ‘shuffle’) and impaired range of movement (making actions such as writing more difficult). A resting tremor, that is a tremor that is always present when sitting or standing, is another very characteristic sign of Parkinson's disease.
Managing Parkinson's Disease
There is currently no cure for Parkinson’s disease although the symptoms can be eased using different therapies.
Medication
A medication called levodopa (also called L-dopa) is given to most patients with the disease. Levodopa is a precursor to dopamine and, when taken up by neurons in the basal ganglia, it is converted to dopamine and replaces the dopamine which has already been lost. The parts of the basal ganglia can then communicate more, although not completely, effectively. Note that dopamine cannot be given directly as it is unable to cross the blood-brain barrier (see more in Section 2.5. Glial Cells).
Other drugs, such as monoamine oxidase-B inhibitors, are used to decrease the activity of enzymes that break down dopamine, and dopamine receptor agonists, such as ropinirole (although these have a major side effect of impulsive and risky behaviour).
Surgery
If Parkinson’s disease cannot be controlled with medication, people may be offered surgery. A surgeon can set up deep-brain stimulation (DBS). During surgery, electrodes are implanted into parts of the basal ganglia. After surgery, these electrodes produce high frequency stimulation that blocks some of the natural electrical signals in the basal ganglia, thereby reducing the symptoms of the disease. It is kind of like a pacemaker of the brain.
Physiotherapy and occupational therapy
Regular physiotherapy exercises can help free up tight and stiff muscles. Physiotherapists can also help improve a patient’s ability to walk. Occupational therapists can help patients adjust their lifestyles and their homes so they can cope better with the difficulties that Parkinson’s disease brings about. This might include installing handrails in the home to aid mobility, learning ways to cope with anxiety and depression, and developing new ways to communicate when handwriting and speech become difficult.
