THE BASAL GANGLIA AND PARKINSON'S DISEASE
Parkinson's Disease is a condition which affects the sufferer's movement. Such movement impairments include the ability to walk, sit still with comfort and speak fluently. In addition to being an especially challenging disorder to live with, Parkinson’s Disease also offers insight into how the brain works in non sufferers. One of the brain’s primary functions is to help us move. Certainly, from an evolutionary point of view, human beings, like other animals, have to move to live. We need to feed, reproduce, build shelters and escape threats. None of these things can be achieved without movement. To generate movement, the brain has a sub cortical structure called the basal ganglia.
The basal ganglia itself comprises of several parts operating in a circuit. The substantia nigra projects excitatory signals to the putamen, which in turn sends inhibitory signals to the globus pallidus and these inhibit the thalamus (the ventral lateral nucleus). The ventral lateral nucleus sends excitatory signals to the supplementary motor area. In essence, the main role of the basal ganglia is to secrete the excitatory neurotransmitter dopamine into the forebrain so that it can facilitate planned movement. Too much dopamine and movement becomes over active (Huntingdon’s Disease); too little dopamine and body movements become under active (Parkinson's Disease).
There have been a number of famous sufferers of Parkinson's Disease, including the actor Michael J. Fox who is best known for playing Marty McFly in the 1980s Back to the Future movies. In a recent interview with the BBC, Michael J. Fox discussed coping with Parkinson's Disease after he was first diagnosed aged 29. Imagine being an actor in an action movie like Back to the Future. Let's say that the director wants you to run as though being chased and then come to a stop at a particular marker. Running in a particular direction, towards a particular point, requires a degree of planning even though running is something that seems to come naturally. Running takes coordination: you have to slow down or otherwise you would run straight into people or objects. If you ever see a toddler running you will notice how often they fall over because they are still learning the necessary balance. So when an actor runs as part of his role, it actually requires sophisticated movement which is controlled by the motor cortex, with help the supplementary motor area; meanwhile, the more primitive motivation to move comes from the basal ganglia.
Talking also requires movement because the lips, jaw and tongue combine to make sounds. Without sufficient dopamine in the substantia nigra, even speech becomes more difficult. Approximately 80% of dopaminergic neurons are lost in Parkinson’s sufferers. In healthy brains, cells are supposed to die routinely - indeed some cancers occur because cells that should die proliferate. For the Parkinson’s sufferer the challenge is to move despite the fact that the necessary dopaminergic neurons that normally facilitate movement are depleted.
Like running, talking is something that, for most people, is achievable without really having to think much about it. In truth, the jaw, lips and tongue actually work hard to produce the movement necessary to explain something difficult. Again, it is something that infants have to learn as they pick up more complex vocabulary. The Parkinson’s sufferer though takes longer to make common speech sounds; without the normal level of dopamine, each movement has to be planned in a conscious way, which means that they are likely to get tired more quickly. Before the onset of symptoms, these would have happened automatically or without as much conscious planning.
As the BBC interview attests, Michael J Fox retains a positive outlook. Similarly, the BBC also featured an interview with a Parkinson's sufferer, Andy Wright, who is a keen tennis player. Andy Wright demonstrates that, whilst movement is difficult, it is still possible for a Parkinson's sufferer to play energetic sports: as Andy Wright says, 'keep moving is the thing'. You will know that it is much harder to speak French or play a musical instrument if you have stopped practising. The movements required to complete such tasks can get forgotten. Movement is fundamental to being healthy, physically and mentally, because when you move the brain must either form new connections or existing ones. In answer to the question, what is the brain for, a reasonable answer is to say that it facilitates movement. In that sense, those with Parkinson's Disease are no different to anyone else.
The interviews with Michael J. Fox and Andy Wright are from the BBC website.
The Michael J. Fox Foundation researches Parkinson's Disease https://www.michaeljfox.org/