Dr. C. George Boeree
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Basal Ganglia
The basal ganglia are a collection of nuclei found on both sides of the thalamus, outside and above the limbic system, but below the cingulate gyrus and within the temporal lobes. Although glutamate is the most common neurotransmitter here as everywhere in the brain, the inhibitory neurotransmitter GABA plays the most important role in the basal ganglia.
The largest group of these nuclei are called the corpus striatum ("striped body"), made up of the caudate nucleus ("tail"), the putamen ("shell"), the globus pallidus ("pale globe"), and the nucleus accumbens ("leaning"). All of these structures a double ones, one set on each side of the central septum.
The caudate begins just behind the frontal lobe and curves back towards the occipital lobe. It sends its messages to the frontal lobe (especially the orbital cortex, just above the eyes), and appears to be responsible for informing us that something is not right and we should do something about it: Wash your hands! Lock your door! As these examples are meant to suggest, obsessive compulsive disorder (OCD) is likely to involve an overactive caudate. On the other hand, an underactive caudate may be involved in various disorders, such as ADD, depression, aspects of schizophrenia, and just plain lethargy. It is also involved in PAP syndrome, a dramatic loss of motivation only recently discovered (see below).
The putamen lies just under and behind the front of the caudate. It appears to be involved in coordinating automatic behaviors such as riding a bike, driving a car, or working on an assembly line. Problems with the putamen may account for the symptoms of Tourette's syndrome.
The globus pallidus is located just inside the putamen, with an outer part and an inner part. It receives inputs from the caudate and putamen and provides outputs to the substantia nigra (below).
The nucleus accumbens is a nucleus just below the previous nuclei. It receives signals from the prefrontal cortex (via the ventral tegmental area) and sends other signals back there via the globus pallidus. The inputs use dopamine, and many drugs are known to greatly increase these messages to the nucleus accumbens.
Another nucleus of the basal ganglia is the substantia nigra ("black substance"). Located in the upper portions of the midbrain, below the thalamus, it gets its color from neuromelanin, a close relative of the skin pigment. One part (the pars compacta) uses dopamine neurons to send signals up to the striatum. The exact function isn't known, but is believed to involve reward circuits. Also, Parkinson's disease is due to the death of dopamine neurons here.
The other part of the substantia nigra (the pars reticulata) is mostly GABA neurons. It's main known function is controlling eye movements. It is also involved in Parkinson's, as well as epilepsy.
As you can see, quite a few serious problems are strongly associated with the basal ganglia. Some, such as ADHD, Tourette's, obsessive-compulsive disorder, and schizophrenia, will be covered in other parts of this text. Others are somewhat less psychological and more physical, but are still important....
Parkinson's disease
Parkinson's is characterized by tremor (shaking), rigid muscles, difficulty making quick, smooth movements, and difficulty standing and walking. Many people also develop depression and anxiety and, later in life, problems with memory loss and dementia.
It usually develops late in life, but it can occur in younger people. One well-known case is the actor Michael J. Fox. It is very difficult for both the patient and his or her family.
Parkinson's is originates in the death of cells in the substantia nigra and the loss of dopamine and melanin produced by those cells. It progresses to other parts of the basal ganglia and to the nerves that control the muscles, involving other neurotransmitters. Possible causes or contributing factors include environmental toxins, head trauma, and genetics.
There are treatments available that slow the course of Parkinson's and alleviate the symptoms. Most involve replacing or mimicking the lost dopamine and other neurotransmitters. Unfortunately, the disease slowly progresses to where the treatments only work for a few hours at a time. Parkinson's does not directly cause death and many patients live long lives with it.
Huntington's disease
Huntington's is characterized by loss of memory and odd jerking movements called chorea ("dance"). It is a hereditary disease (with a dominant gene) involving cell death in the caudate nucleus. It usually starts in a person's 30s, but may start at any age.
There is no cure, but there are treatments that can reduce the symptoms. It is fatal, although it is complications of the disease that usually cause death, rather than the disease itself. Many Huntington's sufferers commit suicide.
Cerebral palsy
People with cerebral palsy have various motor problems, such as spasticity, paralysis, and even seizures. Spasticity is where some muscles are constantly tight and so interfere with normal movement. This is the reason for the unusual hand and arm positions most of us have seen in people with cerebral palsy.
It is apparently due to brain damage, usually sometime before birth. Causes may include fetal infection, environmental toxins, or lack of oxygen.
(Although cerebral palsy tends to remain relatively stable throughout life, there is no cure and is very difficult to deal with for both the person and his or her family.)
PAP ( or Athymhormic) syndrome
PAP is characterized by an unusual lack of motivation. A dramatic case was that of Mr. M, who, while drowning, simply failed to try to save himself, even though a good swimmer.
Damage to the caudate nucleus means that nothing carries any emotional significance anymore. Drowning? Don't be concerned. People with PAP also ignore the usual social and moral motivations we all take for granted. They don't quite "get" that their lack of action could have significant consequences.
Without the motivating influence of the basal ganglia, the frontal lobe simply stops planning for the future. Oddly, they can still respond to external motivation, such as a loved one's request or an authority's command.
See the April 2005 Scientific American Mind article by Patrick Verstichal and Pascal Larrouy for more on PAP syndrome.
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