The Basal Ganglia
Dr. C. George Boeree
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-like nucleus"), the putamen ("nutshell"), the globus pallidus ("pale globe"), and
the nucleus accumbens
("leaning nucleus"). All of these structures are 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 (from the French: perte d'auto-activation psychique) 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.
© Copyright 2006, C. George Boeree
