BASAL GANGLIA AND CEREBELLUM
The basal ganglia and cerebellum are large collections of nuclei that modify movement on a minute-to-minute basis. Motor cortex sends information to both, and both structures send information right back to cortex via the thalamus. (Remember, to get to cortex you must go through thalamus.) The output of the cerebellum is excitatory, while the basal ganglia are inhibitory. The balance between these two systems allows for smooth, coordinated movement, and a disturbance in either system will show up as movement disorders.
A. The basal ganglia:
What are the basal ganglia? The
name is confusing, as generally a ganglion is a collection of cell bodies
outside the central nervous system. Blame the early anatomists.
The basal ganglia are a collection of nuclei deep to the white matter of
cerebral cortex. The name includes: caudate, putamen, nucleus accumbens,
globus pallidus, substantia nigra, subthalamic nucleus, and historically
the claustrum and the amygdala. However, the claustrum and
the amygdala do not really deal with movement, nor are they interconnected
with the rest of the basal ganglia, so they have been dropped from this
section. Other groupings you may hear are the striatum (caudate + putamen
+ nucleus accumbens), the corpus striatum (striatum + globus pallidus),
or the lenticular nucleus (putamen + globus pallidus), but these groupings
obviously get confusing very quickly, so we will try to avoid them.
The anatomy of these structures
should be a review from the "coronal and horizontal sections"
lab. Here once again are the basal ganglia as they appear when stained
An alternate stain is the acetylcholinesterase (AChE) stain. This technique stains for the enzyme that degrades acetylcholine (ACh), a major neurotransmitter. Areas which use ACh generally stain darkly. Here is a section through monkey brain, stained for AChE.
You can see that the caudate and
putamen are stained, while the globus pallidus remains fairly pale. This
emphasizes their different functions and connections. And those are...?
B. Different functions and connections:
The relationships between the nuclei
of the basal ganglia are by no means completely understood. When dealing
with the brain, you may sometimes be tempted to think that everything is
connected to everything else. Take heart, some fairly simple generalizations
and schematics can be drawn.
The caudate and putamen receive
most of the input from cerebral cortex; in this sense they are the doorway
into the basal ganglia. There are some regional differences: for example,
medial caudate and nucleus accumbens receive their input from frontal cortex
and limbic areas, and are implicated more in thinking and schizophrenia
than in moving and motion disorders. The caudate and putamen are reciprocally
interconnected with the substantia nigra, but send most of their output
to the globus pallidus (see diagram below).
The substantia nigra can be divided
into two parts: the substantia nigra pars compacta (SNpc) and the
substantia nigra pars reticulata (SNpr). The SNpc receives input
from the caudate and putamen, and sends information right back. The SNpr
also receives input from the caudate and putamen, but sends it outside
the basal ganglia to control head and eye movements. The SNpc is the more
famous of the two, as it produces dopamine, which is critical for normal
movement. The SNpc degenerates in Parkinson's disease, but the condition
can be treated by giving oral dopamine precursors.
The globus pallidus can also be
divided into two parts: the globus pallidus externa (GPe) and the
globus pallidus interna (GPi). Both receive input from the caudate
and putamen, and both are in communication with the subthalamic nucleus.
It is the GPi, however, that sends the major inhibitory output from the
basal ganglia back to thalamus. The GPi also sends a few projections to
an area of midbrain (the PPPA), presumably to assist in postural control.
This schematic summarizes the connections
of the basal ganglia as described above.
Although there are many different
neurotransmitters used within the basal ganglia (principally ACh, GABA,
and dopamine), the overall effect on thalamus is inhibitory. The function
of the basal ganglia is often described in terms of a "brake hypothesis".
To sit still, you must put the brakes on all movements except those reflexes
that maintain an upright posture. To move, you must apply a brake to some
postural reflexes, and release the brake on voluntary movement. In such
a complicated system, it is apparent that small disturbances can throw
the whole system out of whack, often in unpredictable ways. The deficits
tend to fall into one of two categories: the presence of extraneous unwanted
movements or an absence or difficulty with intended movements.
C. Lesions of the basal ganglia:
Lesions in specific nuclei tend
to produce characteristic deficits. One well-known disorder is Parkinson's
disease, which is the slow and steady loss of dopaminergic neurons
in SNpc. An instant Parkinson-like syndrome will result if these neurons
are damaged. This happened several years ago to an unfortunate group of
people who took some home-brewed Demerol in search of a high. It was contaminated
by a very nasty byproduct, MPTP ,which selectively zapped the SNpc neurons.
The three symptoms usually associated with Parkinson's are tremor,
rigidity, and bradykinesia. The tremor is most apparent at
rest. Rigidity is a result of simultaneous contraction of flexors and extensors,
which tends to lock up the limbs. Bradykinesia, or "slow movement",
is a difficulty initiating voluntary movement, as though the brake cannot
Huntington's disease, or chorea, is a hereditary disease of unwanted movements