The basal ganglia consist of a number of subcortical structures, the most important of which are the caudate nucleus, the putamen, and the globus pallidus. This group of structures has connections to very many parts of the cortex, but especially to the frontal lobe. The basal ganglia are therefore associated with movement planning, memory, emotion, reasoning and attention. Failure of the basal ganglia to function results in disorders such as Parkinson's and Huntington's disease, two diseases primarily related to the dysfunction of movements.
The caudate nucleus and putamen are known as the striatum, and this is the input nucleus of the basal ganglia. This means that they receive information from other parts of the brain, especially the cortex. The globus pallidus is the output nucleus of the basal ganglia, and so it receives information from the caudate nucleus and the putamen. The information is then transmitted to the thalamus. The thalamus then transmits this information to the motor and frontal areas in the cortex.
We have now explained the structures and connections of the basal ganglia. However, how these different structures work together is a whole different story, and not as easy as you might initially think. Logically, activity of the cortex would cause the basal ganglia to become active. This would then cause the thalamus to become active, and this would drive a movement. But this is not the case.
The part of the thalamus where the globus pallidus projects to is always active. This means that this area sends non-stop signals to the motor cortex. In daily life, of course, this cannot work because there are times when we have to sit still, or we only want to move a small part of our body. To counteract the fact that we are always moving, the globus pallidus has connections to the thalamus. The globus pallidus works with the neurotransmitter GABA, which has mainly inhibitory receptors. This means that the globus pallidus stops all activity of the thalamus (this is called inhibition).
When making a voluntary movement, signals from the cortex come to the caudate nucleus and the putamen. These two structures can then tell the globus pallidus to stop inhibiting some signals. So the globus pallidus is then "turned off" for a while, allowing these movements (the inhibition is thus inhibited, this is called disinhibition).
Author: Myrthe Princen (translated by Thomas von Rein)