Where are the basal ganglia?
The basal ganglia are a group of structures found deep within the cerebral hemispheres. The structures generally included in the basal ganglia are the caudate, putamen, and globus pallidus in the cerebrum, the substantia nigra in the midbrain, and the subthalamic nucleus in the diencephalon. Despite the name, the basal ganglia are not actually ganglia.
What are the basal ganglia and what do they do?
The separate nuclei of the basal ganglia all have extensive roles of their own in the brain, but when referring to them as one network the function most frequently associated with the basal ganglia involves movement.
The basal ganglia receive information from the cortex, much of which is sent first to the caudate and putamen (which together are often referred to as the striatum). After the information is processed by the basal ganglia, it is sent back to the cortex by way of the thalamus. Thus, the pathway from the cortex to the basal ganglia and then back to the cortex via the thalamus forms a loop.
In simplistic terms, the functions of the basal ganglia in motor control are to facilitate movement and inhibit competing movements. For example, when someone tries to make an intentional movement like reaching for a pencil, the basal ganglia help to facilitate the movement by allowing motor plans associated with that movement (reaching and grasping in this case) to be activated. At the same time, the basal ganglia cause motor plans that might counteract the movement (perhaps flexing in this case) to be inhibited. The result is a smooth and fluid movement.
Although exactly how the basal ganglia achieve this fluidity of movement is not completely understood, we can see the importance of the basal ganglia to smooth movement when we look at cases where the basal ganglia are damaged. In Parkinson's disease, for example, dopaminergic neurons of the substantia nigra degenerate. When this happens, the ability of the basal ganglia to inhibit contradictory movements is affected. This causes individuals with Parkinson's disease to have difficulty initiating movements, and results in some of the symptoms associated with Parkinson's disease like rigidity and slow movement.
On the other hand, in a disorder like Huntington's disease, neurons that project to the globus pallidus degenerate, causing the globus pallidus to become unusually active. This leads to excessive activation of movement-related circuits and results in the jerky and writhing involuntary movements seen in Huntington's disease.
A balance between the ability to inhibit and facilitate movement is critical to making normal, smooth movements, and the proper functioning of the basal ganglia is essential to maintaining that balance. The basal ganglia, however, are also thought to have roles in habitual behavior, emotion, and cognition. Thus, in addition to movement disorders, the basal ganglia are now being investigated in attempts to understand disorders like Tourette's syndrome, schizophrenia, and obsessive-compulsive disorder.