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.
The word basal refers to the fact that the the basal ganglia are found near the base, or bottom, of the brain. The use of the word ganglia, however, is a bit of a misnomer according to contemporary neuroscience conventions. The term ganglion is used to describe a cluster of neurons, but it’s typically only used to refer to neurons in the peripheral nervous system (i.e. outside the brain and spinal cord). The word nucleus is generally used to describe clusters of neurons found in the central nervous system. Thus, the basal ganglia might more accurately be considered nuclei.
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 they also are interconnected with one another to form a network that is thought to be involved in a variety of cognitive, emotional, and movement-related functions. The basal ganglia are best-known, however, for their role in movement.
The contributions of the basal ganglia to movement are complex and still not completely understood. In fact, the basal ganglia probably have multiple movement-related functions, ranging from choosing actions that are likely to lead to positive consequences to avoiding things that might be aversive. But the basal ganglia are most often linked to the initiation and execution of movements. One popular hypothesis suggests that the basal ganglia act to facilitate desired movements and inhibit unwanted and/or competing movements.
To understand how this might work, think about the action of reaching out to pick up a pencil. First, consider what’s happening in the moments before you extend your arm. Although it might seem like there would be very little movement-related activity going on in the brain at this point (because you are sitting still), your brain is actually constantly at work to inhibit unwanted movements (like jerking your hand involuntarily up in the air or suddenly turning your head to one side). The basal ganglia are hypothesized to play a critical role in this type of movement inhibition, as well as in the release of that inhibition when you do have a movement that you want to make (reaching for the pencil in this case).
After the movement begins, it’s also important that muscles that would counteract the desired movement remain relaxed. When you extend your arm to reach for the pencil, for example, you don’t want the muscles that flex your arm (to move it back towards your body) to be activated at the same time. The basal ganglia are thought to help to inhibit these types of contradictory movements, allowing for a reaching movement that’s smooth and fluid.
The intricacies of how basal ganglia activity leads to the facilitation of movement are still a bit unclear, but one popular hypothesis (which I’ll call the direct/indirect model for reasons that will be made clear below) suggests that there are different pathways in the basal ganglia that promote and inhibit movement, respectively. The direct/indirect model is centered around connections the basal ganglia (specifically the globus pallidus and substantia nigra) form with neurons in the thalamus. These thalamic neurons in turn project to the motor cortex (an area of the brain where many voluntary movements originate) and can stimulate movement via these connections. The basal ganglia, however, continuously inhibit the thalamic neurons, which stops them from communicating with the motor cortex—inhibiting movement in the process.
According to the direct/indirect model, when a movement is desired, a signal to initiate the movement is sent from the cortex to the basal ganglia, typically arriving at the caudate or putamen (which are referred to collectively as the striatum). Then, the signal follows a circuit in the basal ganglia known as the direct pathway, which leads to the silencing of neurons in the globus pallidus and substantia nigra. This frees the thalamus from the inhibitory effects of the basal ganglia and allows movement to occur.
There is also a circuit within the basal ganglia called the indirect pathway, which involves the subthalamic nucleus and leads to the increased suppression of unwanted movements. It is thought that a balance between activity in these two pathways may facilitate smooth movement.
Again, this is just one perspective on basal ganglia function, and despite the importance the basal ganglia are thought to have in movement, there is still much we need to learn to fully understand their contribution to it. We can see the importance of the basal ganglia to movement, however, when we look at cases where the basal ganglia have been 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 may cause individuals with Parkinson's disease to have difficulty initiating movements, resulting 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, degeneration of basal ganglia circuits causes the inhibitory capabilities of the basal ganglia to be diminished. This may lead to the excessive activation of movement-related circuits, causing 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 also being investigated in attempts to understand disorders like Tourette's syndrome, schizophrenia, and obsessive-compulsive disorder.
Purves D, Augustine GJ, Fitzpatrick D, Hall WC, Lamantia AS, McNamara JO, White LE. Neuroscience. 4th ed. Sunderland, MA. Sinauer Associates; 2008.