The basal ganglia or basal nuclei is a structure located deep within the brain, and it’s made up of a group of nuclei - so millions of nerve cell bodies.
Put simply, the cerebral cortex decides how it wants to move the body and sends that input to the basal ganglia, and then the basal ganglia’s job is to help execute a smooth movement.
The basal ganglia are actually two pairs of deep structures - one on the left side and one on the right side of the brain.
Each pair consists of the globus pallidus, which has the internal globus pallidus and the external globus pallidus, and the striatum - which includes the caudate nucleus and the putamen.
The basal ganglia is linked to other brain structures, like the ventral anterior nuclei and ventral lateral nuclei of the thalamus, as well as the substantia nigra of the midbrain.
The basal ganglia can help start, stop, and control desired movements, while also inhibiting undesired movements.
As an example, when you walk, you have to move one leg at a time - so the basal ganglia help one leg to step forward, while inhibiting the other leg, so that it’s stationary - and that prevents you from falling!
Additionally, the basal ganglia is involved in perception.
Let’s take a look at this picture as an example. You can either see a rabbit - with its two long ears - or a duck, with its beak. And you can choose which animal to see, but you can’t see both simultaneously, because the basal ganglia stimulates the vision of one, while it inhibits the vision of the other one. For this reason, the brain can only perceive one image at a time.
For the basal ganglia to work, nearly the entire cerebral cortex projects onto the striatum.
The striatum then projects onto the thalamus, and from there neurons head back to the cerebral cortex through two pathways: the direct pathway - which is excitatory - and the indirect pathway, which is inhibitory.
So the direct pathway and indirect pathway have to be carefully balanced to control smooth movement.
Now, there are two main neurotransmitters involved in these pathways: the excitatory neurotransmitter glutamate, and the inhibitory neurotransmitter GABA.
In the direct pathway, the cerebral cortex sends excitatory projections to the striatum.
Then, the striatum sends inhibitory projections to the internal globus pallidus.
Then, the internal globus pallidus sends inhibitory projections to the thalamus, which is usually in an active state.
With this setup, if the striatum inhibits the internal globus pallidus, then the internal globus pallidus cannot inhibit the thalamus - the two negatives cancel out. As a result, the thalamus is free to send excitatory projections to the motor cortex and this initiates voluntary movements.
In the indirect pathway, the cerebral cortex sends excitatory projections to the striatum once again.
But this time, the striatum sends inhibitory projections to the external globus pallidus, rather than the internal globus pallidus.