Basal ganglia: Direct and indirect pathway of movement

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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.
Then, the external globus pallidus sends inhibitory projections to the subthalamic nucleus, which are constantly active.
Similar to before, the result is that if the striatum inhibits the external globus pallidus, then the external globus pallidus cannot inhibit the subthalamic nucleus.
Once more, the two negatives cancel out. And as a result, the subthalamic nucleus is free to send excitatory projections out.
It just so happens, that the subthalamic nucleus sends its excitatory projections to the internal globus pallidus. This is in direct opposition to what happens in the direct pathway.
By exciting the internal globus pallidus, it’s able to inhibit the thalamus, which sends excitatory signals to the motor cortex.
But the striatum also receives input from another brain structure - the substantia nigra - which releases a third neurotransmitter called dopamine into the striatum.
Dopamine acts on both the direct and the indirect pathways, but with opposite effects due to the presence of two different receptors: D1 receptors, which are excitatory, and D2 receptors, which are inhibitory.
D1 receptors are expressed by the striatal cells that project to the internal globus pallidus. When dopamine is released by the substantia nigra, it binds to D1 receptors, and that activates the direct pathway that controls the muscle you want to move.
On the other hand, D2 receptors are expressed by the striatal cells that project to the external globus pallidus. When dopamine binds D2 receptors, they inhibit the indirect pathway for that same muscle.
Ultimately, the action of dopamine on the striatum favors the excitatory projections to the motor cortex to reinforce the desired movements.
In other words, if you were to add abnormally high levels of dopamine to the system, the direct pathway for wanted movements becomes more active, and at the same time the indirect pathway for inhibiting unwanted movements becomes less active.
As an example, individuals using cocaine or ecstasy, often have enhancements of movements they want, like dancing at a rave, but they’re less able to control unwanted movements like grinding their teeth.
Alright, as a quick recap, the basal ganglia is made up of the striatum and the globus pallidus. The striatum receives input from the cerebral cortex about a desired movement, and then it sends output to the other basal ganglia structures through the direct and the indirect pathways.
The direct pathway ultimately leads to activation of the thalamus, which then stimulates the motor cortex to initiate wanted movements.
On the other hand, the indirect pathway ultimately leads to inhibition of the thalamus to inhibit undesired movements.