Let’s breakdown the basal ganglia pathways. There are two main pathways- the direct (which is excitatory, meaning it increases movement/thoughts/feelings), and the indirect (which is inhibitory, meaning it decreases movement/thoughts/feelings). I will talk about these in terms of movement.
Firstly, the direct pathway starts with inputs from cortex (premotor and supplementary motor cortex for example, which help plan movements) to the striatum (putamen). The striatum inhibits the internal segment of the Globus Pallidus (GPi), which would normally inhibit thalamus. Since it is being inhibited, it will not inhibit thalamus, which means thalamus can excite cortex (primary motor cortex for example) and cause movement.
Does that make sense? I hope so, because the indirect pathway is a bit more complex! It begins the same way- cortex sends projections to neurons in striatum. These neurons, which are designated for the indirect pathway, now will inhibit the external segment of the Globus Pallidus (GPe), which normally inhibits the subthalamic nucleus (STN). Since the GPe is inhibited, it will no longer inhibit the STN, which means the STN will excite the GPi. The GPi will then inhibit the thalamus and prevent it from exciting cortex.
Summary:(“>” is an excitatory connection, “-” is inhibitory, and parentheses denote a connection that is not happening due to inhibition)
Direct Pathway: Ctx>Striatum-GPi(-)Thalamus>Ctx
Indirect Pathway: Ctx>Striatum-GPe(-)STN>GPi-Thalamus(>)Ctx
In my next post, I will discuss how dopamine neurons from the Substantia Nigra contribute to these pathways.