Direct and Indirect Basal Ganglia Pathway

00:00 Now within the multitude of connections that we saw in the previous slide, the connections can be divided into a direct pathway as well as an indirect pathway. Both of these pathways are going to ultimately influence the state of the cerebral cortex. The direct pathway is the simplest one in that there are fewer connections. So this takes a more direct circuit route to influence the cerebral cortex. Let’s take a look at the components of the direct pathway. First, this direct pathway is going to stimulate or excite the motor cortex. This is how it goes about doing so.

00:49 Here we have the connection that exists between the striatum and the globus pallidus, so striatal pallidal. From the globus pallidus, we’ll have a connection here to the thalamus. Then from the thalamus, we can then excite the cerebral cortex. Then the cerebral cortex can communicate back with its circuitry to the striatum. Now, what I want you to do now is to think about math.

01:25 The connections have been changed in color so that we have red and green. Red would be inhibitory influences, whereas green is an excitatory influence. We want to look at how this direct pathway excites the cerebral cortex, so we’re looking at this communication or circuit, the circuit here and the circuit here. We want to think about math here to think about the fact that the direct pathway excites the cerebral cortex. For each red arrow, you’re going to have a minus one.

02:06 For this final green arrow, you'll have a plus one. Minus one times minus one is plus one.

02:14 Plus one times plus one is a positive or an excitatory influence. There’s also an indirect pathway between the basal ganglia to the cerebral cortex. Activation of the indirect pathway has an opposite influence on the motor cortex than does the direct. That’s because it will ultimately inhibit the motor cortex. Let’s look at the route or the circuitry here. We’re going to readily appreciate, you'll readily appreciate the fact that this is an indirect route to the cerebral cortex.

03:00 Here you have the connection between the striatum and the globus pallidus. The circuit then includes a connection from the globus pallidus to the subthalamic nucleus and then back to the globus pallidus specifically the internal segment, and from here to the thalamus, and then we can influence or inhibit the cerebral cortex. So, much more complicated circuitry here and then in the end, it has a negative influence on the cortex. The cortex does feedback on the striatum to kind of complete this loop. Again, we’re going to think about math in coming to an understanding that this is an inhibitory pathway upon the globus pallidus. And so now, the arrows have changed color so that those that are inhibitory are red and those that are excitatory are green. Anything in red is a minus one. Anything in green will be a plus one. We have a minus one, a minus one is a plus one. Plus one times a plus one here, because it’s excitatory is still plus one.

04:14 Now, we have a negative one. A negative one times a positive one is now a negative one.

04:21 Then a negative one times a positive one remains negative. Now we have a negative influence on the cerebral cortex by just using simple math. Another pathway to bring to your attention is that of the nigrostriatal pathway. We see this pathway between the substantia nigra and the striatum right in through here. This will modulate the direct as well as the indirect pathways. By doing so, it can ultimately excite the direct pathway. When that happens, you excite the cerebral cortex. Direct pathway excites, indirect inhibits the cortex.