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.
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.
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.
For this final green arrow, you'll have a plus one. Minus one times minus one is plus one.
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.
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 external 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.
Now, we have a negative one. A negative one times a positive one is now a negative one.
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.