 G'day and welcome to another video on the basal ganglia. Today we're going to be talking about the direct and indirect pathways. Models used to describe the influence of the basal ganglia on conscious movement. First of all we're going to write out the bits that are involved. We've got the cortex, the motor cortex, which occupies this part of the gray matter. We've got the striatum represented by the putamen solely in this model. We've got the globus pallidus externa, the globus pallidus interna, the thalamus in the midline, but the subthalamic nucleus underneath the thalamus of course. And we've got two parts of the substantia nigra, the pars reticulata and the pars compacta. When we talk about these pathways there are two neurotransmitters that enter the discussion. Glutamate which provides an excitatory signal and GABA which provides an inhibitory signal. The end result is always excitatory output from the thalamus to the cortex. But the role of these pathways is to to modulate the strength of that excitation. The direct pathway increases the level of that excitatory output. So we can think of it as an excitatory pathway. The indirect on the other hand reduces the level of that excitatory output. So it's effectively an inhibitory pathway. I'm going to quickly write out the neurotransmitters released by each of these parts we've got listed on the left. This is only relevant to these particular pathways. So the cortex releases glutamate, striatum, globus pallidus, both parts, release GABA, the thalamus and the subthalamic nucleus release glutamate, the pars reticulata, GABA and the pars compacta as we'll discuss down the track releases dopamine. Okay, so let's get stuck into the direct pathway first. The cortex initially provides excitatory input to the striatum. That's going to increase its level of GABA output. And that GABA output is going to act on the globus pallidus interna and the substantia nigra pars reticulata in the direct pathway. So these both release GABA as you can see on the left. And they both release GABA on the thalamus. But because they've been inhibited by the striatum, that GABA output's going to be decreased, which allows the thalamus to provide a strong excitatory output to the motor cortex, which we'll see as increased muscle movement. Okay, over to the indirect pathway now. As before, the cortex excites the striatum with glutamate. But that's where the similarities end. The striatum then releases GABA on the globus pallidus externa. It's connected to the subthalamic nucleus, but because it's inhibited, there's going to be a reduced inhibitory signal to the subthalamic nucleus, which allows it to provide a strong excitatory signal to the globus pallidus internal. The excited internal globus pallidus provides a strong inhibitory signal to the thalamus. And as a result, we've got a reduced excitatory output to the cortex, which we see as reduced muscle movement. So as we mentioned, the past compactive functions to release dopamine, and it does that on the striatum. In the direct pathway, that's released onto D1 receptors of the striatum. The functional outcome of that is an excitatory signal, so increased activity of the striatum, increased GABA output, which leads to an increased excitatory signal to the cortex, the essential function of the direct pathway. On the other hand, in direct contrast in the indirect pathway, we see dopamine being released on D2 receptors, which has an inhibitory effect. An inhibited striatum, in this case, reduces the inhibition of the thalamus. So what we see on both sides is an increased as a result of dopamine release from the substantia nigra, past compacta. Okay, now let's have a run through of those pathways close up. Okay, I feel like I've said excitatory and inhibitory enough for one day, so thank you for watching and we'll see you next time.