Direct And Indirect Pathway

The direct and indirect pathways are circuits heavily involved in the regulation of movement. The direct pathway is thought to decrease inhibitory basal ganglia output (coming from the GPi/SNpr nuclei), whereas the indirect pathway increases inhibitory basal ganglia output. Therefore one would expect the direct pathway to increase motor activity and movement and the indirect pathway to inhibit it. So, when the direct pathway is activated, cells in the striatum make inhibitory connections with cells in the globus pallidus internal segment. The globus pallidus internal segment cells in turn generate inhibitory connections on cells in the thalamus. The firing of globus pallidus internal segment neurons inhibits the thalamus. This reduces the chances of the thalamus exciting the (neo)cortex. However, when the striatal neurons (contain D1 receptors) of the direct pathway fire, they inhibit the activity of the globus pallidus internal segment neurons. This inhibition releases the thalamic neurons from their inhibition allowing them to fire in a manner that will excite the cortex leading to the increase in motor (cortex) activity. The activity in effect creates a positive feedback loop.
External segment of the globus pallidus neurons produce inhibitory connections to cells in the sub thalamic nucleus leading to excitatory connections within the internal segment of the globus pallidus. When these neurons are active, there is inhibition of thalamic neurons. Further inhibition of the cortex occurs when subthalamic neurons fire, increasing the firing rate of the internal segment of the globus pallidus neurons and inhibiting motor (cortex)