Direct and indirect pathways

There are two paths through the basal ganglia circuitry with reverse effects on firing of thalamic, and therefore cortical, neurons which is as shown in figure below. The direct pathway uses the GABA/SP/DYN medium spiny striatal neurons that inhibits GABAergic outflow of the SNpr and GPi to the thalamus. The Cortical activation of this pathway increases the firing of thalamic neurons (as inhibiting an inhibition is excitation).

Figure: Direct and indirect pathway activation increased or decreased firing of thalamic neurons correspondingly. Here, GPe=globus pallidus pars externa; GPi=globus pallidus pars interna; and SNpr=substantia nigra pars reticulata.

The indirect pathway begins with the GABA/ENK medium spiny neuron outcome to the GPe, inhibitory neurons from which go to the STN. And STN excites inhibitory neurons in the SNpr and GPi which go to the thalamus. The Corticostriate activation of the indirect pathway outcomes in reduced firing of thalamic neurons. This dual circuitry permits the possibility that given movement series might be triggered or suppressed by differential activation of direct or indirect pathways correspondingly.