Figure 6. Direct and indirect pathways.

a. Using the bucket analogy, the change in water level is proportional to inflow minus outflow, for any given time step. The direct pathway reflects the inflow, whereas the indirect pathway acts as the leak or damping in transition control. These two pathways exert opponent influences on the output nuclei. For example, striatonigral projections can decrease SNr firing, whereas the striatopallidal projections can increase SNr firing (Freeze et al., 2013).

b. Illustration of the net effects of the two pathways on downstream structures. For the sake of simplicity, the intrinsic BG circuit connections are omitted here. Note that, because from the striatum to downstream targets there are two inhibitory synapses (striatonigral and nigrotectal), the direct pathway has a net excitatory effect on the target region of SNr outputs (e.g. tectum). Thus the bucket analogy is still useful despite the sign of the signals. Bridging collaterals from striatonigral axons to GPe, the classic indirect pathway (cortex-striatum-GPe-SNr/GPi), and the hyperdirect pathway (Cortex-STN-SNr/GPi) can all enhance damping in the transition control system. The highly plastic bridging collaterals allow adjustment of the damping constant.