The pathways have competing effects on movement : first pathway, stimulated, resulte in an activation of cerebral cortex while a stimulation of second pathway produce an inhibition.
Substantia nigra regulates this pathways with two dopamine receptors: D1 and D2.
Dopamine projections from the substantia nigra, pars compacta, to the striatum have complex, and ultimately opposing effects on activity in the direct and indirect pathways. For example, dopamine acts primarily through dopamine D1 receptors on the neurons that participate in the direct pathway, exciting these neurons. It acts on dopamine D2 receptors on the striatal neurons that are involved in the indirect pathway. The D2 receptors are inhibitory. Therefore, dopamine excites the direct pathway and inhibits the indirect pathway, with a net effect to increase facilitatory inputs to the motor regions. (The Basal Ganglia)
In hypokinesias – like parkinson’s disease - dopamine loss produces an overdone activation of indirect pathways producing in the patient typical symptoms of disorder (shaking, rigidity, slowness of movement and difficulty with walking and gait abnormaties).
(Parkinson's disease)