 This model is supported by experiments demonstrating that optogenetically stimulating direct pathway medium spiny neurons increases locomotion, whereas stimulating indirect pathway medium spiny neurons inhibits locomotion.The balance of direct/indirect activity in movement is supported by evidence from neurodegenerative disorders, including Parkinson's disease (PD), which is characterized by loss of dopamine neurons projecting to the striatum, hypoactivity in direct pathway and hyperactivity in indirect pathway neurons, along with motor dysfunction.This results in loss of normal action selection, as loss of dopamine drives activity in the indirect pathway, globally inhibiting all motor paradigms.