Pars compacta
Other similar neurons are more sparsely distributed in the midbrain and constitute "groups" with no well-defined borders, although continuous to the pars compacta, in a pre-rubral position.[6] The dopaminergic neurons of the pars compacta project many of their axons along the nigrostriatal pathway to the dorsal striatum, where they release the neurotransmitter dopamine.Dopaminergic axons also project to other elements of the basal ganglia, including the lateral and medial pallidum,[7] substantia nigra pars reticulata, and the subthalamic nucleus.Degeneration of pigmented neurons in this region is the principal pathology that underlies Parkinson's disease and this depigmentation can be visualized in vivo with Neuromelanin MRI.Binding to neuromelanin by means of charge transfer complexes may concentrate radical-generating toxins in the substantia nigra.