ventral tier dopamine cells have dendrites that extend ventrally into the pars reticulata. Dopaminergic axons The axons from dopamine neurons emanate from a primary dendrite and project ipsilaterally (on the same side) via the medial forebrain bundle to the dorsal striatum. There is a rough topographical correlation between the anatomical localization of the dopamine cell body within the SNc and the area of termination in the dorsal striatum. Dopaminergic cells in the lateral parts of the SNc project mainly to the lateral and caudal (posterior) parts of the striatum, whereas dopamine cells in the medial SNc project to the medial striatum. In addition, dopamine cells in the dorsal tier project to the ventromedial striatum, whereas the ventral tier neurons project to the dorsal caudate nucleus and putamen. In general, there is a greater density of dopaminergic input to the dorsolateral striatum. Each dopamine neuron has an extremely large unmyelinated axonal arborization which can innervate up to 6% of the striatal volume in a rat. Although all SNc dopamine cells project to both the striosome (or patch) and matrix neurochemical compartments of the striatum, most of the axonal territory of a dorsal tier neuron is in the matrix compartment while the majority of the axonal field of ventral tier neurons is in the striosomes. Nigrostriatal dopamine axons can also give rise to axon collaterals that project to other brain regions. For example, some SNc nigrostriatal dopamine axons send axon collaterals to the pedunculopontine nucleus, ventral pallidum, subthalamic nucleus, globus pallidus, amygdala, and thalamus. A small number of SNc dorsal tier dopamine neurons also project directly to the cortex, although most of the dopaminergic innervation of the cortex comes from the adjacent VTA dopamine neurons. Dorsal striatum The dorsal striatum is located in the subcortical region of the forebrain. In primates and other mammals, it is divided by the anterior limb of a white matter tract called the internal capsule into two parts: the caudate nucleus and the putamen. In rodents, the internal capsule is poorly developed such that the caudate and putamen are not separated but form one large entity called the caudate putamen (CPu). The majority (about 95%) of cells in the dorsal striatum are GABAergic medium spiny neurons (MSNs), also known as spiny projection neurons (SPNs). Approximately half of these MSNs contain dopamine D1 receptors and project directly to the substantia nigra to form the direct pathway of the