Many of the major neurotransmitters are distributed by single tracts of neurons. The cell bodies of these neurons are located deep in the midbrain or brain stem but their axons travel great distances across the limbic system and the cortex, branching off to form synapses with many other neurons along the way. In this way, a neurotransmitter tract acts somewhat like a subway, with the neurotransmitters acting as passengers. In the same way that the No. 1 line in New York City runs through City College, Columbia University, the theater district, and the financial district, each neurotransmitter tract runs through specific areas of the brain that serve specific functions. If you blockaded the No.1 line running down Broadway, you would have reduced activity in the four districts mentioned above. Likewise, if you blockade dopamine activity in the nigrastriatal tract, you will have reduced activity in the basal ganglia and subsequent problems with motor coordination will arise. This is exactly what happens with Parkinson’s disease.