Which structure is the primary source of dopaminergic neurons lost in Parkinson's disease?

• A. Caudate Nucleus
• B. Substantia Nigra
• C. Putamen
• D. Subthalamic Nucleus

Answer: (B)

The key idea is that Parkinson's disease primarily involves the loss of dopaminergic neurons in the substantia nigra pars compacta. These neurons send dopamine-rich signals to the dorsal striatum (the putamen and caudate) via the nigrostriatal pathway, which is crucial for regulating movement through the balance of the basal ganglia circuits. When SNpc neurons degenerate, dopamine in the striatum falls, especially affecting the putamen, tipping the scale toward the indirect pathway and producing the hallmark motor symptoms of Parkinson's (bradykinesia, rigidity, tremor). The caudate and putamen are targets of these dopaminergic projections, not the sources of the neurons themselves, while the subthalamic nucleus is part of the circuit that becomes overactive downstream of dopamine loss. Thus, the structure that serves as the primary source of the affected dopaminergic neurons is the substantia nigra.