Final answer:
The substantia nigra pars compacta is the midbrain structure whose dopaminergic axons synapse in the Basal Ganglia, playing a critical role in movement control and implicated in Parkinson's disease when these neurons degenerate.
Step-by-step explanation:
The midbrain structure whose dopaminergic axons synapse in the Basal Ganglia is known as the substantia nigra pars compacta. The substantia nigra is a subcortical nucleus that resides within the basal nuclei and is critically involved in the control of movement. Dopamine produced in the substantia nigra modulates the function of the striatum as part of the motor pathway. Degeneration of substantia nigra neurons leads to disorders such as Parkinson's disease, characterized by impaired motor control and other systemic effects related to the loss of dopaminergic neurons.
Within the basal nuclei, there are two streams of information processing, the direct pathway and the indirect pathway. Both pathways involve the striatum, influencing the output of the Globus Pallidus internus (GPi)/substantia nigra pars reticulata (SNr), which then projects to the thalamus and back to the cortex. These pathways are essential for regulating motor activity and are modulated by the dopaminergic neurons of the substantia nigra.