A 58-year-old man from Delhi presents with a 2-year history of progressive tremor at rest in his right hand, which worsens with stress and improves with intentional movement. He also reports difficulty initiating walking and a stooped posture. Examination reveals bradykinesia, rigidity (cogwheel type), and a resting tremor of 4–5 Hz. MRI brain is normal. Which circuit within the basal ganglia is primarily dysfunctional, and what is the neurochemical basis of his motor symptoms?