Basal Ganglia Circuits

Which neurotransmitter is released by dopaminergic neurons in the substantia nigra pars compacta (SNc) onto the striatum in the direct pathway of basal ganglia circuits?

In the indirect pathway of basal ganglia circuits, which structure receives inhibitory (GABAergic) input from the striatum and projects to the globus pallidus interna?

Which structural feature best distinguishes the direct pathway from the indirect pathway in basal ganglia circuits?

A 58-year-old man from Delhi presents with a 2-year history of progressive tremor at rest in his right hand, which disappears with intentional movement. He also reports difficulty initiating gait and a stooped posture. On examination, he has a pill-rolling tremor, cogwheel rigidity in both upper limbs, and bradykinesia. His wife notes he has become emotionally flat and speaks in a monotone. MRI brain is normal. Which of the following basal ganglia circuits is primarily affected in this patient's condition?

A 42-year-old woman from Bangalore presents with involuntary, jerky, purposeless movements of her limbs and trunk that worsen with stress and improve during sleep. She has a family history of similar illness in her mother and maternal uncle. Cognitive decline and emotional lability are noted. Genetic testing confirms a CAG trinucleotide repeat expansion. On functional neuroimaging, there is selective loss of medium spiny neurons in the striatum, particularly affecting neurons that express D2 dopamine receptors. Which basal ganglia circuit dysfunction best explains her hyperkinetic movement disorder?