Fig. 2

Neuronal circuits and neurotransmission mechanisms of control in the brains of normal individuals and those with Parkinson’s disease. a: Neuronal circuit in basal ganglia in normal brain. b: Degeneration of substantia nigra pars compacta (SNpc) impairs cortico-striatal circuit in PD brain. Decrease in DA levels in the SNpc and striatum causes loss of control of striatal neuronal firing, leading to withdrawal of inhibitory effects on globus pallidus as well as thalamus, therefore, the thalamus becomes over-excitable, which activates the motor cortex excessively. This ultimately leads to impairment of motor coordination and causes Parkinsonism