Parkinson’s disease is characterized by slowness or absence of movement (bradykinesia or akinesia), rigidity, and a resting tremor (especially in the hands and fingers). Patients have difficulty initiating movements, and once initiated the movements are abnormally slow. The cause of Parkinson’s disease is the loss of the dopaminergic neurons in the substantia nigra pars compacta (Figure 4.10). From one’s knowledge of the effects of the nigrostriatal pathway on the direct and indirect pathways, it becomes straightforward to see why the loss of this pathway results in the poverty of movement symptomatic of Parkinson’s disease. Because the nigrostriatal pathway excites the direct pathway and inhibits the indirect pathway, the loss of this input tips the balance in favor of activity in the indirect pathway. Thus, the GPint neurons are abnormally active, keeping the thalamic neurons inhibited. Without the thalamic input, the motor cortex neurons are not as excited, and therefore the motor system is less able to execute the motor plans in response to the patient’s volition.