A functional division of the basal ganglia, the cluster of neurons responsible for voluntary movement that is also called the globus pallidus. As other brain structures, the pallidum is a paired formation; the right structure controls movements on the left side of the body and the left structure controls movements on the right side of the body.
Each pallidum has two parts, the globus pallidus externus (designated as the GPe) and the globus pallidus internus (GPi). Both receive nerve signals from the putamen, and the GPi also receives inputs from the subthalamic nucleus (STN) other structures of the basal ganglia. The GPi then sends out nerve signals to the thalamus, which channels them to sections of the cerebrum that direct voluntary movement, and to the brainstem, which regulates coordination of movement and sends nerve signals to the muscles. GPi nerve signals are inhibitory to these structures; that is, they slow their actions. In a healthy brain, dopamine helps to regulate the activity of neurons throughout the basal ganglia. It acts as an inhibitory neurotrans-mitter among the neurons in the GPi, where it functions to keep their activity in check. Dopamine affects the GPi via direct and indirect pathways from the putamen, which receive dopamine via the NIGROSTRIATAL FIBERS from the SUBSTANTIA NIGRA that produces it.
In Parkinson’s disease, the GPi malfunctions. As dopaminergic neurons continue to die, as is the hallmark of Parkinson’s disease, the number of nigrostriatal fibers also decreases. There is less dopamine in the brain, and there are fewer fibers to carry it where it needs to go. Without dopamine to curtail them the neurons in the GPi intensify their activity, sending increased inhibitory signals to the thalamus. The resulting decrease in thalamic stimulation to the cerebral motor cortex results in bradykinesia (slowed movements), akinesia (lack of movement or “freezing”), dystonia (extreme rigidity), and other dyskinesias (abnormal movements) of Parkinson’s.
anti-parkinson’s medications that restore dopamine levels reestablish temporary control over neuron activity in the GPi, easing these symptoms. Levodopa and other medications become less effective as Parkinson’s progresses, however, and GPi activity again intensifies. surgery called pallido-tomy that interferes with the GPi’s neuron communication can produce long-term relief of neuromuscular symptoms for some people with late stage Parkinson’s disease.