mm³. This translated in numbers of neurons represents a strong compression with loss of map precision. Some axons from the lateral pallidum go to the striatum. The activity of the medial pallidum is influenced by afferences from the lateral pallidum and from the subthalamic nucleus. The same for the substantia nigra pars reticulata. The subthalamic nucleus sends axons to another regulator: the pedunculo-pontine complex (id). The lateropallido-subthalamic system is thought to play a key role in the generation of the patterns of activity seen in Parkinson's disease. Pathophysiology Chronic stimulation of the STN, called deep brain stimulation (DBS), is used to treat patients with Parkinson disease. The first to be stimulated are the terminal arborisations of afferent axons, which modify the activity of subthalamic neurons. However, it has been shown in thalamic slices from mice, that the stimulus also causes nearby astrocytes to release adenosine triphosphate (ATP), a precursor to adenosine (through a catabolic process). In turn, adenosine A1 receptor activation depresses excitatory transmission in the thalamus, thus mimicking ablation of the subthalamic nucleus. Unilateral destruction or disruption of the subthalamic nucleus — which can commonly occur via a small vessel stroke in patients with diabetes, hypertension, or a history of smoking – produces hemiballismus. As one of the STN's suspected functions is in impulse control, dysfunction in this region has been implicated in obsessive–compulsive disorder. Artificially stimulating the STN has shown some promise in correcting severe impulsive behavior and may later be used as an alternative treatment for the disorder. Function The function of the STN is unknown, but current theories place it as a component of the basal ganglia control system that may perform action selection. It is thought to implement the so-called "hyperdirect pathway" of motor control, contrasting with the direct and indirect pathways implemented elsewhere in the basal ganglia. STN dysfunction has also been shown to increase impulsivity in individuals presented with two equally rewarding stimuli. Research has suggested that the subthalamus is an extrapyramidal center. It holds muscular responses in check, and damage may result in hemiballism (a violent flinging of the arm and leg on one side of the body). The physiological role of the STN has been for long hidden by its pathological role. But lately, the research on the physiology of the STN lead to the discovery that the STN is required to achieve intended movement, including locomotion, balance and motor coordination.