Why can damage to the subthalamic nucleus cause hemiballismus?

Hemiballismus is a hyperkinetic movement disorder characterized by involuntary, violent, flinging movements on one side of the body. It is typically caused by a small lesion or infarct within the subthalamic nucleus.12

Anatomically, the subthalamic nucleus functions as a critical component of the basal ganglia motor circuit, where it serves as an input nucleus that receives information from the cortex and thalamus. Unlike most other nuclei in the basal ganglia that utilize inhibitory neurotransmitters, the subthalamic nucleus is unique for its excitatory, glutamate-releasing projections.34

By influencing the outflow from the basal ganglia to the thalamus, the subthalamic nucleus plays a vital role in regulating movement. When this nucleus is damaged, the loss of its normal regulatory influence disrupts the firing patterns within the basal ganglia's parallel pathways. This disruption leads to the loss of control over motor output, resulting in the characteristic wild, flinging movements associated with hemiballismus.3512