[Physiological bases of involuntary movement].
[Physiological bases of involuntary movement].
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Original abstract
Recent physiological, anatomical and neurobiological works on the basal ganglia give us clues for our understanding of the mechanisms of generation of involuntary movement. These are characteristic neuron circuit of the basal ganglia and signal content of the information transferred through the basal ganglia circuit. Efferent signals mediated by the striatal projection neurons are fed to either the internal segment (GPi) or external segment (GPe) of the globus pallidus or substantia nigra pars reticulata (SNr). Projection neurons of the striatum, GPe, Gapital and SNr are all inhibitory in their action, while those of the subthalamic nucleus (STN) are excitatory. Therefore, projection from the striatum to the GPi or SNr (direct path), output nucleus of the basal ganglia, would produce facilitatory effects on the target of the basal ganglia output through disinhibition. On the other hand, projections of striatum-GPe-STN-GPi pathway (indirect path) would produce active inhibition of the target. GPi activity of monkey rendered Parkinsonism by MPTP is abnormally high and activity of GPe is lower than that before MPTP application, while the GPi activity of STN-lesioned monkey showing limb dyskinesia (hemiballismus) is much lower than normal animal. These observations suggest it is essential to maintain adequate level of basal ganglia inhibitory output for proper muscle tone and stable performance of movement. Indirect pathway and STN system may play an essential role in the stabilization of output level of basal ganglia.