An increasing number of neurophysiological studies have provided important information on the pathophysiology of different types of movement disorders, including hypokinetic disorders such as Parkinson’s disease and hyperkinetic movement disorders such as dystonia, Huntington’s disease and Gilles de la Tourette syndrome. The aim of this chapter is to review the main pathophysiological advances that have been made thanks to neurophysiological techniques designed to investigate both voluntary movement and the excitability and plasticity of brainstem, spinal, and cortical circuits. Studies that highlight the involvement of the sensory system in various types of movement disorders will also be discussed. Motor symptoms in Parkinson’s disease (PD) mainly consist of poverty and slowness of movement (bradykinesia), rigidity, and tremor. Parkinsonian symptoms are thought to occur when degeneration of dopaminergic neurons in the substantia nigra compacta induces functional changes in the cortico-striato-thalamo-cortical circuit . Experimental studies on both parkinsonian animals and humans undergoing functional neurosurgery have demonstrated that neuronal firing is increased (as well as changes in the quality of firing) in the subthalamic nucleus (STN), globus pallidus interna (GPi) and substantia nigra reticulata (SNr), and that activity is reduced in the globus pallidus externa (GPe). This finding points to increased activity in the indirect pathway and decreased activity in the direct pathway, which in turn increases inhibitory output from the GPi and SNr to the thalamus and reduces thalamo-cortical activity .
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