Responses of Neurons in Subthalamic Nucleus During Sequential Reaching in Patients with Parkinson’s Disease
Sequential movements are fundamental to motor behaviour, and many are unique to human experience, such as typing, writing, speech, and music. While much work has focused on the prefrontal cortical regions involved in the organization of multiple movements (see Tanji, 2001), relatively little is known about subcortical nuclei such as the subthalamic nucleus (STN). Central to our current understanding of basal ganglia function is the concept of recurrent neuronal pathways connecting the cortex, basal ganglia (including STN), and thalamus (Alexander et al., 1990). Anatomical evidence has accrued to support the existence of “cortico-striato-pallido-thalamic” loops at least as far as the motor regions of these brain structures are concerned. This evidence has lead to the development of theoretical models of the basal ganglia subserving as many as 5 separate functional loops; two motor loops for somatic and ocular systems, two associative circuits for dorsolateral prefrontal and lateral orbitofrontal systems, and a limbic or cingulate system (Alexander et al., 1990). These models indicate a flow of information through a direct and indirect pathway linking the basal ganglia input at the level of the putamen, to the output, the internal segment of globus pallidus (GPi). In this scheme, the STN receives inhibitory input from the GPe, and sends excitatory output to the GPi along the indirect striatopallidal pathway. Although these models have heuristic value, recent anatomical and neurophysiological evidence indicates that the STN is an important input structure to the basal ganglia, rather than a simple intermediate relay in the indirect pathway. There exists a topographical input to the STN from primary motor (MI), supplementary motor area (SMA), and the dorsal and ventral premotor (PMd,v)that the majority of neurons responded equally on both tasks and only a few neurons showed task-specific effects. This suggests that the STN is not a primary locus of storage of motor memories for sequences, but may trigger the release of this information from the pallidal output. Some limited evidence showing statistically significant decrementing responses with later elements of the sequence may indicate some role in serial order, as has been recently proposed for the motor cortex (Carpenter et al., 1999).
KeywordsBasal Ganglion Supplementary Motor Area Delay Period Subthalamic Nucleus Thalamic Reticular Nucleus
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