Neuronal populations in motor areas of cerebral cortex that act as input to the basal ganglia will be identified by retrograde transport of tritiated acetyl wheat germ agglutinin or wheat germ agglutinin conjegated to horseradish peroxidase. Paired injections in neostriatum and each of several other major targets of motor and premotor cortex will also be performed. The neurons projecting to neostriatum will be compared to those giving rise to other important motor cortex output pathways, and the extent to which neurons in cerebral cortex have branched axons projecting to neostriatum and another structure will be assessed. Corticostriatal neurons from each of the populations defined in this way will also be identified by antidromic activation in intracellular recording experiments. These cells will be stained by intracellular injection of horseradish peroxidase to examine their dendritic branching patterns (as an indication of what inputs they might receive), their patterns of intracortical synaptic connections (inferred from the distribution of their intracortical axonal arborizations) and the size and shape of the area in neostriatum that receives an input from a single axon. In addition, major inputs to these cortical areas will be stimulated and the responses of intracellularly recorded corticostriatal neurons analyzed to determine the extent to which different populations of corticostriatal neurons receive synaptic input from ventral thalamic nuclei, contralateral motor and premotor cortex, and axon collaterals of pyramidal tract neurons. These experiments are designed to help to define the nature of the motor cortex input to the basal ganglia by comparing the neurons that convey it to those carrying cortical efferent activity in pathways whose behavioral importance is more clearly understood. They will also contribute to the process of defining the operations performed by the basal ganglia on its cortical input.
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