The common denominator of virtually all disorders associated with clinical parkinsonism is neuronal loss in the substantia nigra, particularly of dopaminergic neurons in the pars compacta that project to the striatum (Fig. 1). The ventrolateral tier of neurons appears to be the most vulnerable in many parkinsonian disorders and these tend to project heavily to the putamen (1). The more medial groups of neurons send projections to the forebrain and the medial temporal lobe and are less affected. The dorsal tier of neurons may be most vulnerable to neuronal loss associated with aging (1).
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