ABSTRACT
The cell loss of the principal neurons of the striatum does not affect the dopaminergic reinforcing input, which mediates synaptic strengthening and behavior modifi cation. There is also no pathological exaggeration of the dopamine-mediated modifi cation process by which purposeful voluntary action is acquired. Therefore, one would expect a loss of behavioral repertoire there, rather than a loss of the processes of modifi cation of behavior, as occurs in some other disorders considered below. Most obvious are the “positive” motor symptoms, such as chorea, athetosis, and ballismus, refl ecting the loss of ability to inhibit unwanted movement, as a result of loss of neurons of origin of the indirect pathway. Less obvious, but well documented in advanced cases is the “negative” motor symptom, namely akinesia, in which loss of neurons of origin of the direct pathway dominate the picture.
