ABSTRACT
The compartments of the frontal lobes, described in the previous chapter, constitute part of the frontostriatal system; the rest comprise the basal ganglia, which, along with the cerebellum, are commonly referred to as the extrapyramidal system. In this chapter we shall describe the direct (facilitatory) and indirect (inhibitory) pathways that make up each of the ®ve parallel, segregated, frontostriatal circuits. We shall review the likely roles of the basal ganglia in selecting and amplifying wanted movements, action, and thought patterns, and inhibiting unwanted activities, via these two pathways respectively. We shall also address other, probably related, roles that have been proposed for basal ganglia function, such as optimising patterns of motor activity in target attainment, sequencing movement complexes in terms of tonic ``set'' and phasic ``cue'' functions, respectively for controlling movement amplitude and for mediating serial release, and ``chunking'' and ``binding'' functions. Although many of these proposed functions are both hypothetical and mutually overlapping, they go some way towards explaining breakdown of function in the neurodegenerative disorders of ageing, such as Parkinson's and Huntington's diseases, and the neurodevelopmental, ``psychiatric'' conditions with which this book is concerned.
