Functional neuroanatomy of the primate isocortical motor system

The concept of the primate motor cortex based on the cytoarchitectonic subd ivision into areas 4 and 6 according to Brodmann or the functional subdivis ion into primary motor, supplementary motor, and lateral premotor cortex ha s changed in recent years. Instead, this cortical region is now regarded as a complex mosaic of different areas. This review article gives an overview of the structure and function of the isocortical part of the motor cortex in the macaque and human brain. In the macaque monkey, the primary motor co rtex (Brodmann's area 4 or area Fl) with its giant pyramidal or Betz cells lies immediately anterior to the central sulcus. The non-primary motor cort ex (Brodmann's area 6) lies further rostrally and can be subdivided into th ree groups of areas: the supplementary motor areas "SMA proper" (area F3) a nd "pre-SMA" (area F6) on the mesial cortical surface, the dorsolateral pre motor cortex (areas F2 and F7) on the dorsolateral convexity, and the ventr olateral premotor cortex (areas F4 and F5) on the ventrolateral convexity. The primary motor cortex is mainly involved in controlling kinematic and dy namic parameters of voluntary movements, whereas non-primary motor areas ar e more related to preparing voluntary movements in response to a variety of internal or external cues. Since a structural map of the human isocortical motor system as detailed as in the macaque is not yet available, homologie s between the two species have not been firmly established. There is increa sing evidence, however, that a similar organizational principle (i.e., prim ary motor cortex, supplementary motor areas, dorso- and ventrolateral premo tor cortex) also exists in humans. Imaging studies have revealed that funct ional gradients can be discerned within the human non-primary motor cortex. More rostral cortical regions are active when a motor task is nonroutine, whereas more routine motor actions engage more caudal areas.