The current view of NPMA organization
The classical view of voluntary motor control was that, when an action is willed, the frontal association areas pass a command to the "premotor" cortical areas, which in turn select a motor plan and pass it to M1. M1 executes the movement via direct and indirect commands to the spinal motoneurons. Evidence has emerged over the past few decades suggesting that this sequential, hierarchical model of voluntary motor control is inaccurate and should be replaced with a more parallel, distributed model. In a parallel, distributed model, NPMAs and M1 areas are engaged together in generating the motor plan and controlling spinal motoneurons. Whereas the sequential, hierarchical model suggested that destruction of any single area in the hierarchy would prevent voluntary movement, the parallel, distributed model implies that if one center is destroyed, other centers may be able to take over.
What evidence is there that voluntary motor control should be viewed as a parallel, distributed process? It is now known that there are many more frontal cortical motor areas than formerly thought, that these areas are extensively interconnected with M1 and with one another, and that many NPMAs have significant corticospinal projections themselves. Primate studies have identified multiple NPMAs in the frontal lobe (Fig. 2.2). Anterior to the M1 in area 4, area 6 contains a ventral premotor area (PMv), and a dorsal premotor area (PMd) on the lateral surface of the hemisphere, and the supplementary motor area (SMA) on the medial surface. PMv and PMd can be further subdivided into a ventrorostral premotor area (PMvr), ventrocaudal premotor area (PMvc), dorsorostral premotor area (PMdr), and dorsocaudal premotor area (PMdc). Rostral to the SMA, area 6 also contains the pre-supplementary motor area (pre-SMA). Area 24 in the superior and inferior banks of the cingulate sulcus contains at least dorsal (CMAd), rostral (CMAr), and caudal (CMAc) cingulate motor areas. These NPMAs can be distinguished from one another because they have: (1) different cytoarchitechtonic features; (2) separate microstimulation maps; (3) separate reciprocal interconnections with the thalamus, with M1 and/or with one another; and (4) different features of physiologic activity (for reviews see Rizzolatti et al., 1998; Dum and Strick, 2002). Figure 2.3 shows our current road map of the interconnections between these areas. Corticospinal projections arise not only from
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