Chapter 8E - Initiation of Movement
Simple motor responses, particularly ones that are repetitive or well rehearsed often require only activity of motor cortex itself. On the other hand, somewhat more complex movements, requiring some decision typically activate the premotor and supplementary motor as well as the motor region. The premotor cortex is active in the planning stages of a motion, even when no movement ultimately takes place. Therefore, one could conclude that areas such as the premotor cortex are involved in planning movements, while the motor cortex is more involved in its execution. For example, lesions of the supplementary motor area can produce a condition called abulia in which initiation of movement is nearly impossible even though all other elements of the motor system are intact. More complex movements, particularly those where significant planning is required, typically begin with activity in the parietal lobes. For example, there is an area in the lower posterior parietal lobe that is necessary for initiating accurate finger grasp.