Motor Systems

Our understanding of functions of motor system evolved remarkably in the last 20 years. This is the result not only of the accumulation of data on this system but also of a shift in our understanding of its functions. Animal behavior is characterized by its capacity to generate movements. Some movements are automatic reactions to external or internal stimuli (reflexes) instead others are manifestations of a centrally generated intention to act (voluntary movements). Intention may result from bodily needs such as hunger or thirst, or from higher-order deliberations based on one's own beliefs and desires. Nonetheless, the way in which intention to act is generated is irrelevant for the notion of voluntary movement. What counts in voluntary action is that the individual aims to achieve an overarching goal and that this latter determines a series of motor acts leading to its achievement. Because of their capacity to generate voluntary movements, animals move in their environment looking for stimuli that may satisfy their immediate or future needs. When they find something appropriate for their need, they can interact with it. In voluntary behavior, however, stimuli do not determine the response, they only set the occasion for it. Therefore, according to their needs, animals may respond to the same stimulus in different ways. They may approach, avoid, or even ignore it. In this chapter, we will describe how animals, and in particular nonhuman primates and humans, are able to organize goal-directed actions. To this aim, we will first describe the anatomic structure of the motor cortex of monkeys and humans. Then we will describe the cortical and subcortical connections of motor areas of the monkey. Subsequently we will focus on the function of specific circuits subserving sensory-motor transformations.