THE PRECEDING CHAPTERS IN THIS BOOK consider how the brain constructs internal representations of the world around us. These representations are behaviorally meaningful when used to guide movement. Thus, an important function of the sensory representations is to shape the actions of the motor systems. This chapter describes the principles that govern the neural control of movement using concepts derived from behavioral studies and computational models of the brain and musculoskeletal system.
We start by considering the challenges motor systems face in generating skillful actions. We then examine some of the neural mechanisms that have evolved to meet these challenges and produce smooth, accurate, and efficient movements. Finally, we see how motor learning improves our performance and allows us to adapt to new mechanical conditions, such as when using a tool, or to learn novel correspondences between sensory and motor events, such as when using a computer mouse to control a cursor. This chapter focuses on voluntary movement; reflexes and rhythmic movements are discussed in further detail in Chapters 32 and 33.
Voluntary movements are generated by neural circuits that span different levels of the sensory and motor hierarchies, including regions of the cerebral cortex, subcortical areas such as the basal ganglia and cerebellum, and the brain stem and spinal networks. These different structures have unique patterns of neural activity. Moreover, focal damage to different structures can cause distinct motor deficits. Although it is tempting to suggest that these individual structures have distinct functions, these brain and spinal areas normally work together as a network, such that damage to one component likely affects the function of all others. Many of the principles discussed in this chapter cannot be easily attributed to a single brain or spinal area. Instead, distributed neural processing is likely to underlie the computational mechanisms that subserve sensorimotor control.
The Control of Movement Poses Challenges for the Nervous System
Motor systems produce neural commands that act on the muscles, causing them to contract and generate movement. The ease with which we move, from tying our shoelaces to returning a tennis serve, masks the complexity of the control processes involved. Many factors inherent in sensorimotor control are responsible for this complexity, which becomes clearly evident when we try to build machines to perform human-like movement (Chapter 39). Although computers can now beat the world’s best players at chess and Go, no robot can manipulate a chess piece with the dexterity of a 6-year-old child.
The act of returning a tennis serve illustrates why the control of movement is challenging for the brain (Figure 30–1). First, motor systems have to contend with different forms of uncertainty, such as our incomplete knowledge with regard to the state of the world and the rewards we might gain. On the sensory side, although the player may see the serve, she ...