In this article we explore the complexities of what goes on in the brain when one wishes to perform even the simplest everyday movements. In doing so, we describe experiments indicating that the spinal cord interneurons are organized in functional modules and that each module activates a distinct set of muscles. Through these modules the central nervous system has found a simple solution to controlling the large number of muscle fibers active even during the execution of the simplest action. We also explore the many different neural signals that contribute to pattern formations, including afferent information from the limbs and information of motor memories.
Scientists and nonscientists alike rarely stop to consider what is going on in their brains when they perform a voluntary movement such as reaching for an object, throwing a ball, or driving a car. Why? Presumably they may realize that translating something as evanescent as a wish to move into muscle contractions must be an awfully complicated process. Indeed, they are right: the neural processes that subserve even the simplest everyday actions are incredibly complex and only partially understood. In this essay we take up the challenge of explaining what we know about this fascinating and complex topic.
Let us begin with the basic fact that, in general, our movements–even the simplest actions–are accomplished through activation of a large number of muscles. For example, if you are sitting at your desk typing at your computer and decide to turn to pick up a cup of coffee, you will activate, approximately at the same time, the eye muscles, the numerous muscles in the neck, and the muscles of the shoulder, arm, forearm, and fingers. A simple computation would show that your brain has activated at least thirty muscles. But note that each muscle is made up of cells called muscle fibers, and that each muscle fiber receives a neural input via its own nerve fiber (see Figure 1). It follows that the number of elements . . .