An open access publication of the American Academy of Arts & Sciences
Winter 2015

A Hard Scientific Quest: Understanding Voluntary Movements

Emilio Bizzi and Robert Ajemian
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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.

EMILIO BIZZI, a Fellow of the American Academy since 1980 and President of the Academy from 2006 to 2009, is Institute Professor in the Department of Brain and Cognitive Sciences and Investigator at the McGovern Institute for Brain Research at the Massachusetts Institute of Technology. His recent publications include articles in such journals as Frontiers in Computational Neuroscience, Proceedings of the National Academy of Sciences, Neuron, and the Journal of Neurophysiology.

ROBERT AJEMIAN is a Research Scientist at the McGovern Institute for Brain Research at the Massachusetts Institute of Technology. His publications include articles in such journals as Neuron, Cerebral Cortex, the Journal of Motor Behavior, and the Journal of Neurophysiology.

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 . . .

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