Volume 8, Number 5, 2002 THE NEUROSCIENTIST 437 Copyright © 2002 Sage Publications ISSN 1073-8584 In the natural world, some complex systems are discrete combinatorial systems—they utilize a finite number of discrete elements to create larger structures. The genetic code, language, and perceptual phenomena are examples of systems in which discrete elements and a set of rules can generate a large number of meaningful entities that are quite distinct from those of their elements. A ques- tion of considerable importance is whether this funda- mental characteristic of language and genetics is also a feature of other biological systems. In particular, whether the activity of the vertebrate motor system, with its impressive capacity to find original motor solutions to an infinite set of ever-changing circumstances, results from the combinations of discrete elements. The ease with which we move hides the complexity inherent in the execution of even the simplest tasks. Even movements we make effortlessly, such as reaching for an object, involve the activation of many thousands of motor units in numerous muscles. But is this hidden complexity simply the result of combinations of a small number of discrete building blocks? This idea, which is central to our understanding of the neural control of movement, has been offered over the years by a number of investigators. Reflexes (Sherrington 1910), elements of central pattern generators (Grillner 1981), simple movement strokes, spinal force fields (Bizzi and others 1991), and synergistic muscle contractions (Saltiel and others 2001) have variously been suggested as possible building blocks. The problem for the experimentalists that have made these suggestions, however, has been to establish qualitatively and quantitatively the existence of these conjectured building blocks, and most important, to state the rules subserving the combinations of the dis- crete elements into behaviorally significant movements. In the last few years, our group and collaborators have asked a specific question: Are there simple units (motor primitives) that can be flexibly combined to accomplish a variety of motor tasks? We have addressed this funda- mental and longstanding question in experiments that utilize spinalized frogs (Bizzi and others 1991; Giszter and others 1993) and rats (Tresch and Bizzi 1999), whereas other investigators have generated corroborative evidence in cats (Lemay and Grill 2000). With an array of approaches such as microstimulation of the spinal cord, NMDA iontophoresis, and cutaneous stimulation of the hindlimb, we have provided evidence for a modu- lar organization of the frog’s and rat’s spinal cord. In the present context, we use the term module to mean a func- tional unit in the spinal cord that generates a specific motor output by selecting a specific pattern of muscle activation. The original evidence suggesting a modular organiza- tion of frog spinal motor systems came from experi- ments using microstimulation. In these experiments, the motor responses evoked by spinal microstimulation were characterized in terms of force fields. A force field is a mapping that associates each position of the frog’s hindlimb with a corresponding force generated by the neuromuscular system. Force fields were measured by placing the frog’s ankle in different locations in the leg’s workspace and recording at each location the isometric force evoked in response to microstimulation of the same site in the spinal cord. The majority of force fields generated by stimulation of different areas of the lumbar gray were found to converge toward an equilibrium point (Fig. 1). In addition, such convergent force fields (CFFs) could be grouped into only a small number of classes (Giszter and others 1993). In a series of control experi- Modular Organization of Spinal Motor Systems E. BIZZI, A. D’AVELLA, P. SALTIEL, andM. TRESCH Department of Brain and Cognitive Sciences and McGovern Institute for Brain Research Massachusetts Institute of Technology Cambridge The vertebrate nervous system produces a wide range of movement flexibly and efficiently, even though the simplest of these movements is potentially highly complex. The strategies by which the nervous sys- tem overcomes these complexities have therefore been of interest to motor physiologists for decades. In this review, the authors present a number of recent experiments that propose one strategy by which the nervous system might simplify the production of movement. These experiments suggest that spinal motor systems are organized in terms of a small number of distinct motor responses, or “modules.” These dis- tinct modules can be combined together simply to produce a wide range of different movements. Such a modular organization of spinal motor systems can potentially allow the nervous system to produce a wide range of natural behaviors in a simple and flexible manner. NEUROSCIENTIST 8(5):437–442, 2002. DOI: 10.1177/107385802236969 KEY WORDS Spinal motor systems, Spinal force fields, Motor primitives, Spinal interneurons Address correspondence to: Emilio Bizzi, MIT/E25-526, Department of Brain and Cognitive Sciences, 45 Carleton St., Cambridge, MA 02139 (e-mail: ebizzi@mit.edu). REVIEW ■