ReviewMotor systems
Introduction
Traditionally, the field of motor control has not been one to capture headlines or the imagination of the general scientific community. Movements are so natural that it is not immediately obvious what needs to be explained until, of course, something malfunctions. Even when the basic issues concerning the production of normal movements are understood, it is often difficult to communicate current views about underlying mechanisms because many important concepts driving the field are arcane — concepts such as transformation of reference frames, internal modeling of body mechanics, and distributed representation of information in populations of neurons. Thus, knowledge of many advances in the field are usually confined to a relatively small number of investigators with expertise in the application of these concepts. This is unfortunate because there have been numerous developments in motor control that have relevance to our general understanding of how complex systems of nerve cells function as well as application to other fields, such as neurology, cognitive neuroscience, rehabilitation medicine and robotics. In this anniversary review, I focus on progress in four areas of motor control: central pattern generation, the function of proprioceptors, internal models and motor learning, and functional localization and sensory–motor transformations in the cerebral cortex. Clearly, these areas do not cover all aspects of motor control, but they do feature a sufficiently broad range of contemporary issues to illustrate many key advances in motor control over the past 10 years.
Section snippets
Central pattern generation
An enduring theme in the Motor systems (formerly called Neural control) section of Current Opinion in Neurobiology relates to the properties of the neuronal circuits generating rhythmic patterns of motor activity, so-called central pattern generators (CPGs). In every Motor systems section over the past decade, there has been one or more reviews on this theme, accounting for slightly more than 20% of all the reviews on motor control. The basic concept of central pattern generation was firmly
Function of proprioceptors
Muscles throughout the body are richly endowed with proprioceptors. A long-debated issue concerns the role of proprioception in the control of movement. It is generally accepted that one of the primary functions of proprioceptors is to detect unexpected events and initiate rapid compensatory responses. Indeed, there are now numerous examples of this function, particularly in the control of posture and locomotion in humans and other mammals [16], [17]. Over the past decade, however, it has
Internal models and motor learning
A concept that has become increasingly prominent over the past decade is internal modeling of the motor apparatus [31], [32]. A central tenet of this concept is that physical properties of the limbs and body are represented (modeled) in the central nervous system, and these internal models function to produce feedfoward commands to motoneurons. When the system is functioning optimally, these feedforward commands are all that is needed to produce an accurate movement. The idea that accurate
Functional localization and sensory–motor transformations
During the 90s, there was a substantial increase in the number of investigations using either non-invasive imaging techniques in humans or single-unit recording techniques in non-human primates to define more precisely the regions in the central nervous system involved in movement control [42], [48], [49], [50], [51]. These investigations have revealed heterogeneity within previously defined single motor areas [52] and identified new motor regions [49], [51]. Strong homologies have also been
Conclusions
In this review, I have focused on progress in just four areas of motor control physiology. It should be apparent even from this limited review that the field has advanced significantly over the past decade and is poised to take full advantage of powerful new experimental strategies stemming from advances in brain imaging and in molecular biology. Before we get too complacent, however, we should note a development that could severely curtail research on motor systems, namely the increase in
Acknowledgements
I thank Tania Lamb and Jaynie Yang for their valuable comments on a draft of this review. My work is supported by a grant from the Medical Research Council of Canada.
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