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EDITORIALS
HIGHLIGHTED TOPIC
Neural Control of Movement
In the April issue, Dr. P. Matthews introduces the series with a Historical Perspective article entitled "Historical analysis of the neural control of movement from the bedrock of animal experimentation to human studies." Highlighting developments in classical neurophysiology, as well as successive advances arising from the development of new techniques, Dr. Matthews emphasizes the importance of the electromyogram (EMG) and the continuing need to improve its reliability as a quantitative measure of neural output to muscle. Although his mini-review points out limitations of EMG recordings, Dr. Matthews also addresses the optimism generated by new methods for cortical stimulation. Dr. Matthews' historical mini-review pays filial homage to the intellectual and experimental contributions made by Sir Charles Sherrington, under whose shadow Dr. Matthews spent his academic life at Oxford.
The surface EMG is often used to derive central control strategies. Dr. D. Farina and colleagues discuss the limitations associated with such conjectured use of EMG in their mini-review entitled "The extraction of neural strategies from the surface EMG." Despite the many appropriate applications of the surface EMG, experimentalists often do not appreciate the limitations of this analytic technique. Failing to acknowledge such limitations can sometimes lead to erroneous interpretations of results and to publication of conflicting reports in the literature. In their mini-review, the authors highlight some of the critical limitations of surface EMG, with the intent of directing greater attention toward interpretations that enable identification of the physiological mechanisms underlying neural control of movement.
Also in this issue, in a mini-review entitled "Noninvasive stimulation of the human corticospinal tract," Drs. J. Taylor and S. Gandevia address artificial activation of the corticospinal tract and interpretation of evoked potentials. These authors present a critical argument for stimulation of descending motor tracts as a means of studying the behavior of the motor system, illustrating the utility of current techniques and their pitfalls. The authors report that transcranial electrical stimulation between the mastoid processes evokes a single descending volley in large corticospinal axons, resulting in monosynaptic recruitment of motoneurons. Such stimulation has been used to define plasticity in the corticospinal connection to motoneurons. An advantage of this approach is that it allows investigators to observe changes in motoneuronal output independent of changes in excitability of the motor cortex.
In the May issue, Drs. S. Harkema and V. Dietz discuss the physiology of movement and spinal cord injury in their mini-review entitled "Locomotor activity in spinal cord injured persons." The existence of central pattern generators in humans is controversial, despite evidence of individuals with clinically complete spinal cord injury who demonstrate properties of oscillating neural networks. Such properties of neural networks of the human spinal cord, especially the response to specific afferent input related to locomotion to generate more effective stepping patterns, may play an important role in the recovery of walking after severe spinal cord injury. Repetitive locomotor training that takes into account the use of multisensory proprioceptive feedback has been shown to improve walking ability following spinal cord injury.
Also in the May issue, Dr. J. Nielsen addresses sensory feedback and descending motor commands in a mini-review entitled "Sensory-motor integration at spinal level as a basis for muscle coordination during voluntary movement in humans." Experiments involving both animals and humans have documented the convergence of sensory feedback signals and descending motor commands on common interneurons in the spinal cord. Noninvasive electrophysiological experiments in human subjects have proven to be very insightful in elucidating the functional significance of this convergence. Through convergence, peripheral feedback evoked by ongoing movement may contribute significantly to muscle activity, and supraspinal centers may modulate corrective reactions to sudden external perturbations. Although this type of research has so far been applied only to a limited extent in the sport and occupational sciences, its implications for more general application are profound and far-reaching.
In the June issue, Drs. M. Schieber and M. Santello discuss fine motor control in their mini-review entitled "Hand function: neural control and peripheral limits to performance." Behavioral tasks requiring fine coordination of finger movements are numerous and wide-ranging, but understanding the control strategies that underlie these movements is complicated by the intricate biomechanical and neural architectures of the hand. Peripheral and central limitations of neuromuscular control have been identified and may in part underlie the simultaneous coordination of finger motion with forces that provide synergies that reduce the number of independent degrees of freedom under control. These authors review evidence for this concept, placing particular emphasis on limitations of finger movement and forces in behavioral individuation and control of the hand by the motor cortex.
Also in the June issue, in their mini-review entitled "Probing vestibular function with galvanic stimuli," Drs. B. Day and R. Fitzpatrick examine the electrophysiology and anatomy of the vestibular organs and present a model of the afferent signal expected from a galvanic stimulus. Galvanic vestibular stimulation has been used for well over a century to discover and probe vestibular function in humans and animals. The technique is simple and innocuous, producing stereotypical ocular and balance reflexes and reproducible illusions of movement. Despite many proposals, it has never gained favor for use in clinical evaluation. During the past 20 years, however, galvanic vestibular stimulation has experienced a kind of renaissance as a research tool and is now frequently applied in a variety of settings. Despite the technique's popularity, the fact that the nature of the vestibular afferent signal it generates is not understood remains a problem, and behavioral responses therefore cannot be interpreted with certainty.
It should be noted that much of the research discussed in these mini-reviews involves human studies; thus there is direct translation of the results of these studies to neural control in humans. Such studies are made possible by technological advances that have enabled different interventions and measurements. This is important because studies of how the nervous system organizes movement require examination of voluntary contractions, and such studies are most economically and easily acquired in human subjects. Human studies provide a better understanding of the neural basis of movement, which will be essential in the development of new therapeutic strategies for people suffering from movement disorders caused by disease or injury. Given that in recent years such research has received great attention both in the academic literature and in popular media, this Highlighted Topic series is particularly relevant. As always, the Editors and I hope that the publication of this Highlighted Topic series will serve to motivate the future publication of papers addressing this very important area of research in the Journal of Applied Physiology.
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