Clinical Neurology and Neurosurgery 114 (2012) 432–435 Contents lists available at SciVerse ScienceDirect Clinical Neurology and Neurosurgery journal homepage: www.elsevier.com/locate/clineuro Overview of neurophysiology of movement control J.C. Rothwell ∗ UCL Institute of Neurology, Queen Square, London WC1N 3BG, UK article info Article history: Received 19 November 2011 Accepted 28 December 2011 Available online 26 January 2012 Keywords: Corticospinal Motoneurone Transcranial magnetic stimulation abstract The motoneuronal outputs from cortex and spinal cord have quite different patterns of organisation. The cortex consists of a highly intermixed mosaic of small output zones whereas the motoneurones in the cord are located in clearly defined columns of cells, that all project to the same muscle. I describe the pattern of innervation between cortex and cord, indicate the importance of cortical plasticity in allowing flexible control of spinal circuits, and show how these inputs interact. Finally I discuss some of the new methods of stimulating descending motor pathways in humans. © 2012 Elsevier B.V. All rights reserved. 1. Introduction The process of movement has in the past been expressed in terms of a sequential flow of information, often expressed as: Idea—Plan—Select—Move. In this scheme, movements were believed to start with an idea that defined a goal to be reached. This was then converted into a plan of action which detailed the best way to achieve the goal. The muscles to be used were then selected and energised to produce movement. Evidence in favour of the distinction between idea and plan came from observations in apraxic patients who could know what they wanted to do but be unable to formulate the best method to achieve the goal. An exam- ple would be a patient trying to put on a jacket but mistaking the back from the front or the arm holes for trouser legs. The patients were said to know what they wanted to do but were unable to do it. The evidence for conversion of the selected plan of action into a suitable pattern of muscle activity was less clear since many condi- tions have movements that are accompanied by abnormal muscle activity, such as dystonia or Parkinsons disease. However, whether this is truly a faulty conversion of an accurate plan or simply or poor planning is not clear. Although this approach has the benefits of simplicity it probably underestimates the degree to which these elements are performed in parallel, not only in time but in space, being executed simul- taneously in many different areas. The benefits of such a parallel organisation are that it is robust; damage to any one structure (at a particular time) can be compensated, at least to a certain extent, by activity in other structures. One disadvantage is the resulting com- plexity, since a given group of neurones can participate in more ∗ Tel.: +44 020 3008 8745. E-mail address: j.rothwell@ucl.ac.uk than one different task. In addition, if this is to work efficiently there must be good communication and feedback between the elements. 2. Summary of supraspinal motor organisation The motor areas of cortex are defined by the fact that all of them send axons to the spinal cord, and are interconnected with each other. They are the primary motor area (Brodmann’s area 4, mainly located in the anterior bank of the central sulcus), pre- motor cortex (the lateral part of area 6 of Brodmann), which is usually divided into dorsal and ventral in human studies, the sup- plementary motor area (the medial part of Brodmann’s area 6) and 3 regions of the cingulate cortex ventral to the supplementary motor area [1]. The primary motor cortex contributes about 40% of the corticospinal fibres, the cingulate and supplementary areas about 20% each and the premotor areas about 10% [2]. All of these areas of cortex also project to brainstem areas that give rise to reticu- lospinal tracts, giving them an indirect route to spinal cord as well as the direct corticospinal route. The primary motor cortex is thought to have fewer of these indirect connections than other motor areas. The corticospinal tract travels in the lateral column of the spinal cord and has the densest terminations among distal muscles of the arm and leg, but also innervates all sections of the cord. About 90% of the projections are crossed. A smaller proportion project ipsi- laterally. The reticulospinal tracts arise from areas of the pontine and medullary reticular formation and tend to project bilaterally to innervate more proximal and axial muscles, although recent electrophysiological work suggests that they may even have some connections to the most distal muscles of the hand [3]. Consistent with this general anatomical picture, lesion stud- ies in primates show that corticospinal section (pyramidotomy) 0303-8467/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.clineuro.2011.12.053