Editing Motor cortex (section)

==The motor cortex map==
A simple view, that is almost certainly too limited and that dates back to the earliest work on the motor cortex, is that neurons in the motor cortex control movement by a [[Feed forward (control)|feed-forward]] direct pathway. In that view, a neuron in the motor cortex sends an [[axon]] or projection to the spinal cord and forms a [[synapse]] on a [[motor neuron]]. The motor neuron sends an electrical impulse to a muscle. When the neuron in the cortex becomes active, it causes a muscle contraction. The greater the activity in the motor cortex, the stronger the muscle force. Each point in the motor cortex controls a muscle or a small group of related muscles. This description is only partly correct.

Most neurons in the motor cortex that project to the spinal cord [[synapse]] on [[interneuron]] circuitry in the spinal cord, not directly onto [[motor neuron]]s.<ref name=Bortoff&Strick>{{cite journal |author1=Bortoff, G.A.  |author2=Strick, P.L.  | year=1993 | title=Corticospinal terminations in two new-world primates: further evidence that corticomotoneuronal connections provide part of the neural substrate for manual dexterity | journal=J. Neurosci. | volume=13 | pages=5105–5118 | pmid=7504721 | issue=12 | pmc=6576412 | doi=10.1523/JNEUROSCI.13-12-05105.1993 }}</ref> One suggestion is that the direct, cortico-motoneuronal projections are a specialization that allows for the fine control of the fingers.<ref name=Bortoff&Strick/><ref name=Heffner&Masterton>{{cite journal |author1=Heffner, R.  |author2=Masterton, B.  | year=1975 | title=Variation in form of the pyramidal tract and its relationship to digital dexterity | journal=Brain Behav. Evol. | volume=12 | pages=161–200 | doi=10.1159/000124401 | pmid=1212616 | issue=3}}</ref>

The view that each point in the motor cortex controls a muscle or a limited set of related muscles was debated over the entire history of research on the motor cortex, and was suggested in its strongest and most extreme form by Asanuma<ref name=Asanuma>{{cite journal | author=Asanuma, H. | year=1975 | title=Recent developments in the study of the columnar arrangement of neurons within the motor cortex | journal=Physiol. Rev. | volume=55 | pages=143–156 | pmid=806927 | issue=2 | doi=10.1152/physrev.1975.55.2.143}}</ref> on the basis of experiments in cats and monkeys using electrical stimulation. However, almost every other experiment to examine the map, including the classic work of [[David Ferrier|Ferrier]]<ref name=Ferrier/> and of Penfield<ref name=Penfield&Boldrey/> showed that each point in the motor cortex influences a range of muscles and joints. The map is greatly overlapping. The overlap in the map is generally greater in the premotor cortex and supplementary motor cortex, but even the map in the primary motor cortex controls muscles in an extensively overlapped manner. Many studies have demonstrated the overlapping representation of muscles in the motor cortex.<ref name=Cheney&Fetz>{{cite journal |author1=Cheney, P.D.  |author2=Fetz, E.E.  | year=1985 | title=Comparable patterns of muscle facilitation evoked by individual corticomotoneuronal (CM) cells and by single intracortical microstimuli in primates: evidence for functional groups of CM cells | journal=J. Neurophysiol. | volume=53 | pages=786–804 | pmid=2984354 | issue=3 | doi=10.1152/jn.1985.53.3.786}}</ref><ref name=Schieber&Hibbard>{{cite journal |author1=Schieber, M.H.  |author2=Hibbard, L.S.  | year=1993 | title=How somatotopic is the motor cortex hand area? | journal=Science | volume=261 | pages=489–492 | doi=10.1126/science.8332915 | pmid=8332915 | issue=5120|bibcode=1993Sci...261..489S }}</ref><ref name=Rathelot&Strick>{{cite journal |author1=Rathelot, J.A.  |author2=Strick, P.L.  | year=2006 | title=Muscle representation in the macaque motor cortex: an anatomical perspective | journal=Proc. Natl. Acad. Sci. U.S.A. | volume=103 | pages=8257–8262 | doi=10.1073/pnas.0602933103 | pmid=16702556 | issue=21 | pmc=1461407|bibcode=2006PNAS..103.8257R |doi-access=free }}</ref><ref name=Park&etal>{{cite journal | author=Park, M.C., Belhaj-Saif, A., Gordon, M. and Cheney, P.D. | year=2001 | title=Consistent features in the forelimb representation of primary motor cortex in rhesus macaques | journal=J. Neurosci. | volume=21 | pages=2784–2792 | pmid=11306630 | issue=8 | pmc=6762507 | doi=10.1523/JNEUROSCI.21-08-02784.2001 }}</ref><ref name=Sanes&etal>{{cite journal | author=Sanes, J.N., Donoghue, J.P., Thangaraj, V., Edelman, R.R. and Warach, S. | year=1995 | title=Shared neural substrates controlling hand movements in human motor cortex | journal=Science | volume=268 | pages=1775–1777 | doi=10.1126/science.7792606 | pmid=7792606 | issue=5218| bibcode=1995Sci...268.1775S }}</ref><ref name=Donoghue&etal>{{cite journal | author=Donoghue, J.P., Leibovic, S. and Sanes, J.N. | year=1992 | title=Organization of the forelimb area in squirrel monkey motor cortex: representation of digit, wrist and elbow muscles | journal=Exp. Brain Res. | volume=89 | issue=1 | pages=1–10 | doi=10.1007/bf00228996 | pmid=1601087| s2cid=1398462 }}</ref><ref name=Meier&etal>{{cite journal | author=Meier, J.D., Aflalo, T.N., Kastner, S. and [[Michael Graziano|Graziano, M.S.A.]] | year=2008 | title=Complex organization of human primary motor cortex: A high-resolution fMRI study | journal=J. Neurophysiol. | volume=100 | pages=1800–1812 | doi=10.1152/jn.90531.2008 | pmid=18684903 | issue=4 | pmc=2576195}}</ref><ref name=":1">{{Cite journal |last1=Schneider |first1=Cyril |last2=Zytnicki |first2=Daniel |last3=Capaday |first3=Charles |date=2001 |title=Quantitative evidence for multiple widespread representations of individual muscles in the cat motor cortex |url=https://linkinghub.elsevier.com/retrieve/pii/S030439400102105X |journal=Neuroscience Letters |language=en |volume=310 |issue=2–3 |pages=183–187 |doi=10.1016/S0304-3940(01)02105-X |pmid=11585597 |s2cid=26687967}}</ref> To be clear as to what the often used term 'overlapping map' actually means, it is better to state that muscles are represented many times over on the cortical surface in non-contiguous loci, intermingled with the representation of other muscles acting at the same, or at a different, joint.<ref name=":1" />

It is believed that as an animal learns a complex movement repertoire, the motor cortex gradually comes to coordinate among muscles.<ref name=Nudo&etal>{{cite journal | author=Nudo, R.J., Milliken, G.W., Jenkins, W.M. and Merzenich, M.M. | year=1996 | title=Use-dependent alterations of movement representations in primary motor cortex of adult squirrel monkeys | journal=J. Neurosci. | volume=16 | pages=785–807 | pmid=8551360 | issue=2 | doi=10.1523/JNEUROSCI.16-02-00785.1996 | pmc=6578638}}</ref><ref name=Martin&etal>{{cite journal | author=Martin, J.H., Engber, D. and Meng, Z. | year=2005 | title=Effect of forelimb use on postnatal development of the forelimb motor representation in primary motor cortex of the cat | journal=J. Neurophysiol. | volume=93 | pages=2822–2831 | doi=10.1152/jn.01060.2004 | pmid=15574795 | issue=5}}</ref>

[[File:Human motor map.jpg|thumb|right|Map of the body in the human brain.]]

The clearest example of the coordination of muscles into complex movement in the motor cortex comes from the work of [[Michael Graziano|Graziano]] and colleagues on the monkey brain.<ref name=Graziano&2008>{{cite book | author=[[Michael Graziano|Graziano, M.S.A.]] | year=2008 | title=The Intelligent Movement Machine | publisher=Oxford University Press | location=Oxford, UK}}</ref><ref name=Graziano&Taylor&Moore/> They used electrical stimulation on a behavioral time scale, such as for half a second instead of the more typical hundredth of a second. They found that this type of stimulation of the monkey motor cortex often evoked complex, meaningful actions. For example, stimulation of one site in the cortex would cause the hand to close, move to the mouth, and the mouth to open. Stimulation of another site would cause the hand to open, rotate until the grip faced outward, and the arm to project out as if the animal were reaching. Different complex movements were evoked from different sites and these movements were mapped in the same orderly manner in all monkeys tested. Computational models<ref name=Graziano&Aflalo2007>{{cite journal | author=[[Michael Graziano|Graziano, M.S.A.]] and Aflalo, T.N. | year=2007 | title=Mapping behavioral repertoire onto the cortex | journal=Neuron | volume=56 | pages=239–251 | doi=10.1016/j.neuron.2007.09.013 | pmid=17964243 | issue=2| doi-access=free }}</ref> showed that the normal movement repertoire of a monkey, if arranged on a sheet such that similar movements are placed near each other, will result in a map that matches the actual map found in the monkey motor cortex. This work suggests that the motor cortex does not truly contain a homunculus-type map of the body. Instead, the deeper principle may be a rendering of the movement repertoire onto the cortical surface. To the extent that the movement repertoire breaks down partly into the actions of separate body parts, the map contains a rough and overlapping body arrangement noted by researchers over the past century.

A similar organization by typical movement repertoire has been reported in the posterior parietal cortex of monkeys and [[galagos]]<ref name=Stepniewska&etal>{{cite journal | author=Stepniewska, I., Fang, P.C. and Kaas, J.H. | year=2005 | title=Microstimulation reveals specialized subregions for different complex movements in posterior parietal cortex of prosimian galagos | journal=Proc. Natl. Acad. Sci. U.S.A. | volume=102 | pages=4878–4883 | doi=10.1073/pnas.0501048102 | pmid=15772167 | issue=13 | pmc=555725| bibcode=2005PNAS..102.4878S | doi-access=free }}</ref><ref name=Gharbawie&etal>{{cite journal | author=Gharbawie, O.A., Stepniewska, I., Qi, H. and Kaas, J.H. | year=2011 | title=Multiple parietal-frontal pathways mediate grasping in macaque monkeys | journal=J. Neurosci. | volume=31 | pages=11660–11677 | doi=10.1523/JNEUROSCI.1777-11.2011 | pmid=21832196 | issue=32 | pmc=3166522}}</ref> and in the motor cortex of rats<ref name=Haiss&Schwartz>{{cite journal |author1=Haiss, F.  |author2=Schwarz, C  | year=2005 | title=Spatial segregation of different modes of movement control in the whisker representation of rat primary motor cortex | journal=J. Neurosci. | volume=25 | pages=1579–1587 | doi=10.1523/JNEUROSCI.3760-04.2005 | pmid=15703412 | issue=6 | pmc=6726007}}</ref><ref name=Ramanathan&etal>{{cite journal | author=Ramanathan, D., Conner, J.M. and Tuszynski, M.H. | year=2006 | title=A form of motor cortical plasticity that correlates with recovery of function after brain injury | journal=Proc. Natl. Acad. Sci. U.S.A. | volume=103 | pages=11370–11375 | doi=10.1073/pnas.0601065103 | pmid=16837575 | issue=30 | pmc=1544093| bibcode=2006PNAS..10311370R | doi-access=free }}</ref> and mice.<ref>{{Cite journal|title = Distinct Cortical Circuit Mechanisms for Complex Forelimb Movement and Motor Map Topography|journal = Neuron|issn = 0896-6273|pmid = 22542191|pages = 397–409|volume = 74|issue = 2|doi = 10.1016/j.neuron.2012.02.028|first1 = Thomas C.|last1 = Harrison|first2 = Oliver G. S.|last2 = Ayling|first3 = Timothy H.|last3 = Murphy|year=2012|doi-access = free}}</ref> Notwithstanding, direct tests of the idea that the motor cortex contains a movement repertoire have not corroborated this hypothesis.<ref name=":0">{{Cite journal |last=Capaday |first=Charles |date=2022 |title=Motor cortex outputs evoked by long-duration microstimulation encode synergistic muscle activation patterns not controlled movement trajectories |journal=Frontiers in Computational Neuroscience |volume=16 |doi=10.3389/fncom.2022.851485 |issn=1662-5188 |pmc=9434634 |pmid=36062251 |doi-access=free}}{{Creative Commons text attribution notice|cc=by4|from this source=yes}}</ref> Varying the initial position of the forelimb does not change the muscle synergies evoked by microstimulation of a motor cortical point. Consequently, the evoked movements reach nearly the same final end point and posture, with variability. However, the movement trajectories are quite different depending on the initial limb posture and the starting position of the paw. The evoked movement trajectory is most natural when the forelimb lays pendant ~ perpendicular to the ground (i.e., in equilibrium with the gravitational force). From other starting positions, the movements do not appear natural. The paths of the paw are curved with changes and reversals of direction and the passive influence of the gravitational force on the movements is obvious. These observations demonstrate that while the output of the cortical point evokes a seemingly coordinated limb movement from a rest position, it does not specify a particular movement direction or a controlled trajectory from other initial positions. Thus, in natural conditions a controlled movement must depend on the coordinated activation of a multitude of cortical points, terminating at a final locus of motor cortical activity, which holds the limb at a spatial location.<ref name=":0" /> These findings are inconsistent with the idea of the representation of the movement repertoire on the cortical surface.