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Motor cortex | |
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Details | |
Identifiers | |
Latin | cortex motorius |
MeSH | D009044 |
NeuroNames | 2332 |
NeuroLex ID | oen_0001104 |
Anatomical terms of neuroanatomy |
The motor cortex is the region of the cerebral cortex involved in the planning, control, and execution of voluntary movements. The motor cortex is an area of the frontal lobe located in the posterior precentral gyrus immediately anterior to the central sulcus.
The motor cortex can be divided into three areas:
1. The primary motor cortex is the main contributor to generating neural impulses that pass down to the spinal cord and control the execution of movement. However, some of the other motor areas in the brain also play a role in this function. It is located on the anterior paracentral lobule on the medial surface.
2. The premotor cortex is responsible for some aspects of motor control, possibly including the preparation for movement, the sensory guidance of movement, the spatial guidance of reaching, or the direct control of some movements with an emphasis on control of proximal and trunk muscles of the body. Located anterior to the primary motor cortex.
3. The supplementary motor area (or SMA), has many proposed functions including the internally generated planning of movement, the planning of sequences of movement, and the coordination of the two sides of the body such as in bi-manual coordination. Located on the midline surface of the hemisphere anterior to the primary motor cortex.
Other brain regions outside the cerebral cortex are also of great importance to motor function, most notably the cerebellum, the basal ganglia, pedunculopontine nucleus and the red nucleus, as well as other subcortical motor nuclei.
In the earliest work on the motor cortex, researchers recognized only one cortical field involved in motor control. Alfred Walter Campbell [1] was the first to suggest that there might be two fields, a "primary" motor cortex and an "intermediate precentral" motor cortex. His reasons were largely based on cytoarchitectonics, or the study of the appearance of the cortex under a microscope. The primary motor cortex contains cells with giant cell bodies known as "Betz cells". These cells were mistakenly thought to be the main outputs from the cortex, sending fibers to the spinal cord. [1] It has since been found that Betz cells account for about 2-3% of the projections from the cortex to the spinal cord, or about 10% of the projections from the primary motor cortex to the spinal cord. [2] [3] The specific function of the Betz cells that distinguishes them from other output cells of the motor cortex remains unknown, but they continue to be used as a marker for the primary motor cortex.
Other researchers, such as Oskar Vogt, Cécile Vogt-Mugnier [4] and Otfrid Foerster [5] also suggested that motor cortex was divided into a primary motor cortex (area 4, according to Brodmann's [6] naming scheme) and a higher-order motor cortex (area 6 according to Korbinian Brodmann).
Wilder Penfield [7] [8] notably disagreed and suggested that there was no functional distinction between area 4 and area 6. In his view both were part of the same map, though area 6 tended to emphasize the muscles of the back and neck. Woolsey [9] who studied the motor map in monkeys also believed there was no distinction between primary motor and premotor. M1 was the name for the proposed single map that encompassed both the primary motor cortex and the premotor cortex. [9] Although sometimes "M1" and "primary motor cortex" are used interchangeably, strictly speaking, they derive from different conceptions of motor cortex organization.[ citation needed ]
Despite the views of Penfield and Woolsey, a consensus emerged that area 4 and area 6 had sufficiently different functions that they could be considered different cortical fields. Fulton [10] helped to solidify this distinction between a primary motor cortex in area 4 and a premotor cortex in area 6. As Fulton pointed out, and as all subsequent research has confirmed, both primary motor and premotor cortex project directly to the spinal cord and are capable of some direct control of movement. Fulton showed that when the primary motor cortex is damaged in an experimental animal, movement soon recovers; when the premotor cortex is damaged, movement soon recovers; when both are damaged, movement is lost and the animal cannot recover.
The premotor cortex is now generally divided into four sections. [11] [12] [13] First it is divided into an upper (or dorsal) premotor cortex and a lower (or ventral) premotor cortex. Each of these is further divided into a region more toward the front of the brain (rostral premotor cortex) and a region more toward the back (caudal premotor cortex). A set of acronyms are commonly used: PMDr (premotor dorsal, rostral), PMDc, PMVr, PMVc. Some researchers use a different terminology. Field 7 or F7 denotes PMDr; F2 = PMDc; F5=PMVr; F4=PMVc.
PMDc is often studied with respect to its role in guiding reaching. [14] [15] [16] Neurons in PMDc are active during reaching. When monkeys are trained to reach from a central location to a set of target locations, neurons in PMDc are active during the preparation for the reach and also during the reach itself. They are broadly tuned, responding best to one direction of reach and less well to different directions. Electrical stimulation of the PMDc on a behavioral time scale was reported to evoke a complex movement of the shoulder, arm, and hand that resembles reaching with the hand opened in preparation to grasp. [11]
PMDr may participate in learning to associate arbitrary sensory stimuli with specific movements or learning arbitrary response rules. [17] [18] [19] In this sense it may resemble the prefrontal cortex more than other motor cortex fields. It may also have some relation to eye movement. Electrical stimulation in the PMDr can evoke eye movements [20] and neuronal activity in the PMDr can be modulated by eye movement. [21]
PMVc or F4 is often studied with respect to its role in the sensory guidance of movement. Neurons here are responsive to tactile stimuli, visual stimuli, and auditory stimuli. [22] [23] [24] [25] These neurons are especially sensitive to objects in the space immediately surrounding the body, in so-called peripersonal space. Electrical stimulation of these neurons causes an apparent defensive movement as if protecting the body surface. [26] [27] This premotor region may be part of a larger circuit for maintaining a margin of safety around the body and guiding movement with respect to nearby objects. [28]
PMVr or F5 is often studied with respect to its role in shaping the hand during grasping and in interactions between the hand and the mouth. [29] [30] Electrical stimulation of at least some parts of F5, when the stimulation is applied on a behavioral time scale, evokes a complex movement in which the hand moves to the mouth, closes in a grip, orients such that the grip faces the mouth, the neck turns to align the mouth to the hand, and the mouth opens. [11] [26]
Mirror neurons were first discovered in area F5 in the monkey brain by Rizzolatti and colleagues. [31] [32] These neurons are active when the monkey grasps an object. Yet the same neurons become active when the monkey watches an experimenter grasp an object in the same way. The neurons are therefore both sensory and motor. Mirror neurons are proposed to be a basis for understanding the actions of others by internally imitating the actions using one's own motor control circuits.
Penfield [33] described a cortical motor area, the supplementary motor area (SMA), on the top or dorsal part of the cortex. Each neuron in the SMA may influence many muscles, many body parts, and both sides of the body. [34] [35] [36] The map of the body in SMA is therefore extensively overlapping. SMA projects directly to the spinal cord and may play some direct role in the control of movement. [37]
Based on early work using brain imaging techniques in the human brain, Roland [38] suggested that the SMA was especially active during the internally generated plan to make a sequence of movements. In the monkey brain, neurons in the SMA are active in association with specific learned sequences of movement. [39]
Others have suggested that, because the SMA appears to control movement bilaterally, it may play a role in inter-manual coordination. [40]
Yet others have suggested that, because of the direct projection of SMA to the spinal cord and because of its activity during simple movements, it may play a direct role in motor control rather than solely a high level role in planning sequences. [37] [41]
On the basis of the movements evoked during electrical stimulation, it has been suggested that the SMA may have evolved in primates as a specialist in the part of the motor repertoire involving climbing and other complex locomotion. [11] [42]
Based on the pattern of projections to the spinal cord, it has been suggested that another set of motor areas may lie next to the supplementary motor area, on the medial (or midline) wall of the hemisphere. [37] These medial areas are termed the cingulate motor areas. Their functions are not yet understood.
In 1870, Eduard Hitzig and Gustav Fritsch demonstrated that electrical stimulation of certain parts of the dog brain resulted in muscular contraction on the opposite side of the body. [43] This confirmed experimentally the existence of a cortical motor center, which was inferred by Jackson a few years earlier on the basis of clinical observations. [44] Together with Broca's (1861) [45] discovery of a language center in the left hemisphere of the cerebral cortex, the demonstration of a cortical motor center put an end to Flourens' [46] doctrine (1842) that function was widely distributed across the cerebral cortex (i.e., not localized). [47]
A little later, in 1874, David Ferrier, [48] working in the laboratory of the West Riding Lunatic Asylum at Wakefield (at the invitation of its director, James Crichton-Browne), mapped the motor cortex in the monkey brain using electrical stimulation. He found that the motor cortex contained a rough map of the body with the feet at the top (or dorsal part) of the brain and the face at the bottom (or ventral part) of the brain. He also found that when electrical stimulation was maintained for a longer time, such as for a second, instead of being discharged over a fraction of a second, then some coordinated, seemingly meaningful movements could be caused, instead of only muscle twitches.
After Ferrier's discovery, many neuroscientists used electrical stimulation to study the map of the motor cortex in many animals including monkeys, apes, and humans. [1] [4] [5] [49] [50]
One of the first detailed maps of the human motor cortex was described in 1905 by Campbell. [1] He did autopsies on the brains of amputees. A person who had lost an arm would over time apparently lose some of the neuronal mass in the part of the motor cortex that normally controls the arm. Likewise, a person who had lost a leg would show degeneration in the leg part of motor cortex. In this way the motor map could be established. In the period between 1919 and 1936 others mapped the motor cortex in detail using electrical stimulation, including the husband and wife team Vogt and Vogt, [4] and the neurosurgeon Foerster. [5]
Perhaps the best-known experiments on the human motor map were published by Penfield in 1937. [7] [8] Using a procedure that was common in the 1930s, he examined epileptic patients who were undergoing brain surgery. These patients were given a local anesthetic, their skulls were opened, and their brains exposed. Then, electrical stimulation was applied to the surface of the brain to map out the speech areas. In this way, the surgeon would be able to avoid any damage to speech circuitry. The brain focus of the epilepsy could then be surgically removed. During this procedure, Penfield mapped the effect of electrical stimulation in all parts of the cerebral cortex, including motor cortex.
Penfield is sometimes mistakenly considered to be the discoverer of the map in motor cortex. It was discovered approximately 70 years before his work. However, Penfield drew a picture of a human-like figure stretched over the cortical surface and used the term "homunculus" (diminutive of "homo", Latin for "man") to refer to it. It is perhaps for this reason that his work has become so popular in neuroscience. It should be noted, however, that Penfield knew well that the idea of a homunculus was a caricature. He stated, 'It is a cartoon of representation in which scientific accuracy is impossible'. [51] Nearly fifty years before, [52] Sherrington (1906) made the point more cogently stating 'The student looking over the political map map of a continent may little realise the complexity of the populations and states so simply represented. We looking at the brain chart of the text book may never forget the unspeakable complexity of the reactions thus rudely symbolised and spatially indicated´. It is therefore important to realize that while pictures of an orderly representation of limb segments across the cortical surface (such as the one shown above) have endured in textbooks, they are erroneous and misleading. [53]
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 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 neurons. [54] One suggestion is that the direct, cortico-motoneuronal projections are a specialization that allows for the fine control of the fingers. [54] [55]
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 [56] 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 Ferrier [48] and of Penfield [7] 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. [57] [58] [59] [60] [61] [62] [63] [64] 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. [64]
It is believed that as an animal learns a complex movement repertoire, the motor cortex gradually comes to coordinate among muscles. [65] [66]
The clearest example of the coordination of muscles into complex movement in the motor cortex comes from the work of Graziano and colleagues on the monkey brain. [11] [26] 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 [67] 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 [68] [69] and in the motor cortex of rats [70] [71] and mice. [72] Notwithstanding, direct tests of the idea that the motor cortex contains a movement repertoire have not corroborated this hypothesis. [73] 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. [73] These findings are inconsistent with the idea of the representation of the movement repertoire on the cortical surface.
Mammals evolved from mammal-like reptiles over 200 million years ago. [75] These early mammals developed several novel brain functions most likely due to the novel sensory processes that were necessary for the nocturnal niche that these mammals occupied. [76] These animals most likely had a somatomotor cortex, where somatosensory information and motor information were processed in the same cortical region. This allowed for the acquisition of only simple motor skills, such as quadrupedal locomotion and striking of predators or prey. Placental mammals evolved a discrete motor cortex about 100 mya. [75] According to the principle of proper mass, "the mass of neural tissue controlling a particular function is appropriate to the amount of information processing involved in performing the function. [76] " This suggests that the development of a discrete motor cortex was advantageous for placental mammals, and the motor skills that these organisms acquired were more complex than their early-mammalian ancestors. Further, this motor cortex was necessary for the arboreal lifestyles of our primate ancestors.
Enhancements to the motor cortex (and the presence of opposable thumbs and stereoscopic vision) were evolutionarily selected to prevent primates from making mistakes in the dangerous motor skill of leaping between tree branches (Cartmill, 1974; Silcox, 2007). As a result of this pressure, the motor system of arboreal primates has a disproportionate degree of somatotopic representation of the hands and feet, which is essential for grasping (Nambu, 2011; Pons et al., 1985; Gentilucci et al., 1988).
The parietal lobe is one of the four major lobes of the cerebral cortex in the brain of mammals. The parietal lobe is positioned above the temporal lobe and behind the frontal lobe and central sulcus.
A gamma wave or gamma rhythm is a pattern of neural oscillation in humans with a frequency between 25 and 140 Hz, the 40 Hz point being of particular interest. Gamma rhythms are correlated with large-scale brain network activity and cognitive phenomena such as working memory, attention, and perceptual grouping, and can be increased in amplitude via meditation or neurostimulation. Altered gamma activity has been observed in many mood and cognitive disorders such as Alzheimer's disease, epilepsy, and schizophrenia.
The human secondary somatosensory cortex is a region of cortex in the parietal operculum on the ceiling of the lateral sulcus.
A cortical homunculus is a distorted representation of the human body, based on a neurological "map" of the areas and proportions of the human brain dedicated to processing motor functions, and/ or sensory functions, for different parts of the body. Nerve fibres—conducting somatosensory information from all over the body—terminate in various areas of the parietal lobe in the cerebral cortex, forming a representational map of the body.
Supplementary eye field (SEF) is the name for the anatomical area of the dorsal medial frontal lobe of the primate cerebral cortex that is indirectly involved in the control of saccadic eye movements. Evidence for a supplementary eye field was first shown by Schlag, and Schlag-Rey. Current research strives to explore the SEF's contribution to visual search and its role in visual salience. The SEF constitutes together with the frontal eye fields (FEF), the intraparietal sulcus (IPS), and the superior colliculus (SC) one of the most important brain areas involved in the generation and control of eye movements, particularly in the direction contralateral to their location. Its precise function is not yet fully known. Neural recordings in the SEF show signals related to both vision and saccades somewhat like the frontal eye fields and superior colliculus, but currently most investigators think that the SEF has a special role in high level aspects of saccade control, like complex spatial transformations, learned transformations, and executive cognitive functions.
The premotor cortex is an area of the motor cortex lying within the frontal lobe of the brain just anterior to the primary motor cortex. It occupies part of Brodmann's area 6. It has been studied mainly in primates, including monkeys and humans. The functions of the premotor cortex are diverse and not fully understood. It projects directly to the spinal cord and therefore may play a role in the direct control of behavior, with a relative emphasis on the trunk muscles of the body. It may also play a role in planning movement, in the spatial guidance of movement, in the sensory guidance of movement, in understanding the actions of others, and in using abstract rules to perform specific tasks. Different subregions of the premotor cortex have different properties and presumably emphasize different functions. Nerve signals generated in the premotor cortex cause much more complex patterns of movement than the discrete patterns generated in the primary motor cortex.
The supplementary motor area (SMA) is a part of the motor cortex of primates that contributes to the control of movement. It is located on the midline surface of the hemisphere just in front of the primary motor cortex leg representation. In monkeys the SMA contains a rough map of the body. In humans the body map is not apparent. Neurons in the SMA project directly to the spinal cord and may play a role in the direct control of movement. Possible functions attributed to the SMA include the postural stabilization of the body, the coordination of both sides of the body such as during bimanual action, the control of movements that are internally generated rather than triggered by sensory events, and the control of sequences of movements. All of these proposed functions remain hypotheses. The precise role or roles of the SMA is not yet known.
Beevor's Axiom is the idea that the brain does not know muscles, only movements. In other words, the brain registers the movements that muscles combine to make, not the individual muscles that are making the movements. Hence, this is why one can sign their name with their foot. Beevor's Axiom was coined by Dr. Charles Edward Beevor, an English neurologist.
A topographic map is the ordered projection of a sensory surface, like the retina or the skin, or an effector system, like the musculature, to one or more structures of the central nervous system. Topographic maps can be found in all sensory systems and in many motor systems.
Premovement neuronal activity in neurophysiological literature refers to neuronal modulations that alter the rate at which neurons fire before a subject produces movement. Through experimentation with multiple animals, predominantly monkeys, it has been shown that several regions of the brain are particularly active and involved in initiation and preparation of movement. Two specific membrane potentials, the bereitschaftspotential, or the BP, and contingent negative variation, or the CNV, play a pivotal role in premovement neuronal activity. Both have been shown to be directly involved in planning and initiating movement. Multiple factors are involved with premovement neuronal activity including motor preparation, inhibition of motor response, programming of the target of movement, closed-looped and open-looped tasks, instructed delay periods, short-lead and long-lead changes, and mirror motor neurons.
Central facial palsy is a symptom or finding characterized by paralysis or paresis of the lower half of one side of the face. It usually results from damage to upper motor neurons of the facial nerve.
The posterior parietal cortex plays an important role in planned movements, spatial reasoning, and attention.
Electrical brain stimulation (EBS), also referred to as focal brain stimulation (FBS), is a form of electrotherapy used as a technique in research and clinical neurobiology to stimulate a neuron or neural network in the brain through the direct or indirect excitation of its cell membrane by using an electric current. EBS is used for research or for therapeutic purposes.
Vittorio Gallese is professor of Psychobiology at the University of Parma, Italy, and was professor in Experimental Aesthetics at the University of London, UK (2016–2018). He is an expert in neurophysiology, cognitive neuroscience, social neuroscience, and philosophy of mind. Gallese is one of the discoverers of mirror neurons. His research attempts to elucidate the functional organization of brain mechanisms underlying social cognition, including action understanding, empathy, language, mindreading and aesthetic experience.
The primary motor cortex is a brain region that in humans is located in the dorsal portion of the frontal lobe. It is the primary region of the motor system and works in association with other motor areas including premotor cortex, the supplementary motor area, posterior parietal cortex, and several subcortical brain regions, to plan and execute voluntary movements. Primary motor cortex is defined anatomically as the region of cortex that contains large neurons known as Betz cells, which, along with other cortical neurons, send long axons down the spinal cord to synapse onto the interneuron circuitry of the spinal cord and also directly onto the alpha motor neurons in the spinal cord which connect to the muscles.
The neuroscience of music is the scientific study of brain-based mechanisms involved in the cognitive processes underlying music. These behaviours include music listening, performing, composing, reading, writing, and ancillary activities. It also is increasingly concerned with the brain basis for musical aesthetics and musical emotion. Scientists working in this field may have training in cognitive neuroscience, neurology, neuroanatomy, psychology, music theory, computer science, and other relevant fields.
Michael Steven Anthony Graziano is an American scientist and novelist who is currently a professor of Psychology and Neuroscience at Princeton University. His scientific research focuses on the brain basis of awareness. He has proposed the "attention schema" theory, an explanation of how, and for what adaptive advantage, brains attribute the property of awareness to themselves. His previous work focused on how the cerebral cortex monitors the space around the body and controls movement within that space. Notably he has suggested that the classical map of the body in motor cortex, the homunculus, is not correct and is better described as a map of complex actions that make up the behavioral repertoire. His publications on this topic have had a widespread impact among neuroscientists but have also generated controversy. His novels rely partly on his background in psychology and are known for surrealism or magic realism. Graziano also composes music including symphonies and string quartets.
Cortical remapping, also referred to as cortical reorganization, is the process by which an existing cortical map is affected by a stimulus resulting in the creating of a 'new' cortical map. Every part of the body is connected to a corresponding area in the brain which creates a cortical map. When something happens to disrupt the cortical maps such as an amputation or a change in neuronal characteristics, the map is no longer relevant. The part of the brain that is in charge of the amputated limb or neuronal change will be dominated by adjacent cortical regions that are still receiving input, thus creating a remapped area. Remapping can occur in the sensory or motor system. The mechanism for each system may be quite different. Cortical remapping in the somatosensory system happens when there has been a decrease in sensory input to the brain due to deafferentation or amputation, as well as a sensory input increase to an area of the brain. Motor system remapping receives more limited feedback that can be difficult to interpret.
Social cognitive neuroscience is the scientific study of the biological processes underpinning social cognition. Specifically, it uses the tools of neuroscience to study "the mental mechanisms that create, frame, regulate, and respond to our experience of the social world". Social cognitive neuroscience uses the epistemological foundations of cognitive neuroscience, and is closely related to social neuroscience. Social cognitive neuroscience employs human neuroimaging, typically using functional magnetic resonance imaging (fMRI). Human brain stimulation techniques such as transcranial magnetic stimulation and transcranial direct-current stimulation are also used. In nonhuman animals, direct electrophysiological recordings and electrical stimulation of single cells and neuronal populations are utilized for investigating lower-level social cognitive processes.
Eberhard Erich Fetz is an American neuroscientist, academic and researcher. He is a Professor of Physiology and Biophysics and DXARTS at the University of Washington.
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