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Organization of the motor system<\/strong><\/p>\n
What do we mean by “motor program” and by learning a motor skill? The central nervous system does not define in detail the displacement of the effectors (muscles), the rotations of the joints, the muscular forces, the joint pressures and the muscular activations, therefore the term engram <\/strong>or neuronal representation<\/strong> seems to provide a better indication (given the current knowledge ) of how the central nervous system codes for purposeful voluntary movements and has no relation to involuntary (automatic) movements.<\/p>\n Reference to “Biomechanics and motor control – defining central concepts” M.L. Latash and V.M. Zatsiorsky, 2016.<\/p>\n The means available to study the different functional aspects of the motor system are rather ineffective in defining a precise location of all the sub-components of the system itself. Once again the detailed study of the activity of single motor units or of the activity of single neuronal groups only provides us with specific information but does not allow us to hypothesize if not generically a motor control function. The “dynamic” approach to the control system seems to promise good prospects for understanding the motor system. In other words, only by causing a perturbation in the control system, will we be able to trace the strategies that the system is able to implement to compensate for the induced perturbation; in this way we will be able to quantify exactly how much the system is no longer able to implement due to an inefficiency linked, for example, to aging, or rather to a lesion in the system itself (in the control section or in the of the effectors).<\/p>\n Some questions The motor system is the set of structures of the central and peripheral nervous system that allows us to transform a “nerve signal” into muscle contraction.<\/p>\n Works in an integrated way with the sensory and sensitive systems (visual, auditory, somatosensory and postural); the latter provide the representation of the external world, of the context in which the motor system plans, coordinates and executes the set of motor acts aimed at a purpose.<\/p>\n The motor system is hierarchically organized into three levels: spinal cord, brainstem, and cerebral cortex. Each level, thanks to the afferent and efferent connections, is able to organize or regulate complex motor responses. Movement information is processed by different systems, which operate in parallel.<\/p>\n The basal nuclei participate in the planning and execution of the movement, which together with the cerebellum are part of feedback circuits that control both the cortex and the brainstem.<\/p>\n The nuclei of the base are involved in motivational processes and in the choice of behavioral plans that allow adaptation to various environmental conditions.<\/p>\n The cerebellum is involved in the coordination and accuracy of ongoing movements and in learning motor skills.<\/p>\n Cortical motor areas receive cortical and subcortical inputs, process information, and emit inputs to the brainstem and spinal cord.<\/p>\n The efferents are the cortico-bulbar and cortico-spinal pathways.<\/p>\n Cortical motor areas contain neurons that project to other cortical areas and the spinal cord. The main subdivision must be made between the primary Motor areas (M1 or Brodman area 4) and the pre-motor areas (Brodman area 6).<\/p>\n Layer V of the cortex of M1 contains upper motor neurons (Betz cells, which are very large and represent no more than 5% of the cells that project to the spinal cord) and non-Betz cells, also present in the premotor areas. The axons of these neurons descend to the brainstem through the cortical-bulbar tract and to the spinal cord through the cortical-spinal tract which, at the level of the ventral portion of the bulb form the bulbar pyramids and 90% of the fibers cross on the midline giving rise to the lateral cortical-spinal bundle; 10% of the fibers do not cross and give rise to the ventral corticospinal bundle.<\/p>\n The motor cortex contains a spatial map of the contralateral half of the body but not as precise as those of the somatosensory areas: a motor neuron does not correspond to a single muscle but to a group of muscles (muscle field), it follows that an M1 motor neuron codes for an organized movement. the topographical representations of movement are likely organized according to ethologically relevant categories of motor behavior. It is believed that M1 would be particularly concerned with hand and oral behaviors that occur in the “personal space”. A muscle can contract as a result of the stimulation of different cortical areas and therefore different cortical areas project (converge) on each muscle. Within the single large areas (arm, leg, etc…) there is a concentric organization: the sites that affect distal muscles are central to a larger area that also contains sites that affect proximal muscles, while sites peripheral to the central area control only proximal muscles.<\/p>\n The individual cortical motor neurons (Betz cells) project directly to spinal alpha motor neurons and involve more than one motor nucleus and sometimes even muscles of different joints and have phasic-tonic activity: they discharge quickly during the dynamic phase of the movement and reduce the discharge to a continuously lower (tonic) level when a stable strength level is reached.<\/p>\n Motor neurons of M1 code for some movement parameters:<\/p>\n There is therefore a transcortical circuit that allows M1 to elaborate rapid responses (faster than simple reaction time\u00a0 and less rapid than spinal reflexes) with a degree of flexibility that spinal reflexes do not have.<\/p>\n In the control of the movements of the fingers, the cortical neuronal populations promote mechanisms of activation and inhibition of muscles that act on all the fingers of the hand. The very nature of the motor task (power take-off or precision grip) contributes to determining the choice of cortical motor neurons to be used for a given muscle, therefore even if a spinal motor neuron is monosynaptically connected to a cortical motor cell, its activity does not it changes coherently with the variations of the cortical cell, as the multiplicity of connections that the spinal motor neuron receives makes its activity flexible, adapting to the motor task that must be performed.<\/p>\n Some cortical motor neurons modify their firing frequency in proportion to the force to be developed, while others reduce it. However, the latter have an excitatory function on the target muscles, but discharge only when fine movements or gradual changes in strength are performed. Their function could be a more accurate reduction of the recruitment of motor units than that which would be obtained with the simple inhibition of cortical motor neurons of the first type mentioned above.<\/p>\n They are a complex set of interconnected areas of the frontal lobe that lie anterior to the primary motor cortex and control motor functions. The cortical areas (second subdivision of Brodman) forming part of this set are: area &, area 8, area 44\/45 and part of areas 23 and 24 on the medial surface of the hemisphere.<\/p>\n The influence on the movement is exerted through a direct route (projections to the cortico-bulbar and cortico-spinal tracts) and an indirect route through extensive reciprocal connections with M1 and therefore the efferents are similar to those of the primary motor cortex while the afferents are clearly different.<\/p>\n Medial and lateral pre-motor areas are distinguished:<\/p>\n \u00a0Lesions of the frontal lobe in the premotor areas result in difficulty choosing a movement in response to a stimulus, although understanding of instructions and the ability to execute the movements are retained.<\/p>\n They are a collection of interconnected areas of the frontal lobe that lie anterior to the primary motor cortex and control motor functions. The areas that are part of this set according to Brodman are: area 6, area 8, area 44\/45 (on the lateral surface of the frontal lobe), and parts of areas 23 and 24 on the medial surface of the hemisphere.<\/p>\n The influence on movement is exerted through a direct route (projections to the cortico-bulbar and cortico-spinal tracts) and an indirect route (extensive reciprocal connections with M1); the efferents therefore overlap with those of the primary motor cortex, while the afferents are very different.<\/p>\n We can distinguish the pre-motor areas into medial and lateral.<\/p>\n The pre-motor areas are involved in the reaching and grasping movements: these are classic movements aimed at a goal, finalized and require a sensory-motor transformation process (processing of spatial information on the position of the object and of our arm in order to set the movement correctly). The circuits involved in reaching and grasping are parietomotor and act in parallel.<\/p>\n In the reaching movement, the parameters (direction and amplitude of movement) depend on the location of the target with respect to us:<\/p>\n In the grasping movement the parameters depend on the shape and size of the object:<\/p>\n The subcortical systems involved in movement are the cerebellum and the basal nuclei.<\/p>\n It has a homogeneous organization, but each area receives afferents from different portions of the brain or spinal cord. This suggests that the cerebellum performs the same type of computational operations on different afferent signals.<\/p>\n The primary function of the cerebellum is to detect the difference (“motor error”) between a planned movement and the actual movement and, once the error has been detected, send signals\u00a0 (through projections to the upper motor neurons), in order to induce a reduction of the same mistake.<\/p>\n It has 3 functionally distinct regions: the cerebellar cortex, the white matter, and the deep nuclei.<\/p>\n The cerebellar cortex is made up of three layers in which there are five types of cells. Four types of cells are inhibitory (stellate cells, basket cells, Purkinje cells, and Golgi cells). Granule cells are excitatory.<\/p>\n Three pairs of deep nuclei: the fastigium, the interpositon and the dentate.<\/p>\n The cerebellum is connected to the brain stem by three symmetrical pairs of bundles of nerve fibers, called the inferior, middle, and superior cerebellar peduncles. The latter contains most of the efferent connections of the cerebellum.<\/p>\n The cerebellum receives two types of afferents: mossy fibers and climbing fibers. Both are excitatory fibers (main excitatory circuit) and target Purkinje cells, but the connections they establish in the pathway are different and, therefore, generate different responses (inhibitory cortical circuits).<\/p>\n The cerebellum is divided into vestibulo-cerebellum, spino-cerebellum, and cerebro-cerebellum based on the origin of the afferents. The vestibulo-cerebellum relates to the lateral and medial vestibular nuclei, while the spino-cerebellum and cerebro-cerebellum relate respectively to the deep fastigium and interpositon nuclei and the dentate nucleus, through which they project to the thalamus and, finally, to the cortex .<\/p>\n The execution of a movement determines a relative activation of climbing fibers which can generate an error signal (due to the detection of differences between expected and transmitted sensory signals), depressing the parallel fibers (which carry “incongruous” central signals) and therefore correcting the movement. As the movement is repeated the effects of parallel fiber signaling become less and less and the movement becomes more and more correct. This explains why as a result of cerebellar lesions motor learning may fail.<\/p>\n The vestibule-cerebellum has inhibitory efferences directed only towards the medial and lateral vestibular nuclei; in this way it controls the eye movements, the coordination of the movement of the eyes and the head and the maintenance of balance and the upright position during the walk.<\/p>\n Injuries of these pathways determine the inability to use vestibular information to control standing and walking: gait with an enlarged support base to compensate for the deficit that often results in falls to the ground anyway (regardless of whether their eyes are open or closed ); these patients, on the other hand, have no problems in controlling the limbs during the execution of the various types of movement when they are lying down or supported.<\/p>\n The spino-cerebellum emits direct efferences to the cortex, brainstem, cortico-spinal and rubro-spinal systems. Check your posture, locomotion and gaze direction. It is roughly organized in somatotopic maps, but with microscopic recordings they demonstrate a fragmented somatotopy (the afferents coming from a precise peripheral site are distributed divergently and terminate at the level of many areas in relation with circumscribed aggregates of granules). Movement control is operated with feed-forward mechanisms.<\/p>\n Lesions of the interpositus nucleus cause a reduction in the activity of generation of the excitatory postsynaptic potential with consequent reduced stimulation of rubro \/ cortico-spinal neurons, and a reduced neuronal excitability that manifests itself clinically with a reduced muscle tone (cerebellar hypotonia). The other problem is the deficit of anticipatory control on the motility of the (lower) limb ipsilateral to the lesion, the clinical manifestations of which are an oscillatory response (pendular reflexes) to an external perturbation and an irregular oscillation around the target (terminal tremor) at the end of the rejoining movement. The cerebellar dysmetria and ataxia is all the more evident the more joints are involved in the movement as the cerebellum has to mediate a greater number of proprioceptive information.<\/p>\n The lesions of the worm and of the nucleus of the fastygium cause alterations of the language, which becomes slow and with the emission of one word at a time (pronounced language).<\/p>\n The cerebro-cerebellum emits efferences towards the primary motor, pre-motor and pre-frontal cortex. It deals with the planning of complex actions, their mental repetition and the conscious evaluation of errors in movements. Injuries of this path cause decomposition of the movement of the half of the body ipsilateral to the lesion and increase in reaction times.<\/p>\n In general, lesions of the medial position of the cerebellum interfere with the accuracy of execution of motor responses, while lesions of the lateral portion interfere with the temporal succession of serial events.<\/p>\n They are the main components of the reentrant cortico-subcortical circuits that connect the cortex and the thalamus. They affect movement by regulating the activity of upper motor neurons.<\/p>\n They include four formations: the striatum (caudate, putamen, ventral striatum) and the pale globe (internal and external) emit GABAergic initorial projections, the substantia nigra (pars compacta and pars reticulata) that emits dopaminergic projections, and the subthalamic nucleus that emits projections glutamatergic excitatory.<\/p>\n From a functional point of view, two ways: direct and indirect.<\/p>\n The basal ganglia, relating to the cerebral cortex and the thalamus, take part in the skeleton-motor circuit. Within this circuit it is possible to identify sub-circuits organized in a somatotopic and functional way (the dorso-lateral portion of the putamen deals with the leg, the ventro-medial portion of the face and the intermediate portion of the arm). When a subject has to perform a movement and receives the indication on the direction of this movement, the cortex (M1, pre-motor, SMA) modifies the firing frequency generating the “motor attitude”, while putamen and internal pale globe modify the discharge rate to produce two possible responses:<\/p>\n Disturbances in the activity of the indirect route can generate hypokinetic or hyperkinetic problems. Hyperactivity of the indirect pathway causes hypokinetic disorders such as Parkinson’s disease, while hypoactivity of the same indirect pathway causes pathologies such as Huntington’s chorea.<\/p>\n How to design a CDSS: Clinical Decision Support System Movement control is a process that takes place mostly autonomously (not consciously) but whose effects \/ results are indispensable for interaction and environmental survival. Motor behavior is the result of a long period of years of learning the motor strategies that spontaneously or specifically addressed (training), are acquired as procedural baggage.<\/p>\n \u00a0<\/p>\n\n\n
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The motor system generates three types of movement:<\/h4>\n
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Three basic laws govern voluntary movements:<\/h4>\n
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Organization of the motor system<\/h2>\n
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CORTICAL MOTOR AREAS<\/h4>\n
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Anatomical and functional organization<\/h3>\n
PRIMARY MOTOR AREA<\/h3>\n
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PRE-MOTOR AREAS<\/h3>\n
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The pre-motor areas<\/h4>\n
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Subcortical systems involved in movement<\/h2>\n
Cerebellum<\/h4>\n
Anatomy<\/h5>\n
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Basal ganglia<\/h4>\n
Anatomy<\/h4>\n
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