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Body movement and the brain

HelpLe système extrapyramidalLe cortex moteur et ses voies descendantesPyramidal Motor System Corticospinal tract
Motor SystemsControl of Movement by the BrainThe Spinal Cord/Brainstem Interactive Atlas
Apostolos P. Georgopoulos
Neural coding of finger and wrist movementsThe motor cortex and the coding of forceOrganization of adult motor cortex representation patterns following neonatal forelimb nerve injury in ratsImmediate and delayed changes of rat motor cortical output representation with new forelimb configurations
Time course of motor cortex reorganization following botulinum toxin injection into the vibrissal pad of the adult rat

Even spinal reflexes, such as the knee-jerk reflex and the leg-withdrawal reflex, which might appear simple at first glance, actually reveal all the complexity of the spinal system for controlling movement. The activity of the alpha motor neurons that innervate the muscles is subject to the triple influence of sensory inputs, spinal interneurons, and the pyramidal tract.

For some time, the primary motor cortex was thought to contain a detailed representation of every muscle in the body, so that the activation of a particular pyramidal cell led to the activation of a single group of motor neurons. But this view has been called into question by recent studies suggesting that the pyramidal cells actually control groups of muscles. The activation of these cells would therefore enable an entire limb to be mobilized to accomplish a given action.

Moreover, recordings of the activity of the motor cortex neurons during a body movement show that activation begins before the movement and continues throughout its execution. This neural activity may be encoding two main aspects of the movement: its force and its direction.

Experience : Pattern d’activité des neurones du cortex moteur chez le singe



Eight Problems with Mirror Neurons


In addition to receiving inputs from the premotor and supplementary motor areas, the pyramidal neurons of the primary motor cortex receive information directly from somatosensory areas 3, 1, and 2. The other major source of incoming axons is the thalamus, or more precisely the caudal part of the ventrolateral nucleus (designated VLc), which relays information from the cerebellum.

The primary motor cortex projects its axons mainly to the corticospinal tract, which is composed of the lateral and ventromedial systems.

The lateral system comprises two main neural pathways, the larger of which is the lateral corticospinal tract. Arising mainly in Areas 4 and 6 of the frontal lobe, which together constitute the motor cortex, this is the longest neural pathway and one of the largest in terms of the number of axons it contains (1 million). The other axons in this tract arise mainly in the somatosensory areas and the parietal lobe.

After crossing the internal capsule, the midbrain, and the pons, the axons arising in the cortex join together at the medulla oblongata to form a dense bundle of nerve fibres. This bundle is shaped somewhat like a pyramid extending along the ventral surface of the medulla, which is why it is called the pyramidal tract at this location. (In contrast, all of the other tracts that arise in the subcortical structures or the brainstem and follow a different route are often referred to as the extrapyramidal tracts.)

Just before entering the spinal cord, the pyramidal tract decussates. In other words, the fibres from the left hemisphere of the cortex now cross over into the right lateral column of the spinal cord, and vice versa. The axons of this tract ultimately synapse on the motor neurons and interneurons of the dorsolateral portion of the ventral horn of the spinal cord.

The rubrospinal tract is the second tract in the lateral system. It arises from the neurons of the red nucleus, in the midbrain. This nucleus receives information from the frontal cortex , a region that also projects massively to the corticospinal tract. Indeed, over the course of primate evolution, the role of the rubrospinal tract, an indirect pathway, has diminished, while the corticospinal tract has assumed more and more of the responsibility for motor control.


The other major descending pathway, the ventromedial system, is composed of four tracts that originate in various areas of the brainstem and contribute chiefly to postural control and certain reflex movements. The originating neurons of these tracts receive sensory information related to balance, body position, and the visual environment.

The vestibulospinal tract originates in the vestibular nuclei that receive information from the inner ear. This tract helps to maintain the head in the correct position relative to the shoulders, which is essential for continuing to look in a given direction while the body is moving.

The tectospinal tract arises in the superior colliculus (tectum) in the midbrain. The superior colliculus receives some visual information directly from the retina, as well as somatosensory and auditory information. Through the representation of the environment formed by the superior colliculus, the tectospinal tract contributes to visual orientation.

The pontine (medial) and medullary (lateral) reticulospinal tracts arise from the reticular formation nuclei in two main parts of the brainstem: the pons and the medulla oblongata. The reticular formation receives inputs from many sources and extends the entire length of the brainstem, from the pons to the medulla. These two tracts help to maintain posture. The axons originating in the pons enhance the spinal antigravity reflexes, while the axons originating in the medulla have the opposite effect, releasing the muscles involved in these reflexes and thus facilitating other movements.

In the mid-1990s, scientists studying Area F5 in the ventral premotor cortex of monkeys found that certain neurons in this area sent out action potentials not only when the monkeys were moving their hands or mouths, but also when they were simply watching another animal or a human being who was making such a gesture. These neurons were dubbed mirror neurons because of the way that a visually observed movement seemed to be reflected in the motor representation of the same movement in the observer.

In addition to mirror neurons, which are activated both when you perform an action yourself and when you see someone else performing it, another kind of neurons, called canonical neurons, become activated when you merely see an object that can be grasped by the prehensile movement of the hand whose movements they encode—as if your brain were foreseeing a possible interaction with this object and preparing itself accordingly.

What these two types of neurons have in common is that they are both activated by an action regardless of whether you are carrying that action out, anticipating carrying it out, or watching someone else carrying it out. Because mirror neurons thus help us foresee the consequences of our own actions, some have argued that these neurons may be the cellular substrate for our ability also to understand the meaning of other people's actions.

This understanding of other people's actions is the foundation for all social relations, and especially for communication between individuals. The discovery of mirror neurons may thus be particularly useful for explaining how we can imagine other people's intentions and state of mind. Lastly, the fact that Area F5 in monkeys is regarded as the homologue for Broca's area in humans suggests that mirror neurons also are involved in human communication.

Link : The mirror neuron system and action recognitionExperience : Grasping objects and grasping action meanings: the dual role of monkey rostroventral premotor cortex (area F5).Link : Same brain patterns whether doing or watchingLink : Schema Design and Implementation ofthe Grasp-Related Mirror Neuron SystemLink : Hearing Sounds,UnderstandingActions:Action Representationin Mirror NeuronsLink : MIRROR NEURONS and imitation learning as the driving force behind "the great leap forward" in human evolutionLink : MIRROR NEURONS and imitation learning as the driving force behind "the great leap forward" in human evolution (Commentaries)Link : Du Contrôle Orofacial des Gestes dans la communication chez les primates jusqu'à la Parole humaineLink : Sons entendus, actions comprises: la représentation des actions dans les neurones miroirs



Long-Term Depression in the CerebellumThe cerebellum: cortical processing and theoryLe cervelet : organisation anatomique et fonctionsCervelet (cortex cérébelleux)

The Purkinje cells are the most characteristic type of neurons in the cerebellum. The dendrites of each Purkinje cell have a very distinctive pattern: their branches all lie in one plane in which they assume the shape of a fan. The fan-shaped dendrites of adjacent cells lie parallel to each other and are separated by a distance of about 0.1 mm. They deploy into the molecular layer of the cerebellar cortex. The axons of the Purkinje cells synapse on the neurons of the dentate nuclei of the cerebellum. These nuclei relay the information to the thalamus, which then projects to the cortex and the striatum.

The dendrite branches of each Purkinje cell receive synapses from the branch terminations of a single afferent climbing fibre. This fibre is the axon of a neuron in the inferior olive, a nucleus in the medulla oblongata. The inferior olive integrates the information from the muscle proprioceptors. Each climbing fibre winds closely around the dendrites of its corresponding Purkinje cell, so that the activation of this fibre will cause a massive excitation of this cell.

In contrast, the second major source of inputs to the cerebellum, the mossy fibres, act in a highly diffuse fashion. These fibres are the axons of neurons in the pontine nuclei that receive information from the cerebral cortex. The mossy fibres carry this information to synapses with the small granular cells in the deep layer of the cerebellum. There are so many of these granular cells that they are thought to account for half of all the neurons in the brain!

The axons of these granular cells ascend into the surface layer of the cerebellum (the cerebellar cortex) where they branch into T shapes to form the parallel fibres. The parallel fibres then run perpendicular to the Purkinje cell dendrite fans, thus crossing many Purkinje cells and connecting them into a single contact. Though each parallel fibre makes only one contact with each Purkinje cell that it crosses, it makes contact with a huge number of such cells along its path, which measures just a few millimetres. Likewise, each Purkinje cell receives over 100 000 synapses from 100 000 different parallel fibres.

At first, this configuration was believed to be the basis for the cerebellar clock. Because the incoming message from the parallel fibres takes an increasing amount of time to traverse the succeeding dendritic levels, a time lag develops. It was thought that the cerebellum might use this lag to co-ordinate the sequence of movements. But now the most commonly accepted interpretation of this dual afferent system is that it provides the ideal basic structure for an elementary learning mechanism called long-term depression.

This depression occurs when the dendrite branches of a Purkinje cell are activated by the climbing fibre and the parallel fibres simultaneously. The result is a long-term reduction in the efficiency of the synapse between these parallel fibres and the dendrites of this Purkinje cell.



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