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Body movement and the brain
Sub-Topics
Making a Voluntary Movement

Experiment
HelpExperience : Regional cerebral blood flow changes in human brain related to ipsilateral and contralateral complex hand movements--a PET study.
Original modules
Experiment Module: Activity Pattern of Neurons in the Motor Cortex of MonkeysActivity Pattern of Neurons in the Motor Cortex of Monkeys

If someone's motor cortex is destroyed (by a stroke, for example), he or she loses the ability to make precise movements, especially of the hands and fingers. Learning of new movements is not strongly affected by damage to the cerebral cortex. The memory of motor sequences learned previously is also largely spared, though these movements will be executed more clumsily. These observations show that it is the cerebellum rather than the cortex that plays an important role in learning and remembering of movements, also known as procedural memory.


Area 4 of the precentral gyrus is not the only area in the cortex that contributes to the pyramidal system. But is the one where movements can be successfully triggered by lower-intensity electrical stimuli. In other words, electrical stimuli that are insufficient to produce movements when applied to other areas of the cortex are sufficient to do so when applied to Area 4.


THE MOTOR CORTEX
THE BASAL GANGLIATHE CEREBELLUMTHE ACTIVATION SEQUENCE FOR THE MOTOR AREAS

So many different structures in the brain are involved in motor functions that some people even say that practically the entire brain contributes to body movements. Though the motor cortex is usually associated with Areas 4 and 6, the control of voluntary movements actually involves almost all areas of the neocortex.

 

The primary motor cortex is the anatomical region composed of Area 4 of the precentral gyrus. Its location was confirmed in the mid-20th century in brain operations performed by neurosurgeons such as Dr. Wilder Penfield, in Montreal. While performing operations to alleviate patients' epileptic symptoms, Penfield stimulated various areas of the cortex to identify vital ones that should not be removed. In this process, he discovered that stimulations applied to the precentral gyrus triggered highly localized muscle contractions on the contralateral side of the body and that there was a somatotopic representation of the corresponding parts of the body in Area 4 in the primary motor cortex (see box below) .

Penfield also showed that cortical Area 6, just rostral to Area 4, has two other somatotopic representations that induce complex movements when stimulated. The first is in the lateral portion of Area 6 and is called the premotor area (PMA). It helps to guide body movements by integrating sensory information, and it controls the muscles that are closest to the body's main axis.

The second somatotropic representation is in the supplementary motor area (SMA), in the medial part of Area 6. The SMA is involved in planning complex movements and in co-ordinating movements involving both hands.

Note, however, that the primary motor cortex, the PMA, and the SMA are not the only parts of the cortex that are involved in generating voluntary movements. The prefrontal cortex and the posterior parietal cortex also play important roles in this regard.

Dr. Penfield's experiments in stimulating the cortex enabled him to develop a complete map of the motor cortex, known as the motor homunculus (there are also other kinds, such as the sensory homunculus). The most striking aspect of this map is that the areas assigned to various body parts on the cortex are proportional not to their size, but rather to the complexity of the movements that they can perform. Hence, the areas for the hand and face are especially large compared with those for the rest of the body. This is no surprise, because the speed and dexterity of human hand and mouth movements are precisely what give us two of our most distinctly human faculties: the ability to use tools and the ability to speak.

History : Mapping the Motor Cortex: Part II



    

Linked
Link : basal gangliaLink : SUBTHALAMIC NUCLEUS & SUBSTANTIA NIGRALink : La boucle "squeletto-motrice"Link : NEUROANATOMIE FONCTIONNELLE
History
History : A BRIEF HISTORY OF THE BASAL GANGLIA
Original modules
Tool Module : Your "Mental Stopwatch"Your "Mental Stopwatch"

The functions of the basal ganglia are complex and still largely unknown. People who have Parkinson's disease, characterized by trembling and by difficulty in initiating movements, show a deficiency of dopamine in their basal ganglia. Because these structures play an important role in determining various aspects of movement, their malfunctioning results in the motor problems associated with Parkinson's disease.

Some abnormalities also are found in the basal ganglia of people who have Huntington's Disease or Tourette Syndrome. These patients experience involuntary movements that cause all sorts of grimaces, tics, and spasms.

Link : Basal Ganglia and Related StructuresLink : Huntington DiseaseLink : Tourette Syndrome


THE BASAL GANGLIA
THE MOTOR CORTEXTHE CEREBELLUMTHE ACTIVATION SEQUENCE FOR THE MOTOR AREAS

The term "basal ganglia" refers to a group of several structures in the brain: the caudate nucleus, the putamen, the globus pallidus, and the subthalamic nucleus. The substantia nigra, a midbrain structure that has many interconnections with the basal ganglia, is not actually part of this grouping but is often associated with it.

The basal ganglia are involved in a complex loop that connects them to various areas of the cortex. The information from the frontal, prefrontal, and parietal areas of the cortex passes through the basal ganglia, then returns to the supplementary motor area via the thalamus. The basal ganglia are thus thought to facilitate movement by channelling information from various regions of the cortex to the SMA. The basal ganglia may also act as a filter, blocking the execution of movements that are unsuited to the situation.



Not all of the circuits involving the basal ganglia are motor circuits, however. Many are instead involved in memorizing and in cognitive and emotional processing. A great deal about the basal ganglia remains unknown. They seem to play a far larger role than just their contribution to motor control.


    

Linked
Lien : The Treasure at the Bottom of the BrainLien : cerebellum

The cerebellum also acts as a learning and memorizing machine, thanks to its modifiable neural connections that continuously compare everything they are programmed to do with the results that they are actually achieving. When this comparison does not allow the expected result to be achieved satisfactorily, the cerebellum's activity modifies the sequence of movements in a compensatory manner to make them more effective. This procedural memory thus develops automatically with practice, without the help of any conscious control.

The cerebellum also appears to play a major role in learning how to co-ordinate the various segments of the body. The movement of each segment of your body affects the next, because of its mass. The cerebellum therefore apparently learns how to calibrate its commands to the muscles in terms of strength and duration in order to correct in advance for the effects of these interactions along the path of motion.

THE CEREBELLUM
THE MOTOR CORTEXTHE BASAL GANGLIATHE ACTIVATION SEQUENCE FOR THE MOTOR AREAS

 

 

The cerebellum appears to play several roles. It stores learned sequences of movements, it participates in fine tuning and co-ordination of movements produced elsewhere in the brain, and it integrates all of these things to produce movements so fluid and harmonious that we are not even aware of them.

To do all this, the cerebellum maintains close communications with the cortex. The motor, somatosensory, and posterior parietal areas of the cortex project massive numbers of axons to the nuclei of the pons, located in the brainstem. The neurons of the pons then project their axons into the cerebellum. This corticopontocerebellar tract forms an extremely dense nerve bundle containing about 20 million axons, just about 20 times more than the pyramidal bundle!

The two hemispheres of the cerebellum then sends signals back to the motor cortex via interconnections in the ventrolateral nucleus (VLc) of the thalamus. The cerebellar hemispheres thus influence the muscles of the arms and legs via the cortex and the lateral motor system.

 

 

The two hemispheres of the cerebellum are not divided neatly in two like the two hemispheres of the cerebrum. The medial portion constitutes what is known as the cerebellar vermis. This vermis does not display any lateralization. It projects axons to the brainstem which, via the ventromedial system, help to maintain posture.

More recently, the cerebellum has also been discovered to play a role in sensory information processing as well as in cognitive functioning.


    

Linked
Link : The anatomy of movement
Researcher
Research : Cerebellar Coordination of Eye and Hand Movements
Original modules
Experiment Module: Activity Pattern of Neurons in the Motor Cortex of MonkeysActivity Pattern of Neurons in the Motor Cortex of Monkeys

"I move, therefore I am a self, (I think)."

- Rodolfo Llinas

Lien : Book Review - I of the Vortex

The brain mechanisms that go into planning and executing a movement are far more complex than the motor cortex's simply issuing a command and the motor neurons' executing it.

For example, suppose that you go to pick up a glass of water that you think is cool and refreshing, but is actually boiling hot. As soon as you touch the glass, you pull your hand back immediately, by reflex, without thinking about it.

But suppose that next, your child tries to grab this glass, which you already know is hot. In this case, because your child's safety is so important to you, you can consciously overcome the reflex to pull your hand away. Instead, using your voluntary motor control, you grab the glass yourself and put it where your child can't reach it.

Lastly, if someone tells you that the glass is made of fine crystal and not ordinary glass, you will probably handle it more carefully. In other words, your brain will take this information into account and adapt your method of grasping the glass accordingly.

All of these facts demonstrate that the execution of a movement is not simply a matter of the brain's sending a "Go!" command to some motor neurons in the spinal cord, but rather the result of a highly elaborate construct. Moreover. the remarkable adaptability of motor activity demonstrates the involvement of powerful regulatory and feedback mechanisms.

THE ACTIVATION SEQUENCE FOR THE MOTOR AREAS
THE MOTOR CORTEXTHE BASAL GANGLIATHE CEREBELLUM

The information processing that the brain must perform to initiate a voluntary movement can be divided into three steps. The first step is to select an appropriate response to the current situation, out of a repertoire of possible responses. This response, which corresponds to a particular behavioural objective, is determined in a global, symbolic fashion.

The second step is to plan the movement in physical terms. This step consists in defining the characteristics of the selected response as the sequence of muscle contractions required to carry it out.

The third step is to actually execute the movement. It is in this step that the motor neurons are activated that trigger the observable mechanics of the movement.

Consequently, the control messages issued by the motor cortex are themselves triggered by messages from other cortical areas. The motor cortex also communicates closely with subcortical structures such as the basal ganglia and the cerebellum, through the thalamus, which acts as a relay.

In light of what we now know about the sequence in which the motor areas of the cortex are activated, we can deconstruct the classic sequence "Ready? Set. Go!" in terms of localized activity in the brain.

In the "Ready?" phase, the parietal and frontal lobes become active first, with a contribution from the subcortical structures involved in vigilance and attentiveness. The "Set" command then activates the supplementary and premotor cortical areas, where the strategies for movement are developed and maintained until the "Go!" signal is given. The "Go!" signal may come from an outside source, as it does in an actual race, or it may come from inside the person doing the running, who decides for himself or herself that all the conditions are present to start running. The "Go!" command then applies information from subcortical structures such as the basal ganglia that will influence Area 6, and then eventually the primary cortex, which will cause the action to be carried out.

 



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