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Organization of the Motor System.docx

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University of Toronto Mississauga
Ayesha Khan

Organization of the Motor System (Function is to move the body!) Chapter 9 Case Study: *Spinal Cord Injury > Christopher Reeve was thrown from his horse at a riding competition. His spinal cord was damaged at the C1 & C2 level, at the upper end of the spine. > His brain and the remained of his spinal cord were intact and functioning, but they were no longer connected. Therefore, his body below the neck was paralyzed. > Learned to wiggle his toes, move the fingers on his left hand, and distinguish between hot and cold and sharp and dull sensations over his body. With aquatherapy, he learned to kick his legs. - We can consider the entire nervous system to be the motor system, as it functions to move the body. Motor System during a Hand Movement: Picking up a cup of coffee 1) Visual information required to locate the cup. 2) Frontal-lobe motor areas plan to reach and command the movement. 3) Spinal cord carries information to the hand. 4) Motor neurons carry message to muscles of the hand & forearm. 5) Sensory receptors on fingers send message to sensory cortex “the cup has been grasped” 6) Spinal cord carries sensory information to the brain. 7) Basal ganglia judge the grasp force & the cerebellum corrects the movement errors. 8) Sensory cortex receives the message “the cup has been grasped” *Motor cortex is located in the frontal lobe. Basil ganglia provides information to the frontal lobe about the force, and the cerebellum provides information about the accuracy of movements. We want to know: how does the brain & the spinal cord work together to produce movement? What are the major parts of the motor system? - Motor cortex - Motor neurons - Basil ganglia - Cerebellum - Spinal cord - Brainstem What is the neocortex and how does it initiate movement? General regions of the neocortex produce skilled movements. Regions include: - Posterior cortex > Posterior sensory regions specify movement goals and send information to the prefrontal cortex through a number of routes. Direct routes prompt M1 for more automatic movements. Indirect routes go through the temporal cortex and used for movements requiring conscious control. - Prefrontal cortex > Instructions travel from the prefrontal cortex which generate plans for movements to the premotor and the primary motor cortex. - Premotor cortex ( including a ventral & dorsal region called the supplementary motor cor)ex > Contains regularly performed movements that allows it is recognize the movement of others and select similar or different actions. - Primary motor cortex (M1 – Brodmann’s area 4) > Consists of movements that are somewhat more elementary (basic) than those of the premotor cortex. Therefore, to put it all together, the goal for movement happens in the posterior cortex, the planning of movement happens in the prefrontal cortex, the sequencing of movements happen in the premotor cortex and finally the execution of movements happen in the primary motor cortex. How does E. Roland’s experiment serve as an indicator of neural activity? > It explains the theory of neocortical motor control. It showed the regions of the brain that were active when subjects in a study were performing different tasks. - Tapping finger = activation of somatosensory & primary motor cortex. - Tapping finger in a sequence = also activates the premotor cortex. - Using finger to navigate a maze = also activates the prefrontal cortex (planning) & regions of the temporal and parietal cortex. * Increases in blood flow occur only in regions that contribute to the particular movement – not the entire neocortex. How do we identify the motor cortex with the use of electrical stimulation? > The specialization of body parts for performing skilled movements is widespread among animals. Ex. Some animals rely primarily on their mouth for skilled movement, while humans usually rely on their hands. > 1950’s, Wilder Penfield used pulses of electrical stimulation to map the cortices of patients who were about to undergo neurosurgery. - Most movements were due to the stimulation of the precentral gyrus – also called called the primary motor cortex (M1 – Brodmann’s area 4). - Movement could also be produced by stimulating the dorsal part of the premotor cortex, therefore, this region was called the supplementary motor cortex. - Found a homunculus “little person” spread across the primary motor cortex, and each hemisphere of the brain almost mirrored the other side, because the body is symmetrical. Also located a secondary homunculus in the supplementary motor cortex (premotor cortex). - The most striking feature is the disproportion in the relative sizes for each body part, compared to the actual size of the body part. This is due to the fact that large parts of the motor cortex regulate hand, finger, lip, and tongue movements more than other parts, giving these parts of the body precise motor control. Therefore, the parts of our bodies that we have much less control over, have a much less smaller representation in the motor cortex. * Movements are topographically organized in the motor cortex. > Electrical stimulation of the dorsomedial regions = movements in lower limbs. > Electrical stimulation in the ventral regions = movements in upper body, hands & face. How are there multiple representations in the motor cortex? > Turns out there are many more homunculi than were recognized by Penfield! Maybe as many as 10 different homunculi within the motor and premotor cortex. Also, parts of the homunculi are not arranged as simply as Penfield thought. Ex. Finger movement can be obtained from many points. Also, the points that elicit finger movement may also cause other body parts to move as well. What are movement sequences? > In monkeys, Michael Graziano found that stimulation elicits actions that he calls “ethological categories of movement” as these movements are USEFUL to the monkey. * Ethology means it is done naturally. A = defensive face posture B = hand to the mouth C = manipulation & shaping of the hand D = outward reach with the hand E = climbing/leaping > When a part of the motor cortex is damaged, a sequence of movements is disrupted. > Organization of the motor cortex is not specific to muscles, but rather to these ethological movements. > Activation of the motor cortex results in movements. It is involved in it’s own language; involved in muscle movement that has MEANING. The motor cortex represents 3 types of organization: 1) the part of the body that is to be moved 2) the spatial location to which the movement is directed 3) the function > The cortical map is quite flexible depending on the past experience of the monkey, its recent experience, objects that are available for reach & even just-completed actions. What is the movement lexicon? > Humans have a lexicon (language) or repertoire (actions that are regularly performed) of movement categories in the motor cortex. This is explained by looking at the way different people perform similar skilled movements. > Evidence for the movement lexicon: - One evidence is that humans usually grab small objects with the thumb and their index finger called “the pincer grip”. It starts to become apparent in infants by 3 months of age and by 1 year they start picking up small objects. - Another evidence for the movement lexicon is that most primates use the same grip pattern. - Also people who have small lesions of the motor cortex in the area of the thumb region not only have a weakness in their thumb, but also in the other fingers and the arm. Finding suggests that a lesion does not impair the muscles or the individual digits, but rather the action of reaching for an object and grasping it. - In the case of a lesion that the person is no longer able to use the pincer grip, the person will substitute it for the whole-hand grip. Therefore, the pincer grip is not learned but is a part of the prewired movement lexicon in the motor cortex. A) Pincer grip = fine movements B) Whole-hand grip = result of a lesion in the thumb region. > The repertoire of the premotor cortex is more complex than the primary motor cortex as it sequences movements, therefore, disrupts more complex movements. > 5 months after the premotor cortex has been made smaller, the monkey is unable to make the two complementary movements together (pushing & catching the food). It can do each movement separately, but not together at the same time using two hands. > Therefore, the premotor cortex plays a greater role in organizing whole-body movements, whereas the primary motor cortex controls specific acts. How do movements get coded by neurons in the neocortex? > Cells play a direct role instructing particular muscles to contract, but other studies suggest that cells specify the target of the movement. > Other cells must take part in visualizing movements. So we want to know… how does the neocortex specify movements?! How do motor-cortex cells specify movements and their force & direction? > Edward Evarts trained a monkey to flex his wrist, to move a bar to which different weights of heaviness were attached. An electrode was implanted in the wrist region of the motor cortex, and recorded the activity of neurons there. > Found that neurons began to discharge even before the monkey flexes his wrist, suggesting that he was planning the movement. Neurons continued to discharge during the wrist movement, which shows an execution of the movement. Neurons discharged at a higher rate when the weight was heavier; therefore the motor-cortex neurons increase the force of a movement by increasing their rate of firing. > Shows that the motor cortex also specifies the direction of movement because the neurons fire when the monkey reaches his hand outward, but not when he brings it back to the starting position. This encodes the direction in which the wrist is moving. How was the encoding of movement direction further studied? > Georgopoulos trained a monkey to move a lever in different directions across a table. > Found that each neuron is maximally active when the monkey moves his arm in a particular direction. > The motor cortex seems to calculate the direction & the distance of movements (movement/orientation of hands). > The premotor cortex was found to respond to the movement to be made and the target, indicating that the premotor cortex is especially concerned with the objective of the movement. What is mirroring movement? > Many premotor-area neurons are also found to discharge when a monkey watches another monkey sees other monkeys make the same movement, or even when they see a human make the same movement. There neurons are called “mirror neurons”. - Mirror neurons encode a complete action. - Some of them have very exact requirements and only respond to a particular hand movement, or are only used to pick up a small object. - Other mirror neurons are not so specific, and continue to respond even when the grip pattern changes or the size of the target varies. - All mirror neurons represent actions, either one’s own or others’. - They are used for imitating or understanding the meaning of others’ actions. > Mirror neurons can also fill in the blanks when the monkey is unable to see a part of the movement. > Mirror neurons, in short, enable communication between sender and receiver. > In humans, mirror neurons are found largely in the left hemisphere. > Provide self-awareness, social awareness, and awareness of the intention & actions of others. > Important in gestural & verbal language. What happens when we observe, remember & imagine movements? > Motor cortex becomes activated even when just thinking, observing and remembering and imagining the action/movement. > Mental rehearsal shows more activity in the prefrontal cortex (planning), and less activity in sensory and motor cortex. No activity in the cerebellum (correction of movements). What are the roles of the prefrontal & posterior cortex? > Sensory information may instruct movements in two ways: 1) Direct connections from the parietal cortex to the primary motor cortex: Movements can be made in direct response to sensory stimulation. Ex. Simple & reflexive movements 2) Various sensory systems send information to the prefrontal & motor cortex: Movements that are more complex in action & intention. > Sensory information is very important for movement. > “Deafferentation” = the loss of somatosensory input resulting from damage to the
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