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Guy Proulx (13)

9. Organization of the Motor System.docx

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PSYC 3530
Guy Proulx

9. Organization of the Motor System Tuesday, October 15, 2013 9:15 AM The Neocortex and the Initiation of Movement  1. The posterior sensory regions of the cortex specify movement goals and send information to the prefrontal cortex by a number of routes. More direct routes are used to prompt M1 to execute relatively automatic movements. Indirect routes through the temporal cortex are used for movements requiring conscious control  2. Instructions travel from the prefrontal cortex, which generates plans for movements, to the premotor cortex to the primary motor cortex  3. The premotor cortex contain a repertoire of movements—a lexicon—that allows it to recognize the movement of others and select similar or different actions  4. The lexicon of the primary motor cortex consists of movements that are somewhat more elementary than those of the premotor cortex   Identifying the Motor Cortex with the Use of Electrical Stimulation o In the 1950s, Wilder Penfield used brief pulses of electrical stimulation to map the cortices of conscious human patients who were about to undergo neurosurgery. He and his colleagues found that most of the movements induced by their experiments were triggered by stimulation of the precentral gyrus o Primary motor cortex or M1-- precentral gyrus (Brodmann’s area 4) o Supplementary motor cortex-- the dorsal part of the premotor cortex o Relatively larger areas of the brain control the parts of the body that make the most- complex and finely tuned movements  Multiple Representations in the Motor Cortex o There may be as many as 10 different homunculi within the motor cortex and premotor cortex o Researchers now propose that the motor cortex is organized not for the control of individual muscles but rather for the control of movements, any of which might require the coordinated action of many muscles in different combinations o Different homunculi represent different classes of movement o Graziano proposes that the motor cortex represents three types of organization:  The part of the body that is to be moved  The spatial location to which the movement is directed  The function o Grazinio’s results support the view that humans have a lexicon, or repertoire, of movement categories in the motor cortex o The pincer grip and other skilled movements are not entirely learned but are part of a prewired movement lexicon in the motor cortex o The premotor cortex plays a greater role in organizing whole-body movements than the motor cortex, which controls specific acts o Part of the role of the neocortex in movement must thus be to blend motor reflexes and skilled actions together  Movement Coding by Neurons in the Neocortex o The debate over how the motor cortex contributes to movement is of long standing, with one extreme position suggesting that the motor cortex controls individual muscles and the other extreme suggesting that the motor cortex controls coordinated actions o Motor-Cortex Cells Specify Movements and Their Force and Direction o Motor-cortex neurons increase the force of a movement by increasing their rate of firing o The primary motor cortex appears to specify the movement to be made by the hand as well as the target to which the hand is directed, whereas premotor-cortex neurons appear to make a more abstract contribution and are especially concerned with the objective of the movement o Practice, rather than dreaming, makes perfect, but that dreaming helps o o Mirror neurons do not respond to objects or to isolated hand movements, and they do not respond very well to pictures or video of movements; Mirror neurons encode a complete action o In humans, the mirror neurons are found largely in the left hemisphere  Roles of the Prefrontal and Posterior Cortex o Sensory information may instruct movements in two ways:  1. Direct connections from the parietal cortex to the primary motor cortex suggest that movements can be made in direct response to sensory stimulation. Such movements are likely to be simple and reflexive.  2.The various
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