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The second route that motor commands from the cortex can take out towards the muscles is called the multineuronal pathway. The initial cell bodies of the pathway are also in the motor cortex, but they immediately synapse to long chains of internuncial neurons descending from the motor cortex to other cortical areas, to the lower brain, and on down the cord. The final links in these chains end not directly upon the neurons of the motor units, but rather upon the spinal internuncial circuits which organize the patterns of stereotyped spinal reflexes. Thus instead of bypassing the intermediate net, this pathway channels motor commands all the way through it. Impulses take longer to travel from the cortex to the motor unit, since they pass through many more synapses and are influenced by input from many other sources along their way. This arrangement makes possible the second kind of motor control—the setting into motion of the stereotyped reflex units of movement that are organized in the subconscious levels of the lower brain and spinal cord. For instance, the individual circuits and the overall sequential arrangements which control walking or swallowing are in the cord; the multineuronal pathway has only to stimulate these sequences into activity, and then steer their general course without the cortex having to pay attention to the details involved in each separate step. All coordinated movements require the interaction of both of these pathways. The conscious mind is not competent to direct every single muscular adjustment involved in general movements, and the reflexes are powerless to arrange themselves into new, finely controlled sequences. Successfully integrated, the two of these paths together provide the best aspects of millions of years of repetitive practice and of moment to moment changes in intent or shifts in the surrounding environment. One is basic vocabulary, the other is variable sentence structure; the language of coordinated movement cannot go forward without both working together.
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