The Primary Motor Cortex essentially controls the muscles of the body. When the region of the Primary Motor Cortex corresponding to the right bicep muscle fires, then a neural pathway fires that ends up with the alpha motor neuron of the right bicep muscle contracting (bear in mind there are literally tens of thousands of neurons dedicated to controlling this one muscle). Also - note that the left Primary Motor Cortex controls the right hand side of the body, and the right Primary Motor Cortex controls the left hand side of the body. Finally, note the image below. The number of neurons in the Primary Motor Cortex dedicated to a given body region corresponds to the amount of fine control needed. So, the face requires a lot of fine motor control - think of the array of facial expressions needed - whereas the hip does not to the same extent. So, fine motor control means more neurons means a larger portion of the Primary Motor Cortex dedicated to control.
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