The brain's motor map

There is a strip in the brain that organizes voluntary movement. It is called the primary motor cortex, and it sits in the precentral gyrus, a fold of the frontal lobe. The striking part is that the body is drawn there: there is a map, and that map is called the motor homunculus, which means little man.

The map follows an order. The feet and legs are on the inner face of the fold, the one facing the other hemisphere. Moving up toward the crest come the thighs, the trunk and the shoulder. And on the outer face, the one facing outward, are the hand and the face.

Now the interesting part: the map is distorted on purpose. The space each body part takes up does not depend on how big that part is, but on how much innervation it has, that is, on how many nerves control it.

That is why the fingers and the lower part of the face, which make fine, precise movements, take up the most cortical surface, while the buttocks and the back, which are huge but make coarse movements, take up far less. Much of this was described by a neurosurgeon, Penfield, who electrically stimulated the surface of the brain.