motor homunculus

The motor homunculus is a kind of body map painted across a strip of your brain. There is a band of tissue near the top of each half of the brain called the primary motor cortex, and it is the launch pad for voluntary movement — when you decide to wave or wiggle a toe, the command starts there. The remarkable thing is that this strip is laid out like a tiny picture of your whole body: one spot controls the foot, the next the leg, then the trunk, hand, face, lips, and tongue, in an orderly sequence. Scientists call this kind of orderly body-to-brain layout somatotopic, meaning each body part has its own matching place on the map. If you drew a little person to show which patch of cortex moves which body part, you would get the homunculus — Latin for little man.

But this little person looks wildly out of proportion, like a funhouse-mirror cartoon: it has enormous hands, gigantic lips and tongue, and a huge face, while the trunk, arms, and legs are shrunken and thin. The reason is that brain space is handed out by how much fine control a body part needs, not by its physical size. Your fingers, lips, and tongue perform delicate, precise movements — gripping a pencil, forming words, shaping speech — so they get a large slice of cortex with many neurons to steer them. Your back or thigh, by contrast, makes only coarse, simple movements, so a small patch is enough. The map is also crossed and roughly upside-down: the left motor cortex moves the right side of the body and vice versa, and the toes sit up near the top of the brain while the face sits lower down. Doctors and neuroscientists use this map to understand why a stroke or injury in one tiny spot can weaken just a hand or just one side of the mouth.