The Motor and Somatosensory Cortex

Run your finger along the top of the brain, over the central sulcus, and you cross two neighboring strips. In front sits the motor cortex, which sends commands out to the body's muscles. Just behind it sits the somatosensory cortex, which receives touch, pressure, temperature, and pain signals coming in from the body. Both are organized the same way: as a map, running roughly from toes near the top of the brain down to the face near the side, with each body part's patch of cortex connected to that specific part.

Here's the twist: the map isn't drawn to scale with the body. The back covers huge skin area but barely registers on the strip, while lips, tongue, and hands get oversized patches. That's because cortical space tracks how much fine control or discrimination a part needs, not how big it is. Hands need to manipulate tiny objects and feel subtle textures; a stretch of back just needs to notice pressure. More cortex devoted to a region means more precise, detailed processing for it.

This map isn't carved in stone. When a finger is amputated, its patch of somatosensory cortex stops receiving input but doesn't go dark forever — neighboring regions, like the patch for the adjacent finger, gradually expand into that unused territory. This is cortical plasticity: unused cortex gets recruited by active neighbors. It also explains phantom limb sensation. Because the cortical map for the missing limb still exists and now gets crossed signals from reorganized neighboring areas, the brain can interpret that activity as if it's still coming from the limb that's gone.