On the homunculus

The two cortical projections of the human brain. Click to enlarge. (source)

It occurred to me that perhaps I should explain the link between cortical projections and homunculi that I referenced at the very end of my last post. There are two cortical projections in the brain, one for sensation, one for the motor cortex. They are, essentially, just "pictures" of your body that are made in your brain based on sensory input and motor (voluntary muscle) output. The idea is that all sensory information coming in to the brain (via afferent nerves) heads to a specific area depending on where the information originated. Places with large amounts of sensory input (like the face) will have disproportionately large amounts of real estate in this cortical projection. Conversely, in the motor cortex, all outgoing information (via efferent somatic nerves) on voluntary muscle control will originate from areas of the brain based on where the message is going. Places with lots of fine motor control (like the hands) will have disproportionately large areas of real estate in that cortical projection.

The result is a very comical "mental" image of the body, and this mental picture is referred to as the homunculus (literally "tiny man").

An artist's rendition of the two homunculi. Click to enlarge. (source)