This bite-sized lesson is perfect for introducing your psychology or brain science students to how the sensory motor pathways are mapped out in the brain. In other words, the homunculus.
Ever notice how when the pad of your finger gets hurt, it really, really hurts? What if you bite your tongue? Pretty unpleasant. Have you ever noticed a scratch, cut, or other boo-boo on your leg, back, arm, or somewhere else, and thought “How did I do that”? Why is one so obvious when it happens, but one might not even be noticed? Ask your homunculus.
The word “homunculus” (ho-mun-cue-lus) is Latin for “Little Man”, and this little guy lives in your brain. Actually, there are TWO of them up in there. Let me explain. We already know that different parts of the brain control different things (known as brain localization), but we also have two parallel “maps” of where those functions are controlled. One is for sensory (what we can detect with our senses) and the other for motor (the one that controls all our movements). If we were to draw a map on the brain of where each control area is located, the complete picture would be called the homunculus.
Studies in the 20th century helped clarify which areas of the brain do what, and researchers have figured out that not all body functions are treated equally by the brain.
Let’s look at the guy pictured above. What parts have really big representations? The hands, face, tongue, etc. Injure one of those areas, and WOW, it will hurt. Why? These body parts have abilities and functions that are really complex, and need to be much more sensitive than say, the arm, leg, or back. While those areas are important, they don’t need to be as sensitive as a hand or finger, which need to make fine movements and need to know more details of what they’re coming in contact with. If one of those high-sensitivity areas is damaged, it’s going to impact your life more than the skin on your leg, for example.
If we were to count the sensory receptors (the itty bitty ends of nerve cells that detect touch, etc.) in our hand, they would be packed closely together, all leading to the same area of the brain, so more space is needed to accommodate them all. On the other hand, if we were to count the sensory receptors on the skin on our leg, they would be spaced farther apart, resulting in fewer of them overall. Thus, the more receptors, the more space is needed in the brain to process all that information.
If you remember from a moment ago I told you there were two of these little guys (one for sensory, and one for motor), then it probably goes without saying that we have a similar layout for the motor neurons (nerve cells that send messages from the brain to the movable body parts). The movement (motor) homunculus represents the space devoted to sending information from the brain to the different body parts, with nerves more or less densely packed together, depending on function.
And you thought those hands were big on the sensory side! Not only does our brain need to devote lots of space to the hands in our sensory areas, but we need to make such fine and detailed movements with our hands and fingers, it needs even more space for that. Notice the feet are a little bigger on the sensory side, though. We need our feet to feel things to keep us safe, balanced, etc but not a whole lot of fine movement happens in them. The rest of the body is the same, with more important functions being bigger.
Now, it’s not like we’d see a little guy or two looking up at us if we cut into the head, but if we were to deconstruct each of these models and slap the body parts represented on the place in the brain where it’s controlled, it would look something like this:
This is a little harder to pinpoint what you’re seeing, hence the models.
So now you know, having little people in your head is perfectly normal after all.
For more brain science articles, check out:
A Brain with a Great Big Hole in it
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