The Homunculus

There are a couple of “risks” associated with spending a lot of time learning about the human body: 1) Student syndrome: aka the combo of neck pain, back pain and headaches that stem from sitting with poor posture for too many hours at a desk. 2) Med school syndrome where you can convince yourself very easily that you or someone you know must have this terrible disease you’ve just learned about and all of the doctors have been missing it all along! Fortunately there are also perks to offset these risks, the number one being that every so often, you get to learn a cool new word to pull out at the next party. One of my first “cool words” was “homunculus”. Rumour had it that it could even be tailored to other species; Dog-unculus for dogs and maybe even paltipunculus for the platypus -which of course made this word even better. Though I’m not sure the extrapolation of the word to other species exists, the concept certainly does! Beyond just the word, the whole idea of the homunculus is pretty cool (if you happen to have a bit of a geek streak in you), so if it isn’t in your regular vocabulary yet, let me share with you what it is.

Literally, a homunculus is a translation from latin meaning “little man”.  In anatomical terms, it refers to the “maps” that we have in the somatosensory and somatomotor cortex (regions of our brain). These maps depict the location of and how much space is attributed to each body part. It’s as if we have a tiny person lying down in our brain with our head at one end and feet at the other- except the body proportions of the homunculus are way out of whack! In the homunculus, the size of the body parts are depicted based on how much “brain space” is allotted to them. Depending on the importance of our motor and sensory accuracy, different areas of our body are assigned different amounts of space. When depicted in this manner, we might look a lot like aliens, but anatomically it’s quite a brilliant design.  

Our sensory and motor homunculus look fairly similar but do have subtle differences upon close examination. In the sensory homunculus, we’ve got really big hands, huge lips and a fairly big tongue. Our arms and legs are pretty skinny and our torso is quite small. Our ears are pretty big, eyes bulged out and our feet a bit long - especially compared to our tiny legs. The motor homunculus is similar, but our hands are even more gigantic and our tongue is a bit bigger, lips a bit smaller - but still pretty huge compared to the rest of our body. This design makes good sense when we look at the volume of information being processed from (sensory) or sent out to (motor) the different regions of our body. We simply don’t need as much detailed information coming from and going to our back compared to our hands.

We all experience and are acutely aware of the phenomenon of the homunculus every day. Think of what it feels like to have a bump on the inside of your lip. You’d think when you pull back your lip to inspect it in the mirror that the thing will be giant, but then you have to lean in extra close…eventually identifying the tiny speck that’s been causing you havoc all day! The reason it feels so huge is that we’ve got a lot of brain space designated to receiving sensory information from that area of the body. It might take a bump 10x that size for us to even notice on our legs. In order to coordinate eating and speaking, we need a lot of motor and sensory brain space for our mouth. Our hands also require a lot to be able to manipulate objects and give us feedback about our environment. If someone asks you to “feel” something, you instinctively do that with your hands, or perhaps if it’s extra soft you rub it on the side of your face; you don’t lift up your shirt and “feel it” with your back first. We can also appreciate the homunculus by watching babies. Babies beginning to explore the world gather much more information about an object by putting it in their mouth - instinctively taking advantage of their brain’s superior ability to process the information coming from that region of the body. 

Here’s a little experiment you can do - you’ll just need someone that you can hold hostage for a minute and a couple of small, somewhat pointy objects like two sharp pencils or opened up paper clips. Have your buddy close their eyes so they don’t cheat. Hold the pointy objects a few centimetres apart and then at the same time gently poke them into the skin on the back of one of their hands. Repeat this a few times, slowly moving the objects closer together with each repeated “poke”. Have the person tell you when they are no longer able to distinguish that two separate objects are poking them and only feel one. This is called our two-point discrimination. On the hands and face it’s very sensitive and you can get the two points within a millimetre or two sometimes before the person will only feel them as “one”. Now try that same routine on their back or thigh. You’ll see very quickly that we lose the ability to distinguish the two pokes as separate much quicker - sometimes the pokes are still a couple centimetres apart when we sense only one thing is touching us. In general, the areas that are more “sensitive” have more brain space attributed to them and are the larger areas on the homunculus. The areas that aren’t so sensitive are smaller. 

It’s the same concept for the motor homunculus, except we are considering the space in the brain that will send messages to the muscles of the various regions of the body. Yes, we walk, run and jump and use the muscles of our legs and trunk to keep us upright, but that is very minor detail compared to the dexterity we have in our hands and fingers and the amount of muscle coordination it takes to be able to track smoothly with our eyes. So it makes sense our little motor homunculus map has giant hands and bulging eyes! 

Finally, although the homunculus represents a general map, our brains are all a little bit different and can adapt to meet our functional needs. Plasticity is our brain’s ability to change and adapt in response to stimuli - or lack there of. Someone who does not have hands but instead uses their feet for tasks like typing, dressing, cooking, playing music etc, is going to develop a much larger area of the motor and sensory regions of the brain devoted to the feet - and so if we made a homunculus map of their brain it would look a bit different than one of a concert pianist (who plays with their fingers) and practices 40 hours a week. 

By now I’m sure you’ve “google imaged” your new word, so my apologies if you get nightmares about how you look according to your brain. But hey, now you have a cool new word for your next party, and a new appreciation for why that little nick on your thumb or your lip feels so darn big - even though you can barely see it ;)