Video: Motor and sensory cortical homunculus

Hey there! Did these two startle you? They might look like crazy monsters, but trust me, they won’t bite, because these two are actually your motor and sensory homunculi. They certainly don’t look like you and me, but that’s because their body parts are sized rather in relative proportion to the level of motor or sensory ability that they’re capable of – meaning their body parts have grown in proportion depending on how much they’re able to move or sense with them.

Believe it or not, but these two are actually a representation of something in your own brain. Your motor and sensory cerebral cortices to be precise – that is, the part of your brain that’s telling your hands to wipe your runny nose or another part that is telling you that your socks are too tight. It seems a little strange, but if you stick with me, I’ll tell you everything as we explore the motor and sensory cortical homunculus.

As we work our way through this topic, we’ll follow a structure that looks something a little bit like this. So, we’ll begin with a quick overview of the brain, reminding ourselves of its main gross or macroscopic features. We’ll then look at two of those features in particular – the precentral gyrus and the postcentral gyrus. And on these gyri is where we’ll find the motor cortical homunculus and the sensory cortical homunculus. We’ll identify the different parts of these homunculi and the pattern that they follow. And then to wrap up, we’ll visit a clinical scenario to see how knowing all of this information could help in that setting.

So, let’s get into it.

First, let’s start with the brain as a whole. So over here, we’re looking at a brain from a lateral view, and in this image, we can see three main parts of the brain. The largest part is here, and this is called the cerebrum; this structure over here is called the cerebellum, and the final part of the brain which is partially covered in this view is the brainstem.

The cerebrum is the region we’ll be focusing on today, and this is where we’ll find both the motor and sensory cortical homunculi. From this external view of the brain, we’re specifically looking at the cortex of the cerebrum called the cerebral cortex. The cortex is the most superficial layer of the cerebrum, and it makes up the largest portion of the brain’s gray matter. Some of the most obvious features of the cerebrum are the ridges and grooves that we can see all over it, and collectively the ridges are called gyri whereas one is called a gyrus – and we can see several of them making up the cerebrum.

Between the gyri are sulci which are relatively shallow grooves, a few of which are being pointed out right now. Fissures are the larger grooves that generally divides the cerebral cortex into lobes.

So what part of the cerebral cortex is important for us for this tutorial? We’ll, we’re interested in two very specific gyri, and these gyri lie on either side of this prominent fissure called the central sulcus. The central sulcus separates the frontal lobe from the parietal lobe. So one gyrus that we’re interested in is in part of the frontal lobe while the other is part of the parietal lobe.

The gyrus anterior to the central sulcus and part of the frontal lobe is called the precentral gyrus. And the precentral gyrus is also known as the primary motor cortex, and this area of the cortex is responsible for the initiation of voluntary movement. The gyrus posterior to the central sulcus and part of the parietal lobe is called the postcentral gyrus, and the postcentral gyrus is known as the primary sensory cortex. This area of the cortex is responsible for processing sensory information from the body such as the things that I mentioned before – for example, feeling warm or cool, sensing something touching your skin or feeling an itch.

For both the precentral gyrus and the postcentral gyrus, we can overlay a representational pattern or map called a homunculus, and this map shows us exactly which part of the gyrus is responsible for either the sensory or motor information from which part of the body.

So the word homunculus is actually Latin for “little man”, but no, we’re not suggesting that you have two little freaky men living in your brain controlling how you sense things and move your body. These two are just representations of how our motor and sensory cortices are organized and to help give us an idea of how much motor and sensory cortex is dedicated to each area of our body.

Let’s take this one step further, firstly by looking at the motor cortical homunculus in a little bit more detail.

So if we transpose our motor homunculus friend onto your motor cortex, he would look something like this. On our view of the whole brain, we can see the precentral gyrus highlighted in green which is the primary motor cortex. On the zoomed-in part of the image, the motor cortical homunculus has been laid on top of the primary motor cortex to show us which part of the cortex is responsible for initiating voluntary movement and in which part of the body. And as we mentioned earlier, the parts of the body are not scaled based on size. For example, if you’re looking at the thumb, its huge compared to the knee.

The reason for this is that the amount or area of cortex representing part of the body is proportional instead to the intricacy of motor function in that region. So the hand and the lips, for example, have a greater area of cortex assigned to them than, say, the elbow or your butt, because we tend to pick up things with our hands and not our elbows, and kiss things with our lips and not our – oh, you get the point.

Moving swiftly along, let’s have a look at this motor cortical homunculus in a bit more detail.

So most medially on the precentral gyrus is the area responsible for our motor innervation of the genitals, and this is a relatively small area as there are a few muscles in the genital region that are under voluntary motor innervation.

Moving laterally along the precentral gyrus, we find the area of the gyrus responsible for innervating the lower limb. Specifically, we’ll come across the motor cortex of the toes first, and as we continue to move laterally, we’ll find that we move superiorly in terms of body parts that the cortex is responsible for – and we see that here now as we hit the motor cortex of the ankle next.

Continuing along, we come to the motor cortex of the knee, and this area is just as the gyrus is coming out onto the superior aspect of the brain. Further along is the motor cortex of the hip, and this is where the cell bodies of neurons that communicate with the muscles in the area of the hip reside.

Now, we’ve come across the motor cortex of the trunk. so although the trunk makes up a large part of a person, the motor cortex designated for it is relatively small, and we have a lot of muscles in the area of the trunk but their movements are not very intricate compared to, say, the hand or the face.

As we continue laterally, we’ll now hit the areas responsible for movement of the upper limb. First is the motor cortex of the shoulder – again, a relatively small area – as movements here are not very intricate. Moving along, we can see the motor cortex for the elbow. And now, we’ve made it to the wrist. Notice how the area here is larger than the motor cortex area of the elbow and if you want, try it out. You can move your wrist in a lot more intricate ways than you can your elbow.

The last part of the upper limb is the hand and fingers. So, because of the intricate and highly dexterous type of movements the hand and fingers are capable of, there is a relatively large area of motor cortex dedicated to it, and we can even see the area for each finger separate from the others. We first come across the motor cortex of the little finger, then motor cortex of the fourth finger, the middle finger, the index finger, and finally, the motor cortex of the thumb. The motor cortex of the thumb has the largest area of motor cortex dedicated to it compared to the other digits of the hand.

So now that we’ve reached the motor cortex of the face, and similar to the hand and the fingers, we’re able to move the muscles on our face in quite intricate ways – for example, when pulling our face to express ourselves. And you know what that means? There’s a relatively large area of motor cortex responsible for movements of the muscles of the face and the eyes.

And again, similar to the hands and the fingers, we can see the motor cortex split into more specific areas. First is the motor cortex of the forehead. Next, we’ll see the motor cortex of the brow, and moving along, we’ll travel inferiorly down the face and across the motor cortex of the eye then the nose, then the lips. Of all the parts of the face, the lips have the largest amount of motor cortex dedicated to moving them because we move them a lot for talking, eating and even kissing your beloved goodbye! And, finally, the motor cortex of the chin.

There are only two structures left in our motor cortical homunculus, and the first of these is the tongue, which also has a relatively large area of motor cortex. Lastly, we have the pharynx, which is the most lateral part of the motor cortical homunculus and where our signals designated for the pharyngeal muscles used in swallowing originate.

And that brings us to the end of our motor cortical homunculus.

Now that we’ve covered the motor cortical homunculus, let’s move on to the sensory one.

As we look at the regions on this sensory map, try to relate it back to the area of motor cortex in that area of the body as well. So, the sensory cortical homunculus is the map we can see on this image, and on the view of the whole brain, we can see the postcentral gyrus highlighted in green which is the primary sensory cortex, and on the zoomed-in part of the image, the sensory cortical homunculus has been laid on top of the primary sensory cortex to show us which part of the cortex receives sensory information from which part of the body.

And just like with the motor cortical homunculus, you might notice that the parts of the body are not to scale based on size. Take a look at the lips compared to the hips. In the case of the sensory cortical homunculus, this is because the amount or area of cortex representing a part of the body is proportional to the complexity of sensations received from that organ.

So let’s have a look at the sensory cortical homunculus in a little bit more detail.

So most medially on the postcentral gyrus is the area responsible for receiving sensory information from the genitals. And, of course, this is the same place on the precentral gyrus where we found motor innervation for the genitals. The main difference though is the area dedicated to the genitals. As you can see, it’s greater for the sensory innervation than the motor. And it should hopefully make sense as the genitals have a greater complexity of sensation information compared to the intricacy of their motor innervation.

Moving swiftly on laterally along the postcentral gyrus, we find the area of the gyrus responsible for receiving sensory information from the lower limb. Specifically, we’ll come across the sensory cortex of the toes first, and as we continue to move laterally, we’ll find that we move superiorly in terms of body parts that the cortex is responsible for. And so far, this is the same pattern that we saw on the motor cortical homunculus.

Okay, so, proximal to the toes is the sensory cortex of the foot. So continuing along, we come to the sensory cortex of the leg and the thigh, and this area is just as the gyrus is coming out onto the superior aspect of the brain which is approximately where the motor cortex of the knee is on the motor cortical homunculus. And do you notice how much of the lower limb sensory information is processing this small portion of the brain? That’s because we don’t have very complex sensations coming from our lower limb.

Moving further along the sensory homunculus is the sensory cortex of the hip, and this is where cell bodies of neurons that receive sensory information from the hip area reside.

Now, we’ve come across the sensory cortex of the trunk, and although the trunk makes up a large part of a person, the sensory cortex designated for it is relatively small just like the motor cortex for the hip. Remember, it’s about complexity of sensation which we don’t have very much of in this area. We’re now coming across the sensory cortex of the neck, and this is a region that wasn’t isolated separately in the motor cortical homunculus, but in the sensory one, it definitely is.

Another part of the body not well represented on the motor cortical homunculus is the area we see highlighted right now and this is the sensory cortex of the head. We did see the motor cortex of the face and we’ll see the sensory cortex of the face later on, but in terms of the other parts of the head, this is where the sensory information is processed along the postcentral gyrus.

As we continue laterally, we’ll now hit the areas responsible for the sensation of the upper limb, and first is the sensory cortex of the shoulder. It’s a relatively small area as sensations here are not very complex.

Moving along, we can see the sensory cortex for the elbow. And now we’ve made it to the hand. There are actually individual areas designated to the sensation of each finger, so the part that we see highlighted in green now is for the palm, and the dorsum of the hand not including the fingers.

Compared to a lot of the rest of the body that we’ve seen so far, the hand and fingers have a large amount of cortex responsible for processing sensory information, and this is because our hands are very sensitive to touch, so we need a lot of cortex to deal with that.

Continuing from our medial to lateral approach on the postcentral gyrus, we come across the sensory cortex of the fifth digit. Next, we’ll see the sensory cortex of the fourth finger, then the sensory cortex of the middle finger, the index finger, and finally, the sensory cortex of the thumb. The sensory cortex of the thumb has the largest area of sensory cortex dedicated to it compared to the other digits of the hand just like on the motor cortical homunculus.

Now, we’ve reached the sensory cortex of the face and similar to the hands and fingers, our faces can handle quite complex sensations, so you know what that means. There’s a relatively large area of sensory cortex responsible for processing sensations of the face. Again, similar to the hands and fingers, we can see the sensory cortex split into more specific areas. First is the sensory cortex of the eyes. Next, we’ll see the sensory cortex of the nose, then we find the sensory cortex of the upper lip, the lower lip, the part of the face between the lower lip and the chin, and finally, the sensory cortex of the chin.

So, we’re getting close to the end, and there are only a few more structures left on our sensory cortical homunculus. The first is what we can see highlighted in green right now, and this is the sensory cortex of the teeth.

Moving along, we’ve now hit the sensory cortex of the tongue. Similar to its motor component, there’s quite a large area dedicated to a relatively small structure. Now highlighted in green is the sensory cortex of the larynx, and there are also parts of the pharynx and the thyroid gland in this area, so sensory information from those structures will come to this part of the sensory cortex as well.

And now, we’ve reached the final part of the sensory cortical homunculus. This is the sensory cortex of the intestines, and similar to the teeth, we did not see this on the motor cortical homunculus because there are no voluntary muscles that control the intestines. However, it’s important to know the location of where sensory information from the organ gets processed.

So, one of the big questions now that we’ve come to the end of the tutorial is why is it so important to know about all of these. So, of course, these cortices have clinical aspects so we’re going to have a little bit of a chat about some clinical correlations right now.

So because the sensory and motor cortices are so specifically organized, physicians can better understand where a stroke may have occurred based on the symptoms that the patients present with. So in this image, we’re looking at a transverse section of the brain with an artery which is known as the anterior cerebral artery highlighted. And you can see how it’s emerging between the two hemispheres of the brain, and that means that blood from this artery will mostly supply this more medial part of the precentral gyrus. So, therefore, that’s the part that controls movements of the lower limb.

The middle cerebral artery, however, supplies the lateral side of the precentral gyrus, so the part that controls the movements of the face. So if a patient had a stroke and was then unable to move their face but their legs were still functional, it would indicate a stroke in the middle cerebral artery rather than the anterior cerebral artery.

And that brings us to the end of the tutorial. But before I let you go, let’s have a quick review of what we looked at today.

So we began by looking at the three main parts of the brain – the cerebrum, the cerebellum, and the brainstem. We focused on the cerebrum, specifically the cerebral cortex, and identified some microscopic features of the cerebrum including the gyri or the ridges, the sulci or the grooves, and the fissures – the larger grooves between the lobes of the brain.

We then focused on two specific gyri – the precentral gyrus also called the primary motor cortex which is responsible for the initiation of voluntary movement and the postcentral gyrus also called the primary sensory cortex which is responsible for processing sensory information from the body.

We then saw that within each of these gyri is a pattern or a map of the body called a homunculus, and these maps show us exactly which part of the gyrus is responsible for either the sensory or the motor information from which part of the body.

The first homunculus we looked at was the motor cortical homunculus. And remember the amount of cortex designated to a part of the body is proportional to the intricacy of the movements of that region. So from medial to lateral, the homunculus covers the genitals all the way up to the larynx. And note the large amount of area for the hands and each finger compared to the ankle and the toes.

The second homunculus we looked at was the sensory cortical homunculus. And remembered the amount of cortex designated to a part of the body in the sensory cortical homunculus is proportional to the complexity of sensations received from that region. And from medial to lateral, the sensory homunculus covers the genitals all the way to the intestines.

And that brings us to the end of our tutorial, which means that you no longer need to look at these two little guys.

I hope you enjoyed it. Thanks for joining us. Happy studying!