Penfield’s homunculus: characteristics and functions

Our brain is extraordinary. We have been studying it for years and we have not yet discovered all its possibilities. It is like the universe, infinite and full of surprises. Perhaps that is why when new functions or brain areas are discovered, we try to simplify the discovery. That is what happened with the well-known Penfield Homunculus.

The Penfield Homunculus was first described by Doctor Wilder Penfield between the 1940s and 1950s. This Canadian neurosurgeon sought to explain and cure neurological diseases such as epilepsy. Thus, one of his best-known works was undoubtedly that of neurostimulation.

By applying small and controlled discharges, something very interesting was discovered. In our brain there is a small area that makes up the sensory map of our body. This structure reflects the sensitivity of each of the parts of our anatomy. He decided to represent this area as if it were a human form, giving rise to the Penfield Homunculus.

What makes this representation special is being aware that we have areas in our body that are more sensitive to stimulation than others. Thus giving rise to a deformed, disproportionate man, where the most sensitive areas show greater size than those that are less so.

Now this is not all, Shortly after, the existence of a new area was discovered, which represents the motor map of our body. In this way, we could say that each of us has two “homunculi”, one sensory and the other motor, both very different but with points in common.

Characteristics and functions of Penfield’s Homunculus

Studies such as those carried out by neurologists Di Noto P, Newman L, Wall S, Einstein in 2013, update in depth those first bases on these structures that were established between 1937 and 1954 thanks to the neurosurgeon Wilder Penfield.

It should be noted, first of all, that we should not expect to see two “human” figures embedded in our brain.. Dr. Penfield outlined this similarity by seeing that each sensory area was correlative to each part of our body. For example, in these structures parts such as the hand and each of our fingers are placed next to each other. Let’s see it in detail.

Motor homunculus or primary motor cortex:

The motor homunculus or primary motor cortex is located right next to the sensory homunculus. It is located exactly in the central sulcus of the frontal cortex. This area is the most important for the motor functioning of our body.

Its function is to regulate and control the motor movements of our body. It does so in collaboration with other areas, such as the supplementary motor cortex and the afferents received from the Thalamus. That is why its appearance is slightly different from that of the sensory Homunculus: its mouth, its eyes and especially its hands are enormous due to the greater specificity in the location of receptors and motor nerves.

A curiosity about this area is the following: it develops differently in each of us. This implies that the speed of your development is unique and personal. It depends on which parts of the body are used the most and how you will have better or more trained motor skills in general.

For its part, it is pertinent to mention that This Homunculus is normally presented as a 2D canonical diagram, which limits and oversimplifies Penfield’s original findings. collected from patients undergoing surgery for epilepsy.

This simplification is a limitation that allows us to erroneously assume that lesions in the motor cortex will cause certain deficits in specific muscles. But, in reality this does not happen like that. Well, lesions in the motor cortex produce deficits in synergistic muscle groups.

Emphasizing this distinction is important, since allows us to understand the true functioning of the motor cortex, which is much more complex than the 2D representation of the homunculus.

Sensory homunculus or primary somesthetic cortex:

The sensory homunculus represents the primary somesthetic cortex or what is the same, the tactile, pressure or pain sensitivity of our body. It is located in the parietal lobe, right at its junction with the frontal lobe. Explained another way, the sensory homunculus comprises Broadman’s areas 1, 2, and 3.

In this area our body schema is represented in a contralateral way, or what is the same, in a laterally inverted way.This means that the right representation of our body is represented in the left area of ​​this brain area and the left in the right part. Although it may surprise us, it is something very common in the functioning of our brain.

It should be noted that this sensory area receives most of the information projections from our body through the Thalamus.

Let us remember, the thalamus is the area of ​​integration of the different sensory sources of our brain. Thanks to it, we perceive our world in an integrated way and not separately according to the sense that perceives it.

The sensory Homunculus is also responsible for our proprioception. Thanks to him, we regulate the posture and our body knows the state of our organs and our muscles. And although it may seem strange to us, how we are from inside.

All this makes this area vital for our well-being, both physical and emotional. In fact, thanks to this structure we have that special sensitivity in our face, our lips…

The phantom limb, the main disease of the Penfield Homunculus

Penfield’s Homunculus, as we already know, collects and integrates our entire body representation, whether in a sensory or motor way. Thus, it is interesting to know that an alteration in this area can result in a curious disease: phantom limb.

When you suffer from this disease, The brain continues to feel or perceive the sensations of an amputated body limb.This condition has been detailed by doctors, lone Nikolajsen and Kristian Friesgaard Christensen in an interesting study from 2015. As explained in the work, even though that area of ​​the body no longer exists, that sensory area that represents said part, continues to send the sensation of pain from our brain.

This means that even if the limb is amputated, due to the activity of the sensory homunculus neurons, we cannot stop feeling it. However, It should be said that as this study explains to us, this discomfort usually disappears after two years.

As we see, a discovery driven by curiosity through electrical brain stimulation has opened up a universe of possibilities. Thanks to him, we have realized the importance of each touch on our skin and our brain and emotional development.

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