Penfield’s Sensory And Motor Homunculi: What Are They?
In the field of neurosciences they are very famous cortical or Penfield homunculi, humanized representations of the distribution of nerves and brain structures that are related to motor and sensory functions. Different homunculi have been created for these two aspects since the brain topography varies between the two.
These beings have an appearance similar to that of people, although their limbs are poorly proportioned; Such irregularities are very useful for conceptualizing the differential innervation of body parts, the key aspect in the morphology of homunculi.
Between 1937 and 1954 American neurosurgeon Wilder Penfield and their collaborators developed various representations of a striking aspect of brain topography: the presence of “maps” of neural pathways, both sensory and motor, in the cortex.
The different functions of our body are not represented proportionally on this map, but rather their size depends on the complexity of the corresponding nerves. However, the location of these brain areas does present notable parallels with the external structure of the body
This led Penfield to be inspired by the relative weight of each function in the cerebral cortex to create symbolic images of a “homunculus”, a term from Latin that translates as “little man” and that has been used frequently throughout the world. of history to designate artificial human beings, especially in the context of works of fiction.
Given that there are differentiated brain topographic representations between motor and sensory functions, we can actually find two homunculi with distinctive characteristics that is worth detailing.
What is its shape due to?
Penfield’s homunculus was described as grotesque by its own author because of the irregularity of its morphology: while hands, mouth, eyes, and ears are disproportionately large Compared to the human body, the rest of the homunculus has a weak appearance.
The comparison between the enormous hands and the fragile and thin arms is particularly striking. These characteristics are even more marked in the case of the motor homunculus than in the sensory one because the functions related to movement are less distributed than the sensory ones.
The cause of the peculiar appearance of homunculi is the differences in the innervation of different parts of the body: the more intense and complex the connection between one of them and the brain, the larger the size of the corresponding section in the cerebral cortex.
The sensory homunculus and the somesthetic cortex
The sensory homunculus represents the someesthetic or primary sensory cortex, which is located in the postcentral gyrus, a cerebral gyrus located in the region of the parietal lobe attached to the frontal one. In fact, Penfield was the first to describe this part of the brain, which corresponds to areas 1, 2 and 3 of Brodmann’s model.
In this section of the cortex the representation of the body schema is inverted: The toes are at the top of the lobe, while the mouth is located at the bottom. Likewise, the “topographic map” of each hemisphere of the body is in the opposite half of the brain. The same happens in the case of the motor homunculus.
This homunculus has a slightly less disproportionate appearance than the engine. However, the face and hands are very large compared to the rest of the body because These regions are equipped with many skin receptors ; The density of these cells in a part of the body determines the size of its cortical representation.
The somesthetic cortex receives most of the projections of sensory information that reach the brain through the thalamus, a structure that acts as a connection point between the cortex and other more peripheral regions.
This part of the cerebral cortex is not only concerned with stimulation from the external world, but also also processes information about proprioception, that is, the sensations that the body detects about the relative position of the muscles. This sense is fundamental for movement, posture or balance, among other functions.
The motor homunculus and the primary motor cortex
The cortical representation of the motor nerves and the corresponding cutaneous receptors It is located in the primary motor cortex, in the central sulcus, a region of the frontal lobe that is located just next to the somesthetic cortex; therefore, the two cortical homunculi are very close to each other.
The primary motor cortex is the most important area of the brain for the functioning of the motor system: it receives input from the thalamus and works together with the rest of the regions associated with movement, such as the supplementary motor cortex, to develop and execute motor schemes.
The appearance of the motor homunculus is even more grotesque than that of the sensory one: its mouth, its eyes and especially its hands are enormous compared to the trunk, arms or legs. This is due to the greater specificity in the location of receptors and motor nerves much less numerous than the sensory ones in much of the body.
Since the synaptic connections, which constitute the basis of the nervous system, are modified during life depending on experience and practice, the motor homunculus changes in the same person over time and differs more than the sensory one in the interindividual level.
