These curious maps of the body in the brain are shaped like disproportionate humans.
In the neuroscience field, the cortical or Penfield homunculi are very famous , humanized representations of the distribution of nerves and brain structures that are related to motor and sensory functions. Separate homunculi have been created for these two aspects as the brain topography varies between the two.
These beings have an aspect similar to that of the people, although their members are little proportionate; Such irregularities are very useful to conceptualize the differential innervation of the parts of the body, the key aspect in the morphology of homunculi.
What is the Penfield homunculus?
Between 1937 and 1954, the American neurosurgeon Wilder Penfield and his collaborators developed various representations of a striking aspect of brain topography: the presence of “maps” of nerve pathways, both sensory and motor, in the cortex.
The different functions of our body are not represented proportionally on this map, but 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 has been used frequently throughout of history to designate artificial human beings, especially in the context of works of fiction.
Given that there are topographical representations of the brain that are differentiated between motor and sensory functions, we can actually find two homunculi with distinctive characteristics that are worth detailing.
What is its shape?
The Penfield homunculus was described as grotesque by its own author because of the irregularity of its morphology: while the 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 huge hands and the arms, fragile and thin, 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 the 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 somesthetic 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. In fact, Penfield was the first to describe this part of the brain, which corresponds to areas 1, 2 and 3 of the Brodmann model.
In this section of the cortex the representation of the body diagram is inverted : the toes are in the upper part of the lobe, while the mouth is located in the lower part. 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 looks somewhat less disproportionate than the engine. However, the face and hands are very large compared to the rest of the body because these regions are endowed 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 sensory information projections 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 processes information about proprioception, that is, the sensations that the body detects about the relative position of the muscles. This sense is essential 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 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 aspect 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 sensory nerves in much of the body.
Since the synaptic connections, which form the basis of the nervous system, change during life as a function of experience and practice, the motor homunculus changes in the same person as time passes and differs more than the sensory in the interindividual plane.