Penfield’s sensory and motor homunculi: what are they?

In the field of neuroscience, they are very well known cortical or Penfield homunculi, Humanized representations of the distribution of nerves and brain structures related to motor and sensory functions. Different homunculi have been created for these two aspects because the topography of the brain varies between the two.

These beings resemble humans, although their limbs are disproportionate; such irregularities are very useful in conceptualizing the differential innervation of body parts, a key aspect in homúnculos morphology.

    What is Penfield’s homunculus?

    Between 1937 and 1954 American neurosurgeon Wilder Penfield and his collaborators have developed several representations of a striking aspect of cerebral topography: the presence of “maps” of the 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 presents itself notable parallels with the external structure of the body.

    This led Penfield to take inspiration from the relative weight of each function in the cerebral cortex to create symbolic images of a “homunculus,” a term derived from Latin which translates to “little man” and which has been used frequently in throughout history to designate artificial human beings, especially in the context of works of fiction.

    Since there are cerebral topographic representations differentiated between motor and sensory functions, we can in fact find 2 homunculi with distinctive features which is worth to be detailed.

    What is the reason for its shape?

    Penfield’s homunculus has been described as grotesque by its own author due to the irregularity of its morphology: while hands, mouth, eyes and ears are disproportionately large Compared with 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 functions.

    The cause of the particular aspect of homúnculos are the differences in innervation of different parts of the body: The more intense and complex the connection between one of them and the brain, the larger the section size corresponding to the cerebral cortex.

      The sensory homunculus and the somesthetic cortex

      The sensory homunculus represents the primary somatic or 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 crust the representation of the body diagram is reversed: 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 found in the opposite half of the brain. The same thing happens in the case of the motor homunculus.

      This homunculus seems a little less disproportionate than the motor. However, the face and hands are very large compared to the rest of the body because of it these regions have many skin receptors; the density of these cells in a part of the body determines the size of their cortical representation.

      The somatic 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 stimulating the outside world, but it also processes information on proprioceptionIn other words, 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.

      Motor homunculus and primary motor cortex

      Cortical representation of motor nerves and corresponding skin receptors it is located in the primary motor cortex, in the central groove, A region of the frontal lobe that lies just on the side of the somatic 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 afferents from the thalamus and works with other regions associated with movement, such as the supplementary motor cortex, to execute motor patterns.

      The appearance of the motor homunculus is even more grotesque than that of the sensory: its mouth, its eyes and especially its hands are enormous compared to the trunk, arms or legs. This is due to greater specificity in the location of receptors and motor nerves, Much less numerous than the sensory ones in much of the body.

      Since synaptic connections, which form the basis of the nervous system, change over the course of life based on experience and practice, the motor homunculus changes in the same person over time and differs more than the “sensory”. in the interindividual plane ”.

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