The classical understanding of how the human brain controls voluntary movement, primarily through the primary motor cortex, may not provide a complete picture of the underlying processes. The motor homunculus, a map depicting how this brain region is divided into sections dedicated to each body part capable of voluntary control, might need an update. Recent research has uncovered new areas and organizational differences that might change how we understand movement control.
A New Perspective on the Motor Cortex
In a study published in Nature on April 19, researchers reported the discovery of three new areas within the primary motor cortex that control integrative, whole-body actions. Moreover, the organization of specific body parts on this map differs from what was previously thought.
Although research on monkeys had already hinted at possible discrepancies in the classical homunculus map, this new study provides substantial evidence to support the existence of alternative organizational patterns. The primary motor cortex is now understood to be divided into three distinct sections, each representing different body regions: lower body, torso and arms, and head.
Changing the Map: A More Detailed View of the Primary Motor Cortex
The new and improved motor cortex map reveals that within each of the three sections, the outermost body part of the corresponding region is mapped to the center of that section. In the lower body section, for example, the toes are placed in the middle, with other leg parts, such as the hip, knee, and ankle, radiating outwards from this central point.
This updated organization of the primary motor cortex has been verified across a wide range of people using data from three large functional MRI (fMRI) studies, which included around 50,000 participants. Similar patterns were also observed in existing datasets from macaque monkeys, children, and clinical populations.
The Mystery of Intereffector Regions
In addition to the updated organization of body parts, the researchers also discovered three mysterious spots within the primary motor cortex not associated with specific body parts. Named intereffector regions, these areas connect to an external network involved in action control and pain perception.
The researchers believe that these intereffector regions may play a role in integrating action goals and body movements involving multiple body parts. Meanwhile, the spaces between these regions are likely responsible for the precise movements of isolated body parts.
Conclusion
The new discoveries regarding the primary motor cortex and the organization of body parts within this region challenge the classical view of how the human brain controls voluntary movement. With a more accurate understanding of the motor cortex's organization and the role of intereffector regions, researchers may gain new insights into the complexities of human movement control. This could potentially lead to advancements in the fields of neuroscience, rehabilitation, and movement-related disorders.
E.M. Gordon et al. A somato-cognitive action network alternates with effector regions in motor cortex. Nature. April 19, 2023. doi: 10.1038/s41586-023-05964-2.
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