April 25, 2023
Author: Manish Verma
Editor: Dr. Jitendra Kumar Sinha
Did you know that the nerves in your left leg are connected to the right side of your brain, while the nerves in your right arm are connected to the left side of your brain? In all bilaterally symmetrical animals, including humans and simple worms, the nerves project contralaterally, crossing from one side of the body to the opposite side of the brain. The underlying mechanism behind this phenomenon may be attributed to geometric factors. However, recent advancements in neuroscientific research have shed light on the evolutionary, functional, and anatomical underpinnings of this intriguing phenomenon. In this article, we delve into the multifaceted aspects of crossed connections between the brain and body, exploring their implications on human perception and movement, and unraveling the mysteries that have long captivated our imaginations.
Despite the daily revelations in our understanding of the complex structures of the brain, one of the most conspicuous features of its wiring still perplexes neuroscientists. The nervous system displays a contralateral organization, whereby the left hemisphere of the brain governs the right half of the body and vice versa. This fundamental phenomenon is crucial to the practice of neurological exams and is relied upon by medical practitioners worldwide. It seems like a needlessly complex arrangement – wouldn’t it be easier if everything just stayed on the same side? Interestingly, there’s nothing stopping the right side of the brain from connecting with the right side of the body. In fact, the wiring scheme of our nervous system seems to require a molecular “traffic cop” to direct the nerve fibers across the midline to the opposite side of the body. It’s a fascinating and puzzling phenomenon – why choose the complicated route when the simpler path is right there for the taking?
Contralateral wiring of neural connections is a widespread feature in the animal kingdom, including lowly nematode worms where left-right reversal across the midline is observed. Interestingly, many of the molecules that direct neuronal growth in these worms are conserved in humans. Evolution must have preserved this arrangement for a reason, yet biologists remain uncertain as to its benefits. However, an intriguing explanation has emerged from the realm of mathematics.
Neurons involved in brain-body connections create a virtual map in the cerebral cortex, such that input from one body part corresponds to a specific area of the cortex, known as somatotopy. This body mapping phenomenon extends beyond the physical body to the external world we perceive through our senses, with the 3D environment similarly mapped onto the brain.
The crisscrossed wiring of our nervous system can be explained by the challenge of projecting 3D space onto a 2D surface in the brain, which presents a topological problem. To avoid errors, the most straightforward solution is to direct nerve fibers across the midline, despite the apparent counterintuitiveness of this arrangement. This is because it simplifies the mapping of 3D space onto the 2D surface of the brain, which can result in some curious visual phenomena, such as the sinusoidal path of orbiting satellites.
Troy Shinbrot and colleagues at Rutgers University have postulated that this principle applies to all systems in which a central control mechanism interacts with a 3D environment. In the absence of crossed connections, a geometric singularity would arise that would lead to confusion in the interpretation of left/right and up/down information.
The concept of midline, left and right, is fundamental in certain symmetrical objects and arises from a geometric frame of reference centered on our bilaterally symmetrical body. While radially symmetrical jellyfish do not have left or right, we use this concept to describe their movement. However, this concept can be challenging for humans to grasp, as it requires learning a frame of reference that varies between individuals.
In our world of relative directionality, the orientation of an object can cause confusion between similar shapes such as “d” and “b” or “p” and “q.” These errors arise due to the flipping of the identical shapes along the vertical axis, which swaps left and right, or the horizontal axis, which swaps up and down. Unlike up and down, left and right depend on an object’s frame of reference, and as bilaterally symmetrical creatures, we never mistake the former.
Looking at oneself in a mirror leads to an apparent left-right reversal, but it is actually a front-back transformation. When mapping from different perspectives, especially from 3D onto a bilaterally symmetrical plane, topological problems arise. The brain and body are three-dimensional structures that must be represented on a 2D plane, causing fibers to cross when passing through the midline. The introduction of folds in the cerebral cortex further adds to this complexity.
Source: www.quantamagazine.org
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What if we fold a paper brain and a paper body in opposite directions, connect them without overlapping,
and see what happens?
This would result in a reversal of the vertical z axes, while the horizontal x and y axes remain the same.
This change in geometry would create geometric singularities, leading to ambiguous or undefined properties.
The processing of sensory information would be complicated, requiring the brain to switch between somatotopic maps with opposite z-axis orientations, resulting in an inverted perception of 3D space. The need for such orientation changes would confound the central sensation network, making it challenging to process sensory information.
A concrete example is the representation of the perception of the pad of a finger through two panes of glass labeled “front” and “bottom,” where the perceived “front” label would be inverted if the connections between the fingertip and the brain did not cross. (Figure 3)
Source: www.quantamagazine.org
If you examine the two perception maps closely in Figure 3, you will realize that a simple physical rotation of one map cannot transform it into the other. This means that if the nervous system were to keep uncrossed connections, the brain would need to keep flipping one axis of its body maps as body parts moved. However, this would be too complicated for the brain to handle.
To solve the wiring problem, the simplest way is to have two symmetrical systems of wiring between the brain and the body. Each side of the body would be connected to the opposite side of the brain through the midline.
This idea makes sense in math, but we don’t know for sure if it’s the actual reason our brains and bodies are connected this way. There hasn’t been much research on this topic. While the scientific method can tell us what happens, it’s not always clear why. However, it’s interesting to think about how changing our perspective can help solve mysteries in biology.
Keywords
Contralateral; Crisscrossed; Neural Circuits; Somatotopy,
References
- Shinbrot, T., & Young, W. (2008). Why decussate? Topological constraints on 3D wiring. Anatomical record (Hoboken, N.J. : 2007), 291(10), 1278–1292. https://doi.org/10.1002/ar.20731
- Vulliemoz, S., Raineteau, O., & Jabaudon, D. (2005). Reaching beyond the midline: why are human brains cross wired?. The Lancet. Neurology, 4(2), 87–99. https://doi.org/10.1016/S1474-4422(05)00990-7
- Konkel L. (2018). The Brain before Birth: Using fMRI to Explore the Secrets of Fetal Neurodevelopment. Environmental health perspectives, 126(11), 112001. https://doi.org/10.1289/EHP2268
- Konkel L. (2018). The Brain before Birth: Using fMRI to Explore the Secrets of Fetal Neurodevelopment. Environmental health perspectives, 126(11), 112001. https://doi.org/10.1289/EHP2268