The brain is considered one of the most important organs in the human body because of its ability to process a tremendous amount of information very rapidly. We, as human beings, have a larger brain compared to many other mammals, but how exactly does that affect us? In other words, does having a larger brain affect synaptic connections in the long run? And are those effects beneficial or harmful? These questions are explained via research conducted by scientists in the United States and France, which indicates that due to its large size, the human brain is more prone to developing mental illnesses than other organisms with smaller brains.
The researchers used combined tracing studies—a research method that traces axons from their source (cell body/soma) to the point of termination (synapse) or vise versa—in both macaques and mice in order to better visualize connections within the brain. The results yielded that macaques, due to their larger brain size, had more long-range axonal connections than mice. Once this was established, the researchers combined their tracing studies with a network theory, better known as the exponential distance rule (EDR), which describes the relationship between distances and strength of short and long-range connections in the brain. Essentially, the EDR states that weaker long-distance connections within the brain could potentially explain why bigger brains are more susceptible to mental illnesses, which occur when long-range axonal connections deteriorate over time.
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Consistent with the tracing results, EDR predicted that there are fewer long-range axons and many short-range axons in both macaques and mice. In fact, if the brain of a given animal is bigger, as it the case with humans, the brain will try to minimize the number of long-range axonal connections. This is also true for other mammals with larger brains, and explains that the efficiency of larger brains is maintained by having fewer long-range axonal connections. The trouble with trying to minimize long-range connections in a large brain, however, is that this causes a sort of disconnection between different parts of the brain. Once there’s a disconnection in the brain—that is, axons are unable to travel from one part of the brain to another—it could result in severe mental illnesses such as Schizophrenia or Alzheimer’s disease. Furthermore, differences in brain sizes could help us understand the human connectome—a comprehensive map of neural connections within the brain that explains how brain is “wired”—and provide an in-depth understanding of the human brain and mental illnesses. This knowledge, along with combined tracing studies and EDR, would allow us to better understand the decline of long-range axonal connections as we age and why this decline affects only some individuals’ mental states, but not others.
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