'Gut Feeling' Suddenly Made Sense: Enteric Nervous System Explains Why Gastrointestinal Tract Should Be Our 'First Brain'

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While humans are taught that all organ systems are connected and controlled by the central nervous system, the gastrointestinal tract or simply the gut has evolved to have its own nervous system operating mostly independently from the rest. 

This evolutionary development has led the gastrointestinal tract, a long series joined in a winding tube from the mouth to the anus, to be informally known as the 'mini-brain' or the 'second brain.' However, Professor Nick Spencer from the Flinders University in Australia has another idea: calling the GI tract "the first brain."

Together with a team of researchers from Flinders University, Professor Spencer has led a study that examines for the first time how this independent nervous system and its neurons communicate with each other. They present their findings in the journal Communications Biology in a report titled "Long Range Synchronization Within the Enteric Nervous System Underlies Propulsion Along the Large Intestine in Mice."

A Unique Yet Little-Known Mechanism

In the new study, the Flinders University researchers examine how neurons in the GI tract's nervous system, also known as the Enteric Nervous System, interact to cause propulsion. Propulsion is one of the main six activities in the entire digestive process, which causes food material to move down the gastrointestinal tract.

Researchers also noted that one of the long-standing mysteries in the human body is how the Enteric Nervous System coordinates propulsion along the tract. Furthermore, while it has been understood that the brain is the primary control system in terms of cognition, memory, and even our immune response, a growing number of studies now point to the gut as having something to do with the same aspects.

One study examining the so-called gut-brain-axis (GBA), appearing in the September 2017 issue of Clinics and Practice journal, examines how gut microbiota composition affects mental health.

The Enteric Nervous System: Our "First Brain"

Invertebrate animals are known to have no brains, having simpler nervous systems, such as sea urchins, sea cucumbers, and sea sponges. Chief among them are animals classified under the genus Hydra, which covers brainless invertebrates that have been present since 600 million years ago. These creatures, primarily found in freshwater environments, supposedly support Professor Spencer's "first brain" theory.

In an interview with media outlet Inverse, Prof. Spencer explained, "Hydra have an intrinsic nervous system, remarkably like the intrinsic nervous system, (or ENS) in the gut of vertebrate animals that exist today." He added that while these creatures do not have the brain as we know it, the nervous system still allows them to move and eat.

Their theory about the gut being the first brain is based on the idea that the brain and spinal column in vertebrates evolved much later after the enteric nervous system did. Therefore, the first brain in the gastrointestinal tract developed first before the brain and spine that we have now.

Prof. Spencer also explained that the enteric nervous system is "essential for life" and that vertebrates could not live without it. It is responsible for the propulsion of food material as well as the expulsion of waste materials. Their study demonstrated how neurons in the gastrointestinal tract allow muscles to contract and relax, allowing for the propulsion, absorption, and expulsion of various parts of food products.

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