Why is the gut considered our second brain?

The reason is that in the intestine is the enteric nervous system, a part of the nervous system that contains a very high number of neurons and also presents a certain autonomy, so it is considered that it could act as a second brain.

Regarding the number of neurons, the enteric nervous system has between 200 and 600 million neurons. This is a high number, although far from the 100 billion (with a b) neurons that the human brain has. It is therefore not surprising that the human brain and not the gut is capable of such sophisticated activities as writing poetry or solving mathematical problems. However, the number of neurons in the enteric nervous system is comparable to the number of neurons in a dog’s brain, which is around 500 million neurons, and with 500 million neurons, you all know that there are some pretty smart dogs. So if we base it on the number of neurons, it doesn’t seem so far-fetched to consider the intestine our second brain.

In addition, the enteric nervous system can act autonomously. In mice, it has been shown that if we separate the entire intestine, including the enteric nervous system, from the rest of the animal’s body, the intestine continues to maintain intestinal movements despite being disconnected from the brain. This shows that the enteric nervous system is able to control the intestine autonomously and thus supports the idea that it can function as a second brain.

There are researchers who go further and refer to the gut as our first brain. And I imagine that again you are wondering why. The reason is that, when analyzing the nervous system of very poorly evolved animals such as the hydra or the starfish, it was observed that the nervous systems of these animals, which basically consist of networks of neurons that support the digestive systems, show many similarities with the enteric nervous system that we humans have. This suggests that our enteric nervous system probably evolved from these primary nervous systems. So based on these observations, it does not seem so far-fetched to refer to the gut as our first brain.

However, regardless of whether we refer to the enteric nervous system as our first or second brain, what is really relevant is whether we can refer to the enteric nervous system as a brain. A set of neurons per se does not constitute a brain. In order to form a brain, neurons must have a certain level of organization. In poorly evolved animals, the nervous system consists of neurons that are organized into networks. An example of an animal with this type of nervous system is the hydra. As animals evolve, so do their nervous systems in which neurons group together to form ganglia, resulting in more efficient nervous systems. One animal with this type of nervous system is the starfish. The nervous systems of animals have continued to evolve and increase in efficiency with the clustering of ganglia to form brains. Among the least evolved animals that have brains is the leech.

And what happens in the enteric nervous system: do neurons group together to form ganglia, which form something like a brain?

In the enteric nervous system, neurons are organized into three distinct parts. A sensory part, composed of sensory neurons, which capture stimuli in the lumen of the intestine such as the presence of food and transmit this information to neurons that communicate with the brain. A part that communicates the intestine with the brain formed by ascending neurons that carry information from the intestine to the brain and descending neurons that carry information from the brain to the intestine. Finally, an effector part formed by secretory neurons that control intestinal secretions and motor neurons that control intestinal movements.

Although neurons in the enteric nervous system are well organized, the fact is that their organization is far from that of the brain. Perhaps it is an exaggeration to refer to the gut as a brain, but what is clear is that the gut is much more complex and relevant than many people think.