Why a critical part of the gut may be “the first brain”

Out of all the organs in the body, the gastrointestinal tract is the only organ to have evolved its only fully independent nervous system.

That’s why this tract, which stretches from the mouth to the anus, has earned the nicknames “mini-brain” and “second brain.” But Nick Spencer, a professor at Flinders University in Australia, argues for another moniker: the first brain.

“I was fascinated 23 years ago, when I started research on the enteric nervous system, and I am even more now,” Spencer tells Inverse.

The enteric nervous system (ENS), or intrinsic nervous system, is the nervous system of the gut and home to hundreds of thousands of individual neurons. These neurons are what allow propulsion along the gut. They are essential for the contraction and relaxation of muscles and work without any interaction with the brain.

Through a recent study published in the journal Communications Biology, Spencer and colleagues became the first to discover how the neurons in this body system “talk” to each other to cause propulsion. These findings reveal how an admittedly strange part of the body works, bringing us one step closer to treating a disease caused by a loss of neurons.

“This is why we believe the gut enteric nervous system is the first brain.”

It’s also a reminder of the length of our journey toward understanding our humanity. While the brain has long held the spotlight in our quest to probe the cause and effects of evolution, an increasing number of studies now point to the importance of the gut.

The gut, we know now, influences our emotional state, our longevity, and our immune system. Gut bacteria is even linked to certain brain conditions — making the brain comparisons even more salient. But we still have a ways to go in our comprehension of its influence. Research like this paper by Spencer reinforces our underestimation of this part of the body and its uniqueness.

The first brain — Some invertebrate animals do not have a brain. These are creatures like sea cucumbers, sea urchins, and sea sponges.

Hydra is a genus of brainless invertebrates, and has been around for over 600 million years. They are incredible freshwater creatures, seemingly ageless, who look a bit like a tube with arms sticking out.

Hydra, Spencer explains, support his “first brain” theory.

“Hydra have an intrinsic nervous system, remarkably like the intrinsic nervous system, (or ENS) in the gut of vertebrate animals that exist today,” he explains via email.

“Despite the lack of a formal brain, as we know it, their intrinsic nervous system allows them to swim and feed and ingest food.”

The idea that the enteric nervous system in the gut is the first brain, he explains, is “based on the fact that the brain and spinal cord evolved in vertebrate animals, like fish and humans, well after the enteric or nervous system had evolved.”

“This is why we believe the gut enteric nervous system is the first brain,” he says.

The enteric nervous system — “The enteric nervous system is essential for life,” Spencer says. “No vertebrate animal can live without one.”

It has a number of jobs: It’s critical for the propulsion of stuff through the gut as well as the expulsion of that stuff as waste. It allows, as this study shows, muscles to contract and relax — which is why we can digest and absorb our food. The enteric nervous system also has its own sensory neurons.

“No other organ in the body has its own intrinsic sensory neurons that are contained within the organ and operate independently of spinal afferent or vagal afferent neurons outside the gut,” Spencer explains.

While different neurons in the enteric nervous system have different jobs, Spencer and colleagues found thousands of them synchronize and coordinate the firing, which causes propulsion through the colon, part of the gastrointestinal system. Propulsion begins with swallowing and continues into the process of peristalsis, the involuntary constriction and relaxation of muscles. You choose to swallow a bite of food and your neurons allow the propelling of that food through your system.

Sometimes, however, the digestion doesn’t go as planned.

“It is impossible to know how to fix a problem of the ENS if we don’t know how it works.”

Some animals, like horses, mice, and people, have a disease called Hirschsprung disease. This can prohibit bowel movements and is caused by a developmental loss of neurons in the enteric nervous system, which may be driven by genetic mutations. An estimated 1 in every 5,000 human babies has some degree of Hirschsprung disease, Spencer says. While it can be life-threatening, surgery very often leads to a successful recovery.

While Spencer’s work serves a fundamental purpose — “in science, it is critical to know how the organ works” — it also informs why conditions like Hirschsprung disease happen.

“It is impossible to know how to fix a problem of the ENS if we don’t know how it works,” he says.

“My laboratory is very much focused on trying to understand how the organ works before we try to cure a disease of the organ. It is a bit like being asked to fix the engine in a broken-down car, but you have no idea how the engine works.”

Luckily, we’re on our way toward understanding how the assembly works.