The Second Brain in Your Gut

Synapse #6: Your gut's role in Parkinson's Disease

Your gut is much more than a slimy tube—it is a vibrant, active ecosystem with over 80 trillion bacteria and is innervated with over 500 million neurons. That’s approximately 0.5% of the number of neurons in your brain, but don’t let the relatively small number fool you: the neurons in your gut—collectively known as the enteric nervous system—is in many ways just as complex.

Your Second Brain

The enteric nervous system is sometimes known as your ‘second brain.’ It has earned this title because these neurons are capable of much more than simply contracting the muscles in your gut. In fact, studies have shown that if you sever the connections between the enteric nervous system (ENS) and the brain, the ENS can go on to function on its own.

More than 90% of the serotonin and 50% of the dopamine in your body lie in your gut and the ENS has some of the same glial supportive cells that I talked about in a previous issue of Synapse. This information and more have led scientists to consider the ENS as a complex, though perhaps unconscious, integrator of information just like the brain.

Parkinson’s: A Disease of the Gut?

Parkinson’s Disease is a neurodegenerative disorder characterized by the misfolding and aggregation of a protein called alpha-synuclein into Lewy bodies which cause brain degeneration. There is this long-known clinical oddity in which Parkinson’s patients often have problems with constipation decades before neurological symptoms arise and the disease is diagnosed. Direct evidence for this effect emerged in 2003 when German neuroanatomist Heiko Braak made the observation that the Lewy body buildup in patients with Parkinson’s begins in an area of the brain known to communicate with the ENS.

This observation could have led to a straight-forward conclusion that Parkinson’s simply begins in the area of the brain that controls the ENS—hence the constipation often seen in these patients.

However, some have scientists wondered if the opposite were true: what if Parkinson’s actually starts in the gut?

A few clinical studies have looked for correlations between bowel problems and Parkinson’s. In fact, one study concluded that constipation occurring as early as 20 years before the onset of motor symptoms is associated with an increased risk of Parkinson’s.

However, direct biological evidence for the emergence of Parkinson’s in the gut didn’t arrive until a recent study in Neuron that eloquently showed that misfolded alpha-synuclein, the protein that causes Parkinson’s, can actually travel up from the gut to the brain. To demonstrate this, researchers injected the pathological form of alpha-synuclein into the gut in mice and found that the mice developed Parkinsonian symptoms. However, if the researchers cut the vagus nerve—the nerve principally connected the ENS and the brain—the mice did not develop Parkinson’s. Here’s a graphic depicting their results:

Thus this study provides, for the first time, strong evidence that pathology in the gut is able to travel from the gut to the brain.

Why this matters

The implications for our new understanding of the relationship between the gut and the brain are numerous. For one, the brain is protected from toxins and many types of pathogens with a blood-brain barrier. However, the ENS has no such protection, and therefore anything we eat could potentially affect our brain in good and bad ways.

I described above an extreme case in which a neurodegenerative disease could start in the gut, but what about sub-disease effects? How does nutrition affect our brain development, intelligence, memory, emotions, etc? These effects are likely numerous and very small which makes them difficult to detect in a study but nonetheless provides food for thought (see what I did there?) on the subject of how our environment—including what we eat—makes us who we are.

🔗 Links

  • Neurotechnology can already read minds: so how do we protect our thoughts? For the past two weeks, we’ve taken up the issue of consciousness here on Synapse. I didn’t even touch on the fact that our consciousness is just the tip of the iceberg of our thoughts—most of which are subconscious. We may soon be able to interpret both conscious and subconscious thoughts with neurotechnology and this article does a great job of laying out our urgent need to develop ethical guidelines for one’s right to privacy with regard to his or her thoughts.

  • “In a lot of ways, it’s like a Fitbit in your skull, with tiny wires.” Elon Musk said in a recent update for his Neuralink company which aims to develop a computer-brain interface to ultimately cure diseases such as deafness, depression. and paralysis. Some Neuroscientists, though, are skeptical and believe the company may be more focused on developing a cool technology (such as the ability to unlock a Tesla with your mind) than actually making it medically applicable.

  • How dozens of languages help build gender stereotypes

🖼 Image of the Week

This week’s image is a visualization of the ENS in a section of gut adapted from this study.

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