The gut-brain axis is a bi-directional link between the brain and digestive tract. It links the 2 intimately, defining how emotions and digestion share a reciprocal relationship.
On the one hand, our emotions can impact how we digest our food. A classic example of this is in the relationship between stress and irritable bowel syndrome(IBS). Stress is known to cause flare-ups of IBS.
On the other hand, what goes on in our gut affects our mind and emotional state. For example, we find in animal models that relieving digestive problems has a beneficial effect on depression and anxiety. We also find disturbances in the microbiome of people with depression and anxiety disorders.
The interesting thing about the gut-brain axis is that many believe the gut to be a second brain. It has its own fully autonomous nervous system known as the enteric nervous system(ENS). The ENS can function on its own without input from the central nervous system(CNS).
The ENS receives input from the CNS and the synergy of the 2 improves overall function. And, the gut communicates with most other organ systems in the body to help coordinate things like blood sugar, energy storage, kidney function and much more.
But how does the gut-brain axis work? How are these 2 distant organs sharing information to make you healthy? Let’s take a look.
Neural connections in the gut-brain axis
The vagus nerve-The parasympathetic nervous system
When most people think of the gut-brain axis, they think of the vagus nerve. The vagus nerve directly connects the ENS and CNS and helps the 2 relay information to one another.
Most of the vagus nerve is made up of afferent fibers. These fibers, known as sensory fibers, relay information about what is going on in the gut to the brain.
Neurotransmitters and hormones secreted from the gut relay information to the brain. They let the brain know if food is present, or if you’ve had enough. Molecules that stimulate the afferent pathways mostly come from enteric nerves, enteroendocrine cells in the gut, or metabolites from the microbiome.
The remaining 10% of fibers are efferent, or motor fibers. They carry orders from the brain to the gut. As a result, they carry out orders to the gut to trigger gastric acid secretion or gut motility.
Another important function of the vagus nerve is the anti-inflammatory reflex. The anti-inflammatory reflex is a coordinated effort between the ENS, CNS, liver and spleen to control inflammation.
Without it, inflammatory molecules in the gut could cause an inflammatory cascade every time we eat.
The sympathetic nervous system
The parasympathetic and sympathetic nervous systems coordinate our physiology to achieve homeostasis. Stimulation of the parasympathetic nervous system is associated with “rest and digest”. This means that activating these fibers tends to promote digestive processes.
On the other hand, stimulation of the sympathetic nervous system is associated with fight or flight. This means that activating these fibers tends to suppress digestive function.
But why does this system even exist? It essentially functions to partition resources to our needs. When presented with a stressor, our immediate need is to resolve the stress. Therefore, the sympathetic nervous system directs resources away from our gut and to our muscles to get away or fight.
In contrast, when we attain food, stimulation of the parasympathetic nervous system reverses this process. It shifts resources to our digestive organs to break down and absorb our food.
Though there is bi-directionality here, the sympathetic nervous system informs the gut of what’s going on in the brain. Therefore, it is sort of working in the opposite direction of the vagus nerve. At least to an extent.
This system works great under the conditions we evolved under. However, in our modern world of constant stress and nutritional excess, it’s not working great for many. With the proper attention, it certainly can.
Enteroendocrine cells in the gut-brain axis
We mentioned enteroendorcine cells(EECs) above as interacting with the vagus nerve. But, they also release hormones into the blood circulation. They play a role in many important functions inside the gut and beyond.
There are 10 different enteroendocrine cells throughout the gut that each release different molecules. Some regulate digestive enzyme and bile secretion, others regulate appetite, motility, and blood glucose regulation.
Nutrients in our food as well as metabolites from our microbiome stimulate enteroendocrine cells. The cool thing about these guys is that they provide numerous direct and indirect connections between the gut and brain.
For example, our brain and gut work together to regulate gastric acid secretion. Through the vagus nerve, the brain stimulates gastric acid secretion through EECs. Other EECs in both the duodenum and stomach shut off acid secretion and regulate gastric emptying through the blood.
The microbiome in the gut-brain axis
The microbiome works on each layer of regulation mentioned above as well as through regulation of the immune system. Metabolites generated from the microbiome work locally in the gut to regulate the enteric nervous system.
They also act on the vagus nerve, both directly through their metabolites as well as by stimulating enteroendocrine cells. Furthermore, their metabolites also enter the blood circulation to exert their effects on the brain and other organs.
Finally, they interact directly and indirectly with the immune system in the gut to help promote immune homeostasis. Here, immune cells sense our microbial residents and metabolites from the microbiome regulate their function.
Both functions are crucial to the gut-brain axis. Any impairment in immune function or metabolism disrupts the gut-brain axis. Fortunately, the gut-brain axis works to coordinate both.
Conclusion
The gut-brain axis is an important communication axis that helps promote homeostasis in our body. While it helps explain the link between our emotional state and digestion, it’s also important for regulating things like immune function and metabolism.
We can focus on things like stress reduction and healthier eating practices to help establish a better connection, the coordination between the 2 depends on our entire lifestyle. Factors such as eating patterns, sleep hygiene, circadian rhythms, exercise, social connections, and dietary quality are all important.
Lifestyle is the fertile soil through which each component of the gut-brain axis grows. In future blogs and videos, we’ll cover how each of the components of this system are affected by our lifestyle, and how we can make changes to strengthen this crucial axis.