The enteric nervous system (ENS) — sometimes referred to as the “second brain” — is the nervous system of the gut.
It contains some 500 million neurons and controls important reflexes, such as peristalsis, the contraction of muscles in the gut to enable digestion. It is also responsible for the secretion of digestive enzymes that help break down food.
The ENS is also a critical part of the gut-brain axis, through which the gut communicates with the brain — and the vagus nerve is particularly essential for conveying information about the intestines to the brain.
The gut-brain axis performs several functions. The majority of serotonin, a neurotransmitter associated with mood, is found in the gut, for example.
In a new study published in the journal eNeuro, researchers from Flinders University, in Adelaide, Australia, have identified a new way that neurons in the gut wall can activate neurons that connect to those in the spinal cord.
They found highly coordinated activity in the gut wall neurons, which they suggest is a powerful mechanism to transmit information about what is going on in the gut to the brain.
The gut is unique among the internal organs, in that it has its very own nervous system. This has been a subject of interest for Nick Spencer, senior author of the new study and a professor at the university’s College of Medicine and Public Health.
“The gut has its own nervous system, which can function independently of the brain or spinal cord. Understanding how the gut communicates and controls other organs in the body can lead to important breakthroughs for disease treatment.”
– Prof. Nick Spencer
In the new study, Prof. Spencer and colleagues focused on viscerofugal neurons, which are found in the gut wall and project to neurons in the spinal cord.
They investigated how these neurons work using the mouse colon, which contracts in a cyclical pattern known as the colonic motor complex. Viscerofugal neurons are known to be active during this process, but exactly how has, until now, been unclear.
The researchers recorded the electrical activity of the viscerofugal neurons. They found that the firing of these neurons was associated with changes in the activity of the smooth muscle of the colon.
The neurons fired in a highly synchronized way, which was associated with the parallel activation of neurons in the spinal cord.
This suggests that viscerofugal neurons relay activity from the nervous system of the gut to the sympathetic nervous system — in other words, the spinal cord and brain.
“The new study has uncovered how viscerofugal neurons provide a pathway so our gut can ‘sense’ what is going on inside the gut wall, then relay this sensory information more dynamically than was previously assumed to other organs, like the spinal cord and brain, which influence our decisions, mood and general well-being,” explains Prof. Spencer.
The activation of viscerofugal neurons has previously been thought to require changes in the circumference of the gut wall — by the gut filling up, for instance — but this study shows that the process does not require any such mechanical activity.
These findings may also have clinical relevance, as a growing number of conditions have been associated with changes to the gut.
“There is significant interest in how the gut communicates with the brain as a major unresolved issue because of growing evidence that many diseases may first start in the gut and then travel to the brain, an example of which is Parkinson’s disease,” explains Prof. Spencer.
As the scientist highlights, there is a well-established connection between the gut and Parkinson’s disease. One study, for example, showed that men who experience constipation are over four times more likely to develop the condition.
There is also accumulating evidence to suggest an association between changes in the gut and autism, multiple sclerosis, dementia, and stroke, making studies like this essential in understanding and eventually treating diverse neurological conditions.
