“Gut feeling” is more than just a figure of speech. A new study from ETH Zurich investigated how the vagus nerve—a large nerve that extends from the brain to the abdomen, connecting to organs along the way—conveys fear, or gut feelings, to the brain. The study demonstrated that signals from the gut to the brain via the vagus nerve affect the brain’s response to different types of fear. Without gut feelings, unlearning fear is especially challenging.
The vagus nerve permits communication to and from the brain through two types of nerves: afferent nerves lead into the brain from and efferent nerves lead from the brain into the rest of the body. In a typically functioning nervous system, this two-way communication helps the body and mind control fear responses.
For the study, the researchers snipped the afferent nerve fibers—nerves connecting the guts to the brain—in laboratory rats, rendering the vagus nerve a one-way communication conduit. The researchers then trained the rats, some with the clipped nerve and some with an intact nerve, to associate a particular sound with an unpleasant experience. The researchers conducted a series of behavioral studies with the rats, including eventually re-training them to associate the sound with a neutral stimulus.
The rats with snipped vagus nerves had less innate fear, but retained their learned fear longer than the rats in the control group. Rats without an intact vagus nerve were less afraid of open spaces and bright lights, but when it came to the auditory stimulus, it took significantly longer for these rats to unlearn their conditioned fear response. This suggests that, while gut feelings are not necessary for unlearning conditioned fears, they make it much faster. The researchers also found that a loss of nerve signals from the abdomen was related to increased production of adrenaline in the brain.
“We were able to show for the first time that the selective interruption of the signal path from the stomach to the brain changed complex behavioral patterns. This has traditionally been attributed to the brain alone. The study shows clearly that the stomach also has a say in how we respond to fear; however, what it says, i.e. precisely what it signals, is not yet clear,” study leader Urs Meyer, researcher at ETH Zurich, said of the results.
The findings demonstrate that vagus nerve stimulation could aid treatment of disorders that impair the fear response, like post-traumatic stress disorder or anxiety. Stimulating the vagus nerve could speed up the process of learning not to fear benign stimuli.
This research is published in the Journal of Neuroscience.
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