Brain and cranial nerves
By Nicole Cain, ND, MA
The vagus nerve is the 10th cranial nerve, also called vagrant or vagus nerve, and is the longest nerve in the body. It contains bidirectional motor and sensory fibers, afferent fibers carrying information from the body tissues to the central nervous system, and efferent fibers carrying information from the central nervous system to the rest of the body.1
The vagus nerve originates in the medulla oblongata, the lowest part of the brain, and enclosed in fibrous tissue (the carotid sheath), it runs down the neck. At the base of the neck, it divides into the left vagus nerve and the right vagus nerve, which both join at the enteric nervous system (ENS), the nervous system surrounding the gastrointestinal tract.1
The vagus nerve is a main component of the parasympathetic nervous system, which is responsible for bodily functions at rest. It is also involved in the coordinated communication and regulation of the organs of the body, the central nervous system and the enteric nervous system.2
As a major gut-brain axis pathway, preliminary evidence suggests that vagus nerve stimulation may be a potential approach to alleviating symptoms related to both physical and psychiatric conditions such as post-traumatic stress disorder. (PTSD), depression, anxiety and inflammatory disorders. bowel disease.2 In fact, vagus nerve stimulation by surgically implanted devices is currently prescribed for treatment-resistant depression,3 mood and anxiety disorders and other conditions associated with inflammation.4
Additionally, in a 2022 report, researchers from Leiden University (Netherlands) demonstrated that non-invasive vagus nerve stimulation reduced emotional reactivity and biases – major components of depression – compared to control subjects.5
What’s Happening in Vagus: The Microbiome-Mood Highway
The vagus nerve represents an important link between the gut microbiome and mental health.2 Although the gut microbiota communicates with the body’s nervous system through a variety of means, including hormonal and immune pathways, the fastest channel of communication with the brain is through vagus nerve signaling..6 This signaling consists of several mechanisms including cell-mediated sensing, direct signaling, and indirect signaling.
Vagus nerve-microbiota connection
Source: Nicole Cain
In cell-mediated sensing, signals from the microbiota are transmitted to the vagus nerve via intermediate sensing cells. For example, intermediate sensing enteroendocrine cells (EECs) in the gut interface with vagus fiber cells to carry signals from the microbiota to the enteric and central nervous systems.seven
When gut microbes release a compound or metabolite, the EECs pick up that signal and pass it on to the afferent fibers of the vagus nerve. In an example of mood-altering neurotransmission, EECs directly release serotonin as 5-hydroxytryptamine (5-HT) and activate 5-HT3 receptors on vagal afferent fibers, which transmit the signal to the central nervous system to modulate mood and cravings.8
Direct microbiota signaling
In direct microbiota signaling, gut microbes are able to communicate directly with the afferent fibers of the vagus nerve. For example, butyrate, a short chain fatty acid (SCFA) produced by certain types of microbiota such as Firmicutes,9 with the cross support of Bifidobacteria, can directly stimulate the afferent fibers. SCFAs have also been shown to maintain the integrity of the blood-brain barrier (BBB), providing anti-inflammatory, neuroprotective and brain tissue support.11
Indirect signaling is a process by which signal molecules are able to indirectly affect surrounding cells.12 Afferent fibers from the vagus nerve stimulate vagal efferent signals via what is known as the inflammatory reflex. Vagus efferent fibers are able to inhibit the release of certain inflammatory cells in order to manage inflammation, an important component of depression.7.13
Vague manipulation: direct and indirect
Although direct vagus nerve stimulation has been used successfully in the treatment of mood disorders such as major depression, it is complex, expensive14limited availability and even more limited safety and efficacy data.
There are other non-mechanical ways to stimulate the vagus nerve. Diaphragmatic breathing, or deep slow breathing (DSB), is a method of deep breathing from the diaphragm in which the exhalation time exceeds the inspiration time. When inhaling, the vagus nerve is inhibited, resulting in an increased heart rate, which can lead to an anxious state.15 During exhalation, vagus activity is restored, resulting in a slowing of the heart rate and a reduction in the parasympathetic response associated with stress.16 In a 2021 study,17 the researchers demonstrated that after just five minutes of DSB, participants showed a decrease in physiological stress and reported significantly less anxiety.
Targeted microbiome therapies are a novel approach to vagal stimulation. Several species of gut microbes, including strains of L. rhamnosus, B.longum, L.casei, demonstrated vagus nerve-mediated mood enhancement.18 However, identifying a highly effective psychobiotic formulation can be difficult. Formulations backed by specific clinical evidence from human trials provide the most reliable results.
As part of the gut-brain axis, the vagus nerve plays an important role in mood and mental well-being. Fortunately, non-invasive tools like deep breathing and clinically proven psychobiotics offer a safe and effective option to help fine-tune the system.19