The body’s nervous system can be broadly divided into two parts: the sympathetic nervous system (SNS) and the parasympathetic nervous system (PSNS).
The SNS controls actions that are under voluntary control, eg skeletal muscle contraction – the “fight or flight” response.
The PSNS controls automatic bodily actions that are not under voluntary control, eg slow digestive movements of our gut, beating of our heart, breathing, immune response and other reflexes, even orgasm – the “rest and digest” function.
The vagus nerve (from the Latin vagus, meaning “wandering”) plays a central role in regulating the PSNS. There are two vagus nerves: the left and the right nerve. Each runs from the brainstem to the large intestine, one running down the left-hand side of the body and the other the right-hand side.
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The majority of the nerve cells (neurons) in the vagus nerve are sensory neurons passing messages up to the brain from our organs and are activated by sensory inputs from the environment. The other nerve cells are motor neurons sending messages in the opposite direction. The vagus nerve tells the brain what is happening throughout the body and the brain uses this information to maintain the body’s internal status quo.
The brain operates in dynamic tension between two opposing forces, maintaining both stability and ability to change over time. The brain’s ability to reorganise and rewire its neural connections is termed plasticity, and the vagus nerve plays a vital role here.
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The facility for plasticity varies both with age and disease state, including post-traumatic stress disorder (PTSD) and also, interestingly, dementia. It was long believed the brain became fixed following childhood and adolescence, and further changes were limited to the debilitations of ageing.
But we now know that, although much brain structure and function develop in this early period, there are also many other critical plasticity periods in life. Conditions that trigger PTSD, also known as “shell shock”, may be one example.
PTSD is an anxiety disorder characterised by anger, irritability, depression and physical exhaustion. PTSD affects approximately 6 per cent of civilians and 14 per cent of military veterans. Many PTSD victims (about 50 per cent) can achieve remission through non-pharmacological therapies, while about 40 per cent respond to pharmacological interventions.
But some patients are resistant to both approaches and suffer significant difficulties in negotiating life. However, a recent study by Mark Powers and others, (Brain Stimulation May-June 2025) demonstrates that electrically stimulating the vagus nerve is a feasible beneficial treatment for PTSD.
The Brain Stimulation study combined vagal nerve electrical stimulation, from a small device implanted around the vagal nerve, with a 12-session course of a non-pharmacological treatment called prolonged exposure therapy, involving patients gaining control of their thoughts and feelings by describing their traumas in a “safe space“ to allow them to make memories less painful.
All seven participants who completed this course of therapy achieved remission for at least six months. The interaction between prolonged exposure therapy and vagus nerve stimulation appears to recruit the brain’s plasticity, ie rewiring of the brain, allowing adaptation and healing long after the trauma became embedded in memory.
The concept of vagus nerve stimulation dates back to the late 19th century, when a stimulator to treat epileptic patients was developed. More than 100,000 people have now had vagus nerve stimulation, mostly through chest implants, to treat epilepsy when drug treatments haven’t worked.
Some people report up to 75 per cent reduction in their seizures with an implant, with increasing benefits the longer the implant has been installed. An implant is also approved to treat severe depression and headaches as featured in BBC Science Focus recently.
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Nerve stimulation doesn’t necessarily mean surgery. It’s possible to access the vagus nerve using devices similar to the transcutaneous electrical nerve stimulation machines more commonly used for quieting general pain by transmitting electrical stimulation through your skin.
Vagus nerve stimulation research is currently ongoing with trials covering burns, obesity, high blood pressure, rheumatoid arthritis, Alzheimer’s disease, bowel disorders (eg, irritable bowel syndrome) and more.
Of course, the process of gathering evidence from trials to support the efficacy of vagus nerve stimulation in treating these conditions will, as always, take time.
William Reville is an Emeritus Professor of Biochemistry at UCC
