The Vagus Nerve – A Jack Of All Trades
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Why don’t you place your fingers on the left side of your neck? The vagus nerve is located just below your skin. It’s a super nerve, for want of a better term. It extends from the brain to the stomach and is involved in many vital bodily functions such as breathing, your heartbeat, swallowing, and the production of gastric acid. Approximately 80 percent of its fibres are sensory, meaning they transmit information from our organs directly to the brain.
Moreover, the vagus nerve is easily accessible, which is perhaps one of the reasons why medicine has long since recognised its potential. It is utilised to treat neurological conditions such as epilepsy and tinnitus. When patients fail to respond to conventional medication, doctors turn to the vagus nerve. They use electrical impulses to stimulate the nerve, thus significantly reducing the impact of the respective disease.
Treating spinal cord injuries
At first glance, it may seem that spinal cord injuries have very little in common with the aforementioned conditions, but they all share one essential aspect: impaired or disrupted nerves within the central nervous system. This raises the question whether what experts call vagus stimulation also has potential to treat spinal cord injuries. “It does,” says Dr Michael Kilgard from Dallas. He is a professor at the University of Texas and has been researching the vagus nerve with his colleagues for years. They first focused on tinnitus, then on post-traumatic anxiety disorders and stroke patients. More recent experiments have proven that vagus stimulation can also be used to treat spinal cord injuries. “We started with the most common neurological disorders. Spinal cord injuries are fairly rare, but they cause severe disabilities. With vagus stimulation, we strive to create new neural connections and restore the independence of those affected,” he explains. The will to help seems enshrined in Kilgard’s DNA. He joined the boy scouts as a teenager and was awarded the Eagle Scout, the highest honour of the American association, for his outstanding commitment. He also has a very personal connection to spinal cord injuries. The scoutmaster for his son’s boy scout troop suffered a terrible accident two years ago.
He had decided to relocate to the country to enjoy his well-deserved retirement. While packing the very last box, he fell and broke his neck. He has been paralysed from the neck down ever since.
Kilgard’s aim is to significantly improve the scoutmaster’s life – and that of many others – with vagus stimulation. The team from Dallas has already proven that this is possible with stroke patients. The treatment increased their arm function significantly. “Following a stroke, progress during physical therapy is three times greater when the vagus nerve is stimulated. We expect the effect in treating spinal cord injuries to be similar,” Kilgard states.
How vagus stimulation works
The vagus stimulation procedure itself is relatively simple, but promises great benefits for patients. The first step is to wrap electrodes around the vagus nerve. This is achieved in a surgical procedure by means of a small skin incision. The electrodes are then connected to a stimulation device implanted below the skin at breast level. At least that was the case until now.
Kilgard’s university colleagues have modernized the entire procedure from the ground up. Today, a small chip has replaced the old wire electrodes and the large stimulator. This makes the procedure faster and gentler for the patient. A device called “ReLay” supplies the chip with power and data. When required, it is simplyplaced around the neck in a cuff and then communicates with the chip wirelessly. Furthermore, the bioengineers have developed an app that controls the entire system. While a therapist had to activate the stimulator in the past, sensors and the app now ensure that the vagus nerve is stimulated at exactly the right time.
New nerve connections
When the stimulator is activated, it sends gentle electric impulses to the brain via the vagus nerve. There they stimulate the regions responsible for learning - for example the learning of new movements. This is the moment something truly amazing happens. Researchers call it plasticity. The brain learns that it has to modify its circuits in order to trigger a certain movement. The nerve cells subsequently release neurotransmitters and begin to interconnect again. “In experiments, we were even able to triple the number of neurons in the brain that control the arm,” Kilgard explains. Perfect timing is an important factor. It makes no sense to set the stimulator to a permanent current. Instead, the impulses need to occur when the patient repeatedly performs a certain movement. Only then does the brain know that something important is happening and that it’s supposed to form new nerve connections.
“There is no point in stimulating even the slightest movements. The best are the ones that really count, the top 20 percent so to speak,” says Kilgard, adding: “The sensors and the app check the exercise the patient is performing. If the patient executes the movement correctly, the app activates the stimulation automatically.”
According to Kilgard, first results can be observed after just one week of training and vagus stimulation. Even in this short time, new nerve connections are formed. It doesn’t require many new connections to achieve a positive effect. “In order to improve control and sensation, we only need to make small changes to the circuit. A handful of new connections is enough,” Kilgard is convinced.
Next step: clinical trial
Following many experiments in the laboratory, Kilgard is currently testing the system for safety in a clinical study involving 10 patients. The study is necessary because of the newly developed technology. However, he does not expect complications. “Vagus stimulation for epilepsy has been around for about 20 years. No side effects have occurred, except for a little tickle in the throat from time to time.” At the same time, Kilgard and his colleagues are continually optimising the therapy itself. They are exploring how often stimulation should be applied in order to achieve the greatest effect. They not only want to improve control over arm and leg functions, but also fine motor skills. Furthermore, they are keen on introducing voice control, which would benefit, for example, patients with high-level spinal cord injuries. “Vagus stimulation is safe, effective, and can be applied at home,” says Kilgard with visible pride. He and his team have designed the therapy to be as cost-effective as possible. Vagus stimulation is expected to find widespread application in the treatment of spinal cord injury patients in approximately five years. Who knows what the researchers from Dallas will find out about the vagus nerve – a real jack-of-all-trades – by then?
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