The Importance of the Vagus Nerve

vagus nerve
The Vagus Nerve. BSIP/UIG/Getty Images

The vagus nerve is the longest nerve of the autonomic nervous system and is one of the most important nerves in the body. The vagus nerve helps to regulate many critical aspects of human physiology, including the heart rate, blood pressure, sweating, digestion and even speaking. For this reason, medical science has long sought ways of modulating the function of the vagus nerve.

What Is the Vagus Nerve?

The vagus nerve (also known as the 10th cranial nerve) is a very long nerve that originates in the brain stem and extends down through the neck and into the chest and abdomen.

It supplies innervation to the heart, major blood vessels, airways, lungs, esophagus, stomach, and intestines.

While there are actually two vagus nerves (the left and the right), doctors usually refer to them together as “the vagus nerve.”

The vagus nerve helps control several muscles of the throat and of the voicebox. It plays a major role in regulating the heart rate and keeping the gastrointestinal tract in working order. The vagus nerves also carry sensory information from the internal organs back to the brain.

Why Is The Vagus Nerve Significant?

Perhaps the greatest significance of the vagus nerve is that it is the body’s major parasympathetic nerve, supplying parasympathetic fibers to all the major organs of the head, neck, chest,​ and abdomen. The vagus nerve is responsible for the gag reflex (and the cough reflex when the ear canal is stimulated), slowing the heart rate, controlling sweating, regulating blood pressure, stimulating peristalsis of the gastrointestinal tract, and controlling vascular tone.​

Sudden stimulation of a vagus nerve can produce what is called a "vasovagal reflex," which consists of a sudden drop in blood pressure and a slowing of the heart rate. This reflex can be triggered by gastrointestinal illness or in response to pain, fright or sudden stress. Some people are particularly prone to the vasovagal reflex, and their blood pressure and heart rate changes can cause loss of consciousness - a condition called "vasovagal syncope."

Excessive activation of the vagus nerve is also seen in certain medical conditions, especially the dysautonomias.

Stimulating the vagus nerve can have therapeutic effects (such as stopping episodes of supraventricular tachycardia (SVT) or hiccups), and can help doctors diagnose certain kinds of heart murmurs. Vagal stimulation can be achieved quite easily by employing the Valsalva maneuver.

The Vagus Nerve and the Heart

The right vagus nerve supplies the sinus node, and its stimulation can produce sinus bradycardia. The left vagus nerve supplies the AV node, and its stimulation can produce a form of heart block. It is by producing transient heart block that the Valsalva maneuver can terminate many kinds of SVT.

The Vagus Nerve In Medical Therapy or VNS Therapy

Because the vagus nerve has so many important functions, medical science has been interested in decades in the idea of employing vagus nerve stimulation, or vagus nerve blocking, in medical therapy.

For decades, the vagotomy procedure (cutting the vagus nerve) was a mainstay of therapy for peptic ulcer disease, since this was a way of reducing the amount of peptic acid being produced by the stomach. However, the vagotomy had several adverse effects, and with the availability of more effective treatment has now become much less commonly used.

Today, there is great interest in using electronic stimulators (essentially, modified pacemakers ) to chronically stimulate the vagus nerve in an attempt to treat various medical problems. Such devices (referred to generically as vagus nerve stimulating devices, or VNS devices) have been used successfully to treat people with severe epilepsy that is refractory to drug therapy. VNS therapy is also sometimes used to treat refractory depression.

Because when you have a hammer everything looks like a nail, companies that make VNS devices are investigating their usage in several other conditions including hypertensionmigrainestinnitusfibromyalgia and weight loss.

There is indeed promise in such applications of VNS. However, the true potential of VNS will emerge once the hype is replaced by firm clinical evidence.

Sources:

Henry TR. Therapeutic Mechanisms of Vagus Nerve Stimulation. Neurology 2002; 59:S3.

Morris GL 3rd, Gloss D, Buchhalter J, et al. Evidence-based Guideline Update: Vagus Nerve Stimulation for the Treatment of Epilepsy: Report of the Guideline Development Subcommittee of the American Academy of Neurology. Neurology 2013; 81:1453. 

Shuchman M. Approving the Vagus-Nerve Stimulator for Depression. N Engl J Med 2007; 356:1604.

Continue Reading