Introduction

Have you ever wondered why you don’t have to consciously regulate your breathing pattern, like decreasing your heart rate after running, or when you digest your food? These processes happen involuntarily and are controlled by the vagus nerve, also known as cranial nerve X, as it is the 10th named nerve in the head.  It is the longest of the 12 cranial nerves, and it relays information from various parts of your body, specifically in the chest and abdomen regions, to the brain, and vice versa.¹ By acting on the brain, the vagus nerve can decrease your heart rate, slow your breathing down, and promote the digestion of food.

Overview of the cranial nerves

Cranial nerves are typically numbered from 1 (I) to 12 (XII), depending on where they emerge from (brain or brainstem), from front to back. They can either take coordinated information from the brain to various parts of the body and initiate a behaviour/ muscle function (efferent nerve), or take sensory information to the brain from different parts of the body (afferent nerve). They may even carry out both input and output of the brain as a mixed nerve.²  They are responsible for functions including various sensations, movement, and body functions, which happen automatically without you thinking about it, which is why it is called the autonomic system.

Think of it like the server staff system in a restaurant:

• The customers at the table (an organ) will send their food order (sensory information) via a server (nerve) to the kitchen (the brain). This is like the nerve’s sensory (afferent) function
• The kitchen (the brain) will process the food order and send it out to the customers at the table (an organ) via another server (a nerve), and then the customers will eat their food (motor function/muscle command). This is like the nerve’s motor (efferent) function
• Sometimes you might have the same server who takes your order to the kitchen and delivers the food to your table. This is like mixed nerves that can carry out both functions

Anatomical overview of the vagus nerve

The vagus nerve is the longest of the 12 cranial nerves. It starts from a part of the brainstem, at the base of the brain, known as the medulla oblongata, and goes through the neck, chest, and abdomen.¹ The nerve is made up of the left vagus nerve and the right vagus nerve, which act on the heart chambers. These join to form the vagal trunk as it goes through the chest into the abdomen, which further branches off to other organs in the abdomen. The word “vagus” is of Latin origin, meaning “wandering”, which appropriately represents its extensive distribution and “wandering” course throughout the body.³

Functions of the vagus nerve

The autonomic nervous system

The vagus nerve makes up a big part of the autonomic nervous system, which controls the body’s functions that we don’t actively think about, specifically playing a role in the automatic functions of the chest and abdomen region, including the regulation of heart rate, digestion, and breathing.⁴  

The autonomic nervous system is made up of two parts: the sympathetic nervous system and the parasympathetic nervous system. The vagus nerve is a main part of the parasympathetic nervous system. If the nervous system is what drives the body, the parasympathetic branch acts as the body’s ‘brakes’ and promotes the ‘rest and digest’ functions (e.g., slowing down the heart rate). It does the opposite of the sympathetic nervous system, which acts as the ‘accelerator’ of the body, promoting the ‘fight or flight’ response.⁴

Other important functions

The vagus nerve also plays a role in speech, throat reflexes such as coughing and swallowing, as well as taste and sensation within the mouth and at the back of the throat.⁵ To do this, it interacts with other cranial nerves that have similar functions, such as the 5^th cranial nerve (trigeminal) and the 9^th cranial nerve (glossopharyngeal nerve).⁶ As the vagus nerve is a mixed nerve, it can also take sensory information from various parts of the body, including general sensation from the ear, the throat (pharynx), the voice box (larynx), or information from the heart, lungs and abdominal organs, back to the brain, so it can process the information and send a signal for an appropriate motor response.¹

Control of important organ systems

Cardiovascular system

The vagus nerve is important for slowing down the heartbeat and lowering blood pressure.^7,8 Under a fast heart rate or high blood pressure, baroreceptors in the heart will send sensory information via the vagus nerve to the brain, which interprets this information. In response, the brain will send a signal down the vagus nerve to the heart, to widen blood vessels (vasodilation) reducing blood pressure for easier blood flow and to reduce the speed of electrical signals pulsating through the heart by controlling the sinoatrial node (the heart’s natural pacemaker that generates electric signals) and the atrioventricular node (the gatekeeper that controls the passage of electrical signals through the heart). Therefore, the heart rate and blood pressure will decrease to a stable level. 

Respiratory system

The vagus nerve regulates our breathing, respiratory reflexes, and also acts as a defence mechanism for the airways.^9,10 The vagus nerve receives oxygen signals from the lungs and airways and carries them to the brain, which sends a signal back to the lungs to carry out different functions like promoting exhalation, slowing down respiration to increase oxygen. Additionally, the vagus nerve can send signals to produce more mucus, narrowing the airways (bronchoconstriction) in response to allergens so that it can trap any particles and clear them from the airways. It is also involved in reflexes such as coughing, sneezing, and the Hering-Breuer reflex (which stops the lungs from expanding too much during deep breathing). 

Digestive system

The vagus nerve helps with the “rest and digest” responses of the body.¹¹ The vagus nerve takes information from abdominal organs to the brain, such as whether you have food in your gut or how full your stomach is, satiation, and if you are still hungry. This nerve then carries signals back to the abdomen to stimulate the production of digestive juices within the stomach and pancreas for breaking down food.^12,13 It also helps move food along the gut by stimulating the muscles of the gut to contract (peristalsis).¹²

Clinical relevance and disorders

Damage to the vagus nerve can result in autonomic dysfunction, including conditions such as gastroparesis, acid reflux, vasovagal syncope, and abnormal reflexes, including swallowing disorders (dysphagia).¹ The vagus nerve may be damaged by conditions such as diabetes or due to postsurgical or postinfectious causes. Gastroparesis is the slowing down of digestion and the movement of food through the stomach due to weaker stomach contractions.¹⁴ Vasovagal syncope is when your heart rate and blood pressure drop suddenly, due to your vagus nerve overreacting, causing you to lose consciousness briefly due to low blood pressure. The vagus may overreact due to stress, anxiety, fatigue, or even from seeing blood and needles.

Current research and therapeutic use

VNS (vagus nerve stimulation) involves stimulating the nerve via means such as implanting a small device under the skin in the chest with a lead connected to the vagus nerve. ¹⁵ It acts as a therapeutic tool for conditions such as epilepsy and depression.³ By stimulating the vagus nerve, there is an acceleration in electrical activity in the brain, which helps to reduce how often and how severe seizures occur.¹⁶  

There are also other investigations being carried out testing VNS therapy for other mental health disorders like post-traumatic stress disorder (PTSD), chronic inflammatory conditions like asthma and inflammatory bowel disease (IBD), cardiovascular disorders like cardiac arrest and strokes,  autoimmune disorders (e.g., rheumatoid arthritis), cluster headaches, and chronic pain therapy.¹⁷ This is due to the emerging evidence of the vagus nerve and its role in inflammation, immune system regulation, and its role as a connection between the brain and the gut (gut-brain axis).^18,19 There are also studies showing that the gut can stimulate the vagus nerve and influence brain conditions such as mood and anxiety disorders. There is also evidence that gut bacteria affect the activity of the vagus nerve and help reduce anxiety and improve your mood.¹⁹

Summary

The vagus nerve (cranial nerve X) can rapidly sense and respond to physiological changes, which makes it an important component for the autonomic control of organ systems. Its wide distribution allows it to take information from various important organ systems to the brain and bring information from the brain back to influence their functions, such as pumping blood through the heart, breathing, and digestion.

If this nerve is damaged (e.g., due to diabetes or surgery), autonomic control will be distorted, causing conditions such as gastroparesis (slower food digestion) and vasovagal syncope (fainting due to low blood pressure). Vagal nerve stimulation involves implanting a small device that stimulates the vagus nerve, preventing severe and frequent seizures and mood/anxiety disorders.