There are twelve pairs of cranial nerves in the human body, all of which connect the important systems to the brain. The function performed by each nerve is different. The vagus nerve is one of these cranial nerves. It is the 10th cranial nerve in the body, and has numerous functions. The vagus nerve is also known by the names of cranial nerve X and pneumatic nerve, as it is also found in the stomach and the lungs. It runs from the brain, throat, thorax, lungs, heart, stomach, abdominal muscles, to various other organs. The vagus nerve sends signals throughout the body, and then transmits them back to the brain. It triggers off the parasympathetic nervous system (PNS), and assists in maintaining the health of the immune cells, organs, tissues, and stem cells. It regulates heartbeat, speech, perspiration, blood pressure, process of digestion, production of glucose, and breathing. In addition to giving output to various organs, it makes up around 90% of afferent nerves that are particularly transmitting sensory information about the condition of the body's organs to the central nervous system. The vagus nerve initiates from the rootlets in the medulla of the brain stem. It comes out from the cranium through the jugular foraman, along with the pharyngeal and accessory nerves, and joins the viscera. The nerve assists in communication between the brain and the viscera. Inside the jugular foramen are the jugular and the noose sensory ganglia. The vagus nerve that runs through the side of the skull is the auricular branch. The motor neurons of the vagus nerve supplies nerves to the throat, larynx, pharynx, and esophagus. These nerves help in swallowing, speaking, and coughing. The vagus nerve is responsible for the creation of new brain neurons, and raised brain-derived neurotic factor (BDNF) which is a good food for the brain cells. It helps with the mending of brain tissues. Additionally, triggering the vagus nerve can excite the stem cells to give rise to new cells. This nerve controls the parasympathetic nervous system. This nervous system uses the neurotransmitter oxyacetylene. Oxyacetylene brings about learning, memory, and relaxation. It is used by the vagus nerve to transmit messages of relaxation all around the body. Arousing the vagus nerve sends oxyacetylene throughout the body, thereby relaxing the body and lowering inflammation related to the effects from stress. The vagus nerve comes down vertically inside the carotid sheath to the internal carotid arteries and towards the middle of the internal jugular vein at the root of the neck. Here, the right and left nerves travel through different routes. The right vagus nerve goes through the anterior of the subclass artery as it comes in the thorax. The left one comes down and travels between the left carotid and left subclass arteries, and comes into the thorax. Various branches form in the neck.

• Pharyngeal Branches - They innervate the pharynx and soft palate muscles.
• Superior Laryngeal Nerve - It divides into internal and external branches. The internal branch supplies nerves to the pharyngeal and upper region of the larynx. The external branch supplies nerves to the hypothyroid muscle of the larynx
• Recurrent Laryngeal Nerve - The first division of the vagus nerve is the recurrent laryngeal nerve, which is responsible for innervating the intrinsic laryngeal muscular structure. This nerve lies in between the trachea and esophagus. It descends and splits into left and right parts. The left branch goes with the ligamentum arteriole, and the right goes with the subclass artery. The recurrent laryngeal nerve supplies nerves to most of the larynx muscles. For swallowing, it helps produce vocal cord motion in the food passage, and allows glottis shutdown at the time of cough reflex. Injury to the recurrent nerve leads to vocal cord paralysis.

The pulmonary branches of the vagus nerve can be fractionated into the anterior and posterior regions. The anterior branches run through the anterior surface of the base of the lungs. They link with strands from the sympathetic and form the anterior pulmonary plexus. The posterior branches run along the posterior surface of the base of the lungs. They are connected by the third and fourth thoracic ganglia of the sympathetic trunk to build the posterior pulmonary plexus. Branches from the pulmonary plexus join the branches of the bronchi through the lungs. Involuntary functions of the lungs are governed by the branches of the vagus nerve and other nerves. The nerve unfolds the larynx while inhaling and travels through the muscles of the mouth to enable speech. In the lungs, it constricts the bronchi by causing the muscles to stiffen. One branch of this nerve governs the muscles that move the vocal cords inside the larynx. Injury to this nerve can lead to gruffness of voice. In the thorax, the right vagus nerve creates the posterior vagal trunk, while the left creates the anterior vagal trunk. There are two more branches present. The first is the left recurrent laryngeal nerve, which is found below the aorta. It supplies nerves to the larynx muscles. The second is the cardiac branches which innervates the heart. Parasympathetic fibers for the heart are partly controlled by the vagus nerve and partook by the thoracic ganglia. The right vagus nerve supplies nerves to the SA node, while the left innervates the AV node. The vagal efferent also supply nerves to the atrial muscle. However, the ventricular myocardial is only thinly innervated by vagal efferent. The vagus nerve helps in monitoring and maintaining heartbeat. They are always dynamic, creating a rhythm of about 90 beats per minute. If necessary, neurotransmitters are secreted by the nerve, which helps in reducing the intensity of heartbeat, or decreasing blood pressure. The vagus nerve frames the esophageal plexus. It goes through the diaphragm and enters the abdominal viscera. It forms the celiac, mesenteric, and centering plexuses. It then approaches the stomach to produce hepatic branches and nerves of Latarjet, which innervate the pyloric tantrum. The vagus nerve provides parasympathetic nerves to most of the abdominal organs. It carries branches to the esophagus, stomach, and intestines. This nerve assists in the complex procedures in the digestive system, like sending signals to the stomach muscles for compressing food and thrusting it into the small intestine. In case the vagus nerve is injured, the food may stay in the stomach instead of moving down the intestine, which will affect digestion. The nerve also aids in governing the chemical levels in the digestive system, so that the intestines can work on the food and determine the nutrient intake. Also, when the stomach is full, this sensation is carried to the brain by the vagus nerve. It also helps sense taste and hunger. In diabetic patients, when the sugar levels are high and not controlled, it can cause vagus nerve injury. The vagus nerve can be tested by stimulating the pharyngeal reflex. If the lateral side of the pharynx is knocked, the muscles of the pharynx will squeeze, and can lead the patient to choke. While testing the soft palate innervation, tell the patient to say 'ah'. The soft palate must go up and the uvula must go back. If the nerve has a wound on it, the soft palate and the uvula will not move evenly, and deviate from that damaged part. An inexpensive way of triggering the vagus nerve is to simply inhale. When you take a deep breath through the mouth, loosen and inflate your diaphragm. As a result, the vagus nerve is activated. Following this, exhale through the nose, and you experience the stress going away from your body. The brain heals and you feel relaxed. Activation of the nerve can also reduce inflammation, sharpen memory, regenerate organs and cells, thicken the brain, and boost the immune system. Disorders of the vagus nerve can be divided into two classes: disorders caused by nerves that are hypo-active or non-functioning, and disorders caused by the vagus nerve. An overactive vagus nerve leads to swooning. An under active nerve can cause nausea, pyrolysis, stomach aches, weight loss, and can also lead to a dipping heart rate. On a concluding note, moving away from core medical-based observations, the vagus nerve is the one that makes us feel good while hugging someone. It is also responsible for that fuzzy feeling we experience inside when we witness some occurrence that moves us emotionally.