Vagus Nerve Anatomy & Physiology and why it is important to know:
The vagus nerve is the tenth of our twelve cranial nerves, often referred to as CNX. It makes up over 80% of our parasympathetic nervous system! The vagus innervates and receives messages from nearly every major organ in the body, including the heart, lungs and most of the gastrointestinal tract.
And over 80% of the information communicated on the vagus nerve is afferent. What this means is the most important thing to our parasympathetic nervous system is the state of our organs which is sent along these afferent pathways from the receptors within the organs. This matters because if we understand the function and pathway of the vagus nerve we can influence it and therefore stimulate vagal tone and the parasympathetic nervous system.
First, is a basic overview of the vagus nerve anatomy for those of you who are not well versed in human biology.
For those of you with extensive training in A&P (doctors, therapists, etc.) please see below, "In-depth Vagus nerve A&P".
Basic overview of vagus nerve A&P:
Vagus nerve is the longest nerve in the human body, and you have two of them, one on each side. It starts in the brain stem (top of the spinal cord, just inside the skull) and travels down to the gastro-intestinal system. Unlike what we think of as a "traditional nerve", the vagus nerve does not travel in the spinal cord, it goes from the brainstem directly to the organs it will affect.
After exiting the skull, the vagus nerve travels toward the chest with the carotid arteries. On the way to the chest the chest the vagus nerve provides motor innervation (ability to move) to muscles of the palate, throat and vocal cords.
In the chest the vagus nerve does parasympathetic innervation (slowing) of the heart and lungs. It then goes down to the diaphragm and into the gastro-intestinal system where it does parasympathetic innervation of everything from the stomach to nearly the end of the large intestine (not the final parts). Vagus also innervates all the organs and glands affecting digestion.
In-Depth Vagus Nerve A&P:
Vagus nerve originates in medulla just caudal to the glossopharyngeal nerve (CNIX) as 8-10 rootlets from the dorsal olivary sulcus of the medulla. In the cranium auricular branches send off to supply sensory innervation to the posterior portion of the external auditory meatus & external ear and meningeal branches go to supply the meninges of the posterior cranial fossa.
The vagus nerve exits skull via jugular foramen along with the glossopharyngeal & accessory (CNXI) nerves and jumps into the carotid sheath to travel inferiorly with the internal jugular vein and common carotid artery on its way to the thorax.
Through the neck there are several branches given off. Branches of the vagus nerve in neck:
Pharyngeal Nerve - Branch of vagus Nerve:
The pharyngeal nerve enters the pharynx and splits into the pharyngeal plexus. It gives motor innervation to the following muscles of the pharynx, tongue & soft palate;
Pharynx muscles; Superior, middle and inferior pharyngeal constrictor muscles as well as the longitudinal pharyngeal muscles to perform peristalisis for swollowing. The stylopharyngeous muscle is not innervated by vagus, but rather glossopharyngeal nerve (CNIX).
Tongue muscles; only the palatoglossus muscle. The other tongue muscles are innervated by hypoglossal nerve (CNXII). The palatoglossas muscle acts to raise the posterior portion of the tongue.
Palate muscles; palatophayngeous, salpingopharyngeous, levator veli palatini & the uvula. The only palate muscle not innervated by vagus nerve is tensor veli palatini, innervated by a branch of trigeminal nerve (CNV). These muscles help coordinate movement of the palate for swallowing, breathing and speech as well as opeing of the eustcian tube.
Superior laryngeal nerve - Branch of Vagus Nerve:
The superior laryngeal nerve splits into two branches of its own; internal and external. The internal branch pierces the the thyrohyoid membrane to provide sensory innervation to the laryngeal structures above the true vocal folds. The external laryngeal branch runs inferiorly to innervate the cricothyroid muscle of the larynx (aids in phonation) and inferior constrictor muscle (swallowing).
Vagus may also play a role in sensory innervation to the chemoreceptors of the carotid body. As well branches of vagus may innervate the carotid sinus and baroreceptors at the carotid bifurcation to the internal & external carotid arteries.
Recurrent laryngeal nerve - Branch of Vagus Nerve:
The recurrent laryngeal nerve is going to differ in its path from left side to right sid eof the human body. On the RIGHT side only, the recurrent laryngeal nerve hooks under the right subclavian artery, going from anterior to posterior, and falls into a groove between the trachea and the esophegus. It then ascends to the larynx (voice box) to provide motor innervation to 8 of the 9 intrinsic muscles of the larynx on the right. These muscles control the shape of the rima glottids (open between true vocal cords and arytenoid cartilages) and length & tension of the vocal folds.
The left recurrent laryngeal nerve wraps around the arch of the aorta from anterior to posterior, lateral to ligament arteriosum and goes superiorly in the left tracheo-esophageal groove to the larynx to supply 8 of 9 laryngeal muscles on the left side.
As the vagus nerve enters the thorax at the base of the neck the right and left sides differ their paths here as well. The right side passes anterior to subclavian artery & posterior to the sternoclavicular joint as it goes into the thorax. The left vagus nerve passes inferiorly between the left common carotid artery and the left subclavian arteries posterior to the sternoclavicular joint and enters the thorax.
In the thorax the right vagus nerve forms the posterior vagal trunk and the left vagus nerve forms the anterior vagal trunk. Branches from each contribute to formation of the cardiac, pulmonary and esophegeal plexuses.
Cardiac Branches of the Vagus Nerve:
Go to innervate the heart, regulate heart rate and provide visceral sensation to heart. Vagal control of the heart rate is done through the sino-atrial node. When properly functioning the heart rate should be 60-80 beats per minute.
Left vagus nerve follows the aorta posteriorly behind the left lung to participate in the pulmonary plexus. Then goes medial to meet the esophagus and helps form the esophageal plexus. The anterior esophageal plexus forms the anterior vagal trunk inferiorly and then exits the thorax via the esophageal hiatus, riding the anterior esophagus.
Right vagus goes inferior from the right recurrent laryngeal nerve split at the right subclavian artery and contributes to the superficial & deep cardiac plexuses, then runs posterior to the root of the right lung. Several branches go to the posterior pulmonary plexus. The right vagus then travels medially to the posterior esophagus to form posterior esophageal plexus, and on to form the posterior Vagal trunk. It finally exits the thorax via esophageal hiatus riding the posterior aspect of the esophagus.
Vagus Nerve in the Abdomen:
After entering into the abdomen via the esophageal hiatus in the diaphragm the vagus nerve terminates into branches supplying parasympathetic innervation to the stomach, esophagus, small intestine and large intestine (everything prior to the splenic flexure). In addition to the gastro-intestinal tract, vagus nerve also supplies parasympathetic function to several organs in the abdomen, including; kidneys, liver, spleen, pancreas, gall bladder and ureters. The remaining abdominal organs, mainly those of reproduction, the remainder of the GI tract and the bladder, get their parasympathetic innervation from the pelvic splanchnic nerves, arising from the sacrum.
The vagus Nerve is what is being measured with Heart Rate Variability (HRV). This is referred to as Vagal Tone. The Vagus nerve controls approximately 80% of our parasympathetic nervous system.
To truly understand the function of the vagus nerve, what affects it, how it effects our health & function and how to modulate vagal tone; it is important to understand the anatomy of the Vagus nerve.
Now take a look at how we can stimulate each part of the vagus nerve found on the "How to Improve HRV" page.