VAGUS NERVE COURSE

Vagus Nerve Course (10^th or X cranial nerve – Mixed)

Longest Cranial Nerve:
The vagus nerve is the longest among all the cranial nerves. It extends from the brainstem down to the thorax and abdomen, innervating various organs.

Origin:
The vagus nerve originates from the side (lateral aspect) of the medulla oblongata, which is a part of the brainstem.

Exits the Skull:
Along with two other cranial nerves (the 9th or glossopharyngeal nerve and the 11th or accessory nerve), the vagus nerve exits the skull through an opening called the jugular foramen.

Jugular Ganglion:
Inside the jugular foramen, the vagus nerve forms a small swelling called the jugular ganglion, which acts like a relay station for signals. This ganglion communicates with nearby cranial nerves (the 11th, 12th, and 9th nerves).

Initial Path:
After exiting the skull, the vagus nerve travels downward and slightly backward for a short distance alongside the 11th cranial nerve (accessory nerve).

Separation of Nerves:
As they descend, the vagus and accessory nerves separates from each other. The 12th cranial nerve (hypoglossal nerve) runs between them.

Vagosympathetic Trunk: 
The vagus nerve continues downward, and joins with fibers of the cervical sympathetic nerve.  These sympathetic nerve fibers originate from a structure called the cranial cervical ganglion, which is part of the autonomic nervous system. Together, they form a structure called the vagosympathetic trunk, which carries signals to and from the brain and various organs.

Path Through the Neck:
The vagosympathetic trunk runs under a protective sheath called the carotid sheath, located deep in the neck alongside the carotid artery and jugular vein. It then moves into the jugular furrow, a groove in the neck.

Separation Before the Thorax:
Just before entering the chest (thorax), the vagus nerve separates from the sympathetic nerve fibers, continuing independently to supply thoracic and abdominal organs.

Right Vagus Nerve:

1. Thoracic Pathway:
   In the chest cavity (thoracic cavity), the right vagus nerve travels underneath the right axillary artery (a major blood vessel in the shoulder region) and along the right side of the trachea (windpipe).
2. Branches:
   1. Just behind the aortic arch, it splits into two branches, called the superior and inferior branches.

Left Vagus Nerve:

1. Thoracic Pathway:
   The left vagus nerve also travels in the thorax but follows a slightly different route. It goes beneath the left axillary artery and crosses over the aorta near its origin (where it emerges from the heart).
2. Reaching the Lung:
   The left vagus reaches the hilum of the left lung, which is the central region where the bronchi, blood vessels, and nerves enter or exit the lung.
3. Branches:
   1. It divides into two branches, superior and inferior, at the upper part of the esophagus (the food pipe).

Joining and Further Pathway:

1. Esophageal Plexus Formation:
   1. The superior and inferior branches of both the right and left vagus nerves come together (but separately on each side) along the esophagus. This creates a network of nerves (a plexus) that helps control esophageal function.
2. Descent to the Abdomen:
   1. From the level of esophagus, the nerves continue downward through the caudal mediastinum (the lower part of the chest cavity) and pass through the esophageal aperture (an opening in the diaphragm).
   1. They enter the abdomen as the superior and inferior continuations of the vagus nerve, and innervates various abdominal organs.

Cervical Branches of Vagus Nerve:

(a) Auricular Nerve:

• Origin:
  This branch arises from the jugular ganglion (a small swelling of the vagus nerve located as it exits the skull).
• Connection:
  It connects with the facial nerve (cranial nerve VII).
• Function:
  It innervates to the skin behind the ear and the external auditory meatus (the canal leading to the eardrum).

(b) Pharyngeal and Esophageal Branches:

• Origin:
  These branches arise close to where the auricular nerve originates.
• Pharyngeal Plexus Formation:
  They combine with sympathetic nerve fibers to form the pharyngeal plexus.
• Function:
  This plexus provides innervation to:
  • The pharynx (muscles and mucosa of the throat).
  • The cranial part of the esophagus (the upper section of the food pipe).

(c) Cranial or Superior Laryngeal Nerve:

• Origin:
  This branch arises slightly behind the pharyngeal and esophageal branches.
• Pathway:
  • It travels along the side wall of the pharynx (throat region).
  • Runs alongside the glossopharyngeal nerve (cranial nerve IX).
  • Enters the larynx (voice box) through an opening in the thyroid cartilage called the thyroid foramen.
• Function:
  It controls parts of the larynx, contributing to voice production and swallowing.

(d) Aortic Nerve:

• Origin:
  • The left vagus nerve gives off the aortic nerve just in front of the thoracic inlet (the entry to the chest cavity).
  • The right aortic nerve arises from the right vagus nerve at the level of the 2nd rib.
• Pathway:
  The aortic nerve runs parallel to the vagus nerve trunk.
• Function:
  It provides sensory innervation to the aorta, monitoring changes in blood pressure and oxygen levels.

Thoracic branches of the vagus nerve:

(a) Recurrent Laryngeal Nerve:

• Right Recurrent Laryngeal Nerve Pathway:
  • Originates behind the first rib.
  • Loops (or “winds”) around a small artery in the chest.
  • Gives off sensory branches to the trachea (windpipe) and esophagus (food pipe).
  • Travels upward (cranially) along the ventral surface of the trachea, crossing dorsal aspect of the common carotid artery.
  • Reaches the 8th tracheal ring, moves to the back (dorsal surface) of the trachea, and continues upward.
  • Finally, it supplies the larynx (voice box) with motor and sensory fibers.
• Left Recurrent Laryngeal Nerve Pathway:
  • Originates at the 4th intercostal space (between the ribs).
  • Loops around the aortic arch (a large blood vessel emerging from the heart).
  • Runs upward along the ventral surface of the esophagus.
  • Ascends further between the trachea and esophagus.
  • Divides into 2–3 smaller branches near the 3rd tracheal ring, below the thyroid gland.
  • These branches supply the pharyngeal muscles and the larynx.

Key Function:

The recurrent laryngeal nerves are critical for controlling the muscles of the larynx (voice production) and for providing sensation to the trachea and esophagus.

(b) Cardiac Nerves:

• Right Side Pathway:
  • The right cardiac nerves branch off the right vagus nerve at the level of the 3rd rib.
• Left Side Pathway:
  • The left cardiac nerves originate from the left vagus nerve at the level of the 5th rib.
• Formation of the Cardiac Plexus:
  • These nerves from both sides join with sympathetic fibers to form the cardiac plexus, a network of nerves that controls heart functions such as heart rate and blood pressure.

(c) Pulmonary Branches:

• Origin:
  • Both the right and left vagus nerves give off pulmonary branches at the level of the tracheal bifurcation (where the trachea splits into the right and left bronchi leading to the lungs).
• Formation of the Pulmonary Plexus:
  • These branches combine with sympathetic nerves to form the pulmonary plexus.

• Function:
  • The pulmonary plexus innervates the lungs, regulating their functions, including bronchoconstriction, bronchodilation, and secretion of mucus.

Abdominal branches of the Vagus Nerve:

1. Superior Esophageal Continuation:
   1. The superior branch of the vagus nerve continues downward (caudally) along the esophagus.
   1. It communicates with the splanchnic nerve (which carries sympathetic signals to the abdomen).
   1. It forms the hepatic plexus (a network of nerves for the liver) and the reticulo-omasal plexus (nerves supplying parts of the stomach in ruminants).
   1. It sends branches to the right side of all stomach compartments, ensuring parasympathetic innervation to regulate digestive activity.
   1. Additionally, it contributes fibers to the celiac plexus, which controls abdominal organs like the pancreas, spleen, and intestines.
2. Inferior Esophageal Continuation:
   1. The inferior branch of the vagus also helps form the hepatic and reticulo-omasal plexuses.
   1. It innervates the stomach, assisting in digestion and secretion processes.
3. Other Visceral Innervations:
   1. Vagus nerve fibers extend to the mucous membrane of the intestine, aiding in intestinal motility and secretion.
   1. They also innervate organs such as the kidneys, adrenal glands, and other abdominal viscera, influencing their function in response to the body’s needs.

Effects of Lesions on the Vagus Nerve:

1. Dysphagia:
   1. Damage to the vagus nerve can lead to difficulty in swallowing due to impaired motor control of the pharyngeal and esophageal muscles.
2. Changes in Voice:
   1. The vagus innervates the larynx through its recurrent laryngeal branch. Lesions can cause hoarseness or other voice changes.
3. Palatal Movements:
   1. The vagus helps control movements of the soft palate. A lesion might result in the absence of certain movements, affecting speech and swallowing.