Vague Nerve

Generality

The vagus nerve, or pneumogastric nerve, is one of the twelve cranial nerves of the human being, to be precise the tenth.

Present in pairs (or pairs), the cranial nerves are fundamental nerve structures that arise at the level of the brain and can have a sensitive, motor or both (therefore mixed) function.

From the site: neurowiki2013.wikidot.com

The vagus nerve is the main representative of the nerve fibers that make up the parasympathetic nervous system: according to some studies, it would constitute about 75% of the latter.

Inside the human body, it goes a long way: from its birth in the medulla, it goes through the jugular hole, passes through the neck, goes down into the chest and, from here, reaches the abdomen.

In its course towards the abdomen, it establishes numerous innervations: with the external auditory canal, with the trachea, with the stomach, with the lungs, with the stomach, with the intestine etc.

The functions performed are obviously parasympathetic.

Short review of what a nerve is

To fully understand what a nerve is, it is necessary to start from the concept of neuron .

Neurons represent the functional units of the nervous system . Their task is to generate, exchange and transmit all those (nervous) signals that allow muscle movement, sensory perceptions, reflex responses and so on.

Typically, one neuron consists of three parts:

• A body, where the cell nucleus resides.
• The dendrites, which are equivalent to receiving antennas of nerve signals generally coming from other neurons.
• Axons, which are cellular extensions having the function of spreading the nervous signal. The axon covered with myelin (myelin sheath) is also called nerve fiber .

A bundle of axons forms a nerve .

Nerves can carry information in three ways:

• From the central nervous system (CNS) to the periphery . The nerves with this property are called efferent. The efferent nerves control the movement of the muscles, so I am in charge of the motor sphere.
• From the periphery to the CNS . Nerves with this capacity are called afferent. The afferent nerves signal to the CNS what they detected in the periphery, therefore they cover a sensitive (or sensory) function.
• From the SNC to the periphery and vice versa . Nerves with this double capacity are called mixed. Mixed nerves have a dual function: motor and sensory.

What is the vagus nerve?

The vagus nerve, or pneumogastric nerve, is the tenth pair of cranial nerves of the twelve total, present in the human being, and the principal representative of the nerve fibers that constitute the parasympathetic nervous system .

From the site: Mosby's Medical Dictionary, 8th edition. © 2009, Elsevier

It is a mixed nerve, so it has a motor function and a sensory function.

WHAT ARE THE CRANIAL NERVES?

Cranial nerves (or encephalic nerves ) are a group of nerves that originate from some structures of the brain and that can carry motor, sensory or mixed information.

Present in pairs, they are altogether twelve pairs. To identify each pair, are the Roman numerals I to XII.

Except for the I and II pair - which arise, respectively, from the telencephalon and diencephalon - the remaining ten pairs emerge from the brainstem .

Through their axons, the cranial nerves establish contact with the musculature, the glands and the sense organs of the head and neck.

Within this framework, the vagus nerve (or X pair ) is an exception: unlike the other cranial nerves it makes contact with different thoracic and abdominal organs.

Please note: in medical language, it is customary to refer to the various pairs of existing cranial nerves with singular names. Thus, the use of the term "vagus nerve" to indicate the X pair of cranial nerves is quite normal, as is the use of the term "olfactory nerve" to refer to the first pair of cranial nerves and so on.

This is not a mistake, but a simple convention, which aims to facilitate the study of these important nervous structures.

WHAT IS THE PARASIMPATIC NERVOUS SYSTEM?

The parasympathetic nervous system constitutes, together with the sympathetic nervous system, the so-called autonomous or vegetative nervous system ( SNA ), which carries out a control action on the involuntary control functions.

Originating in the central nervous system (some of its nerves originate from the brainstem, others from the spinal cord), the parasympathetic nervous system fulfills several functions: it stimulates rest, relaxation, rest, digestion and energy storage.

As can be appreciated from the figure below, he presides over the adaptation system known as " rest and digestion " (in English is " rest and digest "), which characterizes the moments of routine and the quiet activities of everyday life.

The stress of the parasympathetic nervous system can involve: the narrowing of the pupil (miosis), the increase of digestive secretions (from salivary, gastric and biliary to enteric and pancreatic ones), the increase of peristaltic activity (promoting the motility of the intestinal wall), decrease in heart rate, contraction of bronchial muscles (ie bronchi), relaxation of the bladder sphincter (thus facilitating urination), dilation of blood vessels present on skeletal muscles and stimulation of erection.

All this means that the nerve fibers of the parasympathetic nervous system are distributed to the eyes, to the salivary glands, to numerous abdominal organs (stomach, intestine, liver etc.), to the heart, to the lungs, to the blood vessels and to the genital organs.

From a comparison between the parasympathetic and the sympathetic system, one can immediately notice that the latter carries out an activity diametrically opposed to the first, since it has stimulating, exciting and contracting functions (after all it presides over the adaptation system known as "attack and Run ").

ORIGIN OF THE NAME

The term "vague" derives from the Latin word " vagus ", which means "vagabond", "wandering". Probably, the doctors coined this name in reference to the long and intricate path that the vagus nerve takes within the human body.

The reason behind the use of the term "pneumogastric" is definitely different. This adjective refers to two important innervations of the vagus nerve: the innervation of the lungs ("pneumo") and the innervation of the stomach ("gastric").

Anatomy

The vagus nerve emerges from the medulla oblongata (NB: it is one of the three sections of the brain stem), precisely in the area between the bulbar pyramids and the inferior cerebellar peduncle.

From here, it extends through the jugular hole, penetrates into the so-called carotid sheath of the neck (which contains the common carotid artery and the internal jugular vein) and reaches, in sequence, the viscera of the thorax and those of the abdomen .

Along its way it gives rise to different nerve branches, which innervate various organs and tissues including: skin of the external auditory canal, mucosa and muscles of pharynx and larynx, trachea, bronchi, lungs, heart, large blood vessels, esophagus, stomach and intestine.

The nerve branches - which are to all effects of the nerves - take the name of branches, when they contribute to the formation of more complex nervous structures, called plexuses .

In the plexuses, the axons (or fibers) of other important nerves of the human body flow together.

SPECIAL FEATURES? OF THE RIDE OF THE NERVO VAGO OF RIGHT AND OF THE NERVO VAGO OF LEFT

Starting from the base of the neck (therefore also at the thoracic and abdominal level), the right vagus nerve and the left vagus nerve follow a different course, which distinguishes them from one another.

Briefly, the main differences are:

• Between neck and chest

  The right vagus nerve resides: behind the anastomosis between the internal jugular vein and the subclavian vein; in a medial position with respect to the pleural dome (ie the parietal pleural tract that covers the apices of the lungs); in a lateral position with respect to the common carotid artery; in front of the subclavian artery.

  The vagus nerve on the left resides: behind the brachiocephalic venous trunk; in a medial position with respect to the pleural dome, the subclavian artery and the phrenic nerve; in a lateral position to the common carotid artery.

• Chest area

  The right vagus nerve resides: behind the brachiocephalic venous trunk, the superior vena cava and the anonymous artery; in a medial position with respect to the mediastinal pleura (the mediastinal pleura); in a lateral position with respect to the trachea and the laterotracheal lymph nodes; behind the esophagus (glued to the latter through the orifice, oesus, esophageal).

  The vagus nerve on the left resides: behind the brachiocephalic venous trunk and the common carotid artery; in front of the left subclavian artery and the esophagus; in a lateral position with respect to the trachea; in a medial position with respect to the mediastinal pleura and the phrenic nerve; in front of the esophagus (with the latter passing through the orifice, or esophageal orifice).

• Abdomen area

  The right vagus nerve passes behind the cardia - the valve that separates the esophagus from the stomach - and ends at the celiac plexus, which resides near the celiac artery.

  The left vagus nerve travels anteriorly to the abdominal portion of the esophagus, then crosses the edge of the cardia and, finally, ends with the branches of the anterior gastric plexus and the hepatic branches.

NERVO VAGO DIRAMAZIONI: NERVI, BRANCHE AND PLESSI

At the level of the jugular hole, there is a single branch: the so-called auricle . This innervates the skin of the external auditory canal.

At the level of the neck, the branches are: the pharyngeal branches (which contribute to the formation of the pharyngeal and intercarotid plexus ), the superior laryngeal nerve (which forms the upper laryngeal branch and the inferior laryngeal branch), the recurrent laryngeal nerve (also called inferior ) and the superior cardiac nerve (which gives rise to 2-3 branches).

At the level of the thorax, the branches are: the inferior cardiac branch (the right and left branches converge in the so-called cardiac plexus ), the anterior bronchial branches (which contribute to the formation of the anterior pulmonary plexus ), the posterior bronchial branches (which contribute to to the posterior pulmonary plexus ) and the anterior and posterior esophageal branches (ending in the esophageal plexus ).

At the level of the abdomen, the branching system is quite complicated; in fact it includes:

• The gastric branches of the right vagus nerve, which give rise to the posterior gastric plexus, and the gastric branches of the left vagus nerve, which form the anterior gastric plexus .
• The celiac branches, which mainly originate from the right vagus nerve and form the so-called celiac plexus (or solar plexus ).
• The hepatic branches, which derive mainly from the left vagus nerve and give rise to the so-called hepatic plexus .

Salient features of the vagus nerve in brief.

• The vagus nerve represents the longest cranial nerve of the human being and the only one that innervates thoracic and abdominal organs.
• The vagus nerve comprises almost 75% of all parasympathetic fibers; belongs to the category of mixed nerves.
• The vagus nerve is born at the length of the medulla; from here, it crosses the jugular hole and descends first into the chest and then into the abdomen.
• With its nerve fibers, the vagus nerve innervates: the external auditory canal, pharynx, larynx, trachea, bronchi, lungs, heart, large blood vessels, esophagus, stomach, spleen, pancreas, stomach and intestine. In some of these anatomical elements, it has an exclusively sensitive function; in others, only motor; in others again, both sensitive and motor.
• The main functions of the vagus nerve are: increased digestive secretions (including salivary, gastric, pancreatic, biliary and enteric), decreased heart rate (bradycardia), increased activity of peristalsis (especially in the intestine), contraction of the bronchial muscles and dilation of the innervated arterial vessels (carotids, aorta, etc.).

Functions

The vagus nerve is, to be precise, a mixed somatic and visceral nerve .

In fact, it has: groups of sensitive nerve fibers connected to external anatomical elements (such as the skin of the external auditory canal); groups of sensitive nerve fibers that carry information starting from different viscera of the human body present between the neck and abdomen; and, finally, groups of motor nerve fibers that innervate the voluntary and involuntary musculature of numerous viscera present between the neck and abdomen.

Doctors and neurology experts (the medical branch that deals with the nervous system) considered it appropriate to divide the various axons of the vagus nerve into five main categories, shown in the table below:

EFFECTS OF VAGUE NERVE

In light of what has been stated so far, it is now possible to describe more easily the parasympathetic effects of the vagus nerve.

This long nervous structure, once stimulated, can determine the increase of digestive secretions (including the salivary, gastric, pancreatic, biliary and enteric), the decrease in heart rate ( bradycardia ), the increase in peristalsis activity ( especially in the intestine), the contraction of the bronchial muscles and the dilation of the innervated arterial vessels (carotids, aorta, etc.).

NERVO VAGO E CUORE

The vagus nerve mediates the entire parasympathetic activity of the heart.

Through the lower right cardiac branch, it innervates the atrial sinus node (SA node), which, among the heart marker centers, is the most important (dominant center marker).

By means of the lower left cardiac branch, on the other hand, it innervates the atrioventricular node, a pedestal center of the secondary heart (therefore less important than the previous one).

The balanced stimulation of the lower cardiac branches is essential for the heart rate not to suffer an excessive reduction.

In fact, following their hyperstimulation - in particular the lower left cardiac branch - it is possible that an atrioventricular block condition is established.

The vagus nerve and the discovery of acetylcholine

The study of cardiac activity in relation to vagus nerve stimulation led to the discovery, by a German pharmacologist named Otto Loewi, of a now known chemical neurotransmitter: acetylcholine .

With his experiments (which allowed him to also obtain the Nobel Prize), Loewi showed that:

• The vagus nerve induces bradycardia, through the release of acetylcholine.
• Some acetylcholine receptor inhibitors (muscarinic receptors) interrupt various functions of the vagus nerve, including the bradycardia one.