The vagus nerve is a mixed nerve. Its motor fiber starts from the nucleus ambiguus and runs in parallel with the glossopharyngeal nerve. After passing through the brain stem, it goes out of the skull cavity through the jugular vein hole to supply all the muscles of the pharynx, larynx and soft palate except the soft palate muscle and the stem pharynx muscle.Sensory neurons are divided into superior ganglion and inferior ganglion near jugular foramen.The peripheral branches of the superior ganglia conduct a part of the general sensation of the external auditory canal, tympanic membrane and auricle;The nucleus of the spinal tract of the brainstem, the central branch of which enters the trigeminal nerve.The peripheral branches of the inferior ganglia conduct the sensation of the pharynx, larynx, trachea, esophagus and internal organs;The central branch enters the nucleus of solitary tract.The parasympathetic nerve originates from the dorsal nucleus of the vagus nerve at the bottom of the fourth ventricle and distributes in internal organs.
containbodyMovement, visceral movement, visceral sensation and somatic sensationfibre。
1. Somatic motor fiber: the skeletal muscle that controls the throat can move at will.
2. Visceral motor parasympathetic fiber: the main component of vagus nerve, distributed in visceral organs in the chest (such as trachea, bronchus, lung, heart, etc.) and visceral organs in the abdominal cavity (such asliver, pancreas, spleen, kidney, adrenal gland and digestive tube from stomach to transverse colon) to regulate the activities of these organs.
3. Visceral sensory fibers: transmit the sensory impulses of visceral organs in the chest and abdomen.
4. Somatosensory fibers: transmit the general sensation of auricle, external auditory canal and pleura.
When the main vagus nerve is injured, the symptoms are rapid heart rate, nausea, vomiting, deep and slow breathing, etc. Due to paralysis of the pharyngeal muscles, hoarseness, language difficulties, dysphagia, etc. may occur.
Physiological characteristics
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The vagus nerve sends out multiple branches in the neck, chest and abdomen, innervating the organs in the neck, chest and most of the organs in the abdominal cavity. It regulates the three systems of circulation, respiration and digestion by conducting the sensory impulses of organs and organs and controlling the activities of myocardium, smooth muscle and glands.
（1） Circulation system:
The preganglionic fibers of the parasympathetic nerves innervating the heart walk in the vagus nerve trunk. The cell bodies of these preganglionic neurons are located in the dorsal and doubtful nuclei of the vagus nerve in the medulla oblongata.Postganglionic fibers innervate the sinoatrial node, atrioventricular junction, atrial muscle, atrioventricular bundle and its branches. The right vagus nerve mainly innervates the sinoatrial node, the left vagus nerve mainly innervates the atrioventricular junction area, and the vagus nerve also innervates the ventricular muscle, but the number of its fiber terminals is far less than that in the atrial muscle.Acetylcholine,Ach) acts on the M-type cholinergic receptor on the myocardial cell membrane, which can cause heart rate to slow down (i.e. negative frequency effect), atrioventricular node conduction to slow down (i.e. negative conduction effect), and atrial muscle contractility to weaken (i.e. negative force effect). It also has a direct inhibitory effect on ventricular muscle, but atrial muscle is more sensitive to Ach than ventricular muscle.In addition, Ach released from vagal nerve fiber ends binds to M-type cholinergic receptor of vascular smooth muscle, leading to nitric oxide release and vasodilation.The afferent nerve fibers of the aortic arch baroreceptor walk in the vagus nerve trunk, enter the medulla oblongata, and reach NTS. When arterial blood pressure rises, the afferent impulses of the baroreceptor increase. Through the related cardiovascular central integration, the cardiac vagus tension, cardiac sympathetic and sympathetic vasoconstrictor tension are reduced. The effects are slow heart rate, reduced cardiac output, and reduced peripheral vascular resistanceWhen the arterial blood pressure decreases, it will be reversed when the arterial blood pressure increases, so as to rapidly regulate the arterial blood pressure.The sensory signals of the chemoreceptors in the aorta are also transmitted to the NTS via the vagus nerve, which changes the cardiovascular activity in hypoxia, asphyxia, blood loss, arterial hypotension and acidosis.
（2） Respiratory system:
The bronchial branch of vagus nerve and sympathetic nerve together form the pulmonary plexus and send out fine branches to innervate the bronchi and lungs.The vagal nerve endings release Ach to bind to the M-type cholinergic receptors on the epithelium and secretory cells of the airway surface, causing an increase in the frequency of ciliary oscillation and airway mucus secretion, and binding to the M-type cholinergic receptors on the bronchial smooth muscle, causing bronchospasm, bronchoconstriction, and increased airway tension.In the lung, the vagus nerve participates in the dilation reflex of the lung. When the lung is dilated, the respiratory tract is stretched, the stretch receptor is stimulated, and the vagus nerve enters the medulla oblongata along the afferent impulse of the vagus nerve, accelerating the transformation of the inspiratory process into the expiratory process, and increasing the respiratory rate.In addition, the chemoreceptors in the aorta are stimulated when the arterial partial pressure of oxygen decreases, the arterial partial pressure of carbon dioxide or the concentration of hydrogen ions increases, and sensory signals are transmitted to NTS through the vagus nerve, which reflexively causes the deepening and acceleration of respiration.
（3） Digestive system:
In the neck and chest, the superior laryngeal nerve branch of the vagus nerve mainly innervates the cricothyroid muscle, tightens the vocal cords, and conducts the general visceral sensory impulses in the distribution area.The motor fibers of the recurrent laryngeal nerve branch of the vagus nerve innervate all laryngeal muscles except cricothyroid muscle. The sensory fibers are distributed in the laryngeal mucosa below the glottic fissure.The pharyngeal branch of the vagus nerve innervates the activities of the pharyngeal constrictor muscle and the soft palate muscle and the sensation of the pharyngeal mucosa.Therefore, when one side of the vagus nerve is injured, the pharyngeal reflex on the affected side may appear due to the paralysis of all the laryngeal muscles on the affected side and the sensory conduction disorder of the pharyngeal mucosa, and the cough reflex on the affected side may disappear when the throat is stimulated. The clinical manifestations are hoarseness, language disorder, dysphagia or choking.When bilateral vagus nerves are injured, dysphagia, deep and slow breathing, severe breathing difficulties or asphyxia occur.In the abdomen, the preganglionic fibers of the vagus nerve enter the gastrointestinal tissue and form synapses with the neurons of the myenteric plexus and submucosal plexus. The postganglionic fibers innervate gland cells, epithelial cells, blood vessels and smooth muscle cells of the digestive tract.Ach released from the postganglionic fiber terminals of the vagus nerve activates M-type cholinergic receptors, leading to the contraction of the digestive tract and the increase of gland secretion, while the digestive tract sphincter relaxes.
