(from Latin, vagus: wandering) The vagus nerve, the tenth cranial nerve (see CRANIAL NERVES), is named for its extensive distribution and many sensory and motor branches in the neck, thorax and abdomen. It is the primary brainstem nerve controlling the upper gastrointestinal tract, respiratory system and the heart. As well, the vagus nerve innervates the pancreas, liver and gall bladder. Functionally, it is involved in the coordination of swallowing, respiration, cardioinhibition, and secretomotor function of the DIGESTIVE SYSTEM. The vagus nerve has two sensory ganglia, containing PSEUDOUNIPOLAR NEURONS, located immediately external to jugular foramen via which the nerve reaches the periphery. The superior JUGULAR GANGLION contains general somatic afferent neurons that innervate the DURA MATER (see MENINGES) and skin of the ear and external acoustic meatus. Neurons of the inferior NODOSE GANGLION provide general visceral afferent input from the upper alimentary tract (oesophagus, stomach, small intestine), respiratory system (larynx, trachea, lungs) and heart. Vagal sensory information from CHEMORECEPTOR, BARORECEPTOR and MECHANORECEPTOR systems is relayed primarily to the nucleus of the solitary tract and area postrema in the BRAINSTEM. A few special visceral afferent neurons relay taste information from the epiglottis.
General visceral efferent (PARASYMPATHETIC) axons originate mainly in the dorsal motor nucleus of the vagus nerve which has a longitudinal columnar viscerotopic organization. The parasympathetic preganglionic neurons project to parasympathetic ganglia in most of the viscera that have vagal sensory innervation. Preganglionic neurons in the dorsal motor nucleus of the vagus also project to interlobular ganglia of the pancreas to regulate insulin secretion. Special visceral efferent (branchiomotor) neurons originate in the NUCLEUS AMBIGUUS and project to the striated muscles of the pharynx, larynx and esophagus. The nucleus ambiguus also contains preganglionic cardioinhibitory neurons that course in cardiopulmonary nerves to intrinsic cardiac ganglia on the heart. Their function is to reduce heart rate and force of contraction of the myocardium. The exact location of cardiac preganglionic neurons remains somewhat controversial but the preponderance of evidence indicates that they originate primarily in the external formation of the nucleus ambiguus and, possibly, to a lesser extent, in the dorsal motor nucleus of the vagus nerve. Vagal motor neurons serve as a final common pathway for autonomic regulation, especially subserving visceral responses to EMOTION and PAIN. The vagus nerve also mediates satiety signals and sensations of gastric pain. Fever and sickness behaviour induced by activation of the IMMUNE SYSTEM by proinflammatory cytokines are mediated by vagal afferents associated with intra-abdominal vagal paraganglia.
DAVID A.HOPKINS
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