The vagus nerve, also known as the tenth cranial nerve, cranial nerve X, or simply CN X, is a cranial nerve that interfaces with the parasympathetic control of the heart, lungs, and digestive tract. It comprises two nerves—the left and right vagus nerves—but they are typically referred to collectively as a single subsystem. The vagus is the longest nerve of the autonomic nervous system in the human body and comprises both sensory and motor fibers. The sensory fibers originate from neurons of the nodose ganglion, whereas the motor fibers come from neurons of the dorsal motor nucleus of the vagus and the nucleus ambiguus. The vagus was also historically called the pneumogastric nerve.
The autonomic nervous system (ANS), formerly referred to as the vegetative nervous system, is a division of the peripheral nervous system that supplies smooth muscle and glands, and thus influences the function of internal organs. The autonomic nervous system is a control system that acts largely unconsciously and regulates bodily functions, such as the heart rate, digestion, respiratory rate, pupillary response, urination, and sexual arousal. This system is the primary mechanism in control of the fight-or-flight response.
The parasympathetic nervous system is one of the three divisions of the autonomic nervous system, the others being the sympathetic nervous system and the enteric nervous system. The enteric nervous system is sometimes considered part of the autonomic nervous system, and sometimes considered an independent system.
The enteric nervous system (ENS) or intrinsic nervous system is one of the main divisions of the autonomic nervous system (ANS) and consists of a mesh-like system of neurons that governs the function of the gastrointestinal tract. It is capable of acting independently of the sympathetic and parasympathetic nervous systems, although it may be influenced by them. The ENS is nicknamed the "second brain". It is derived from neural crest cells.
The medulla oblongata or simply medulla is a long stem-like structure which makes up the lower part of the brainstem. It is anterior and partially inferior to the cerebellum. It is a cone-shaped neuronal mass responsible for autonomic (involuntary) functions, ranging from vomiting to sneezing. The medulla contains the cardiac, respiratory, vomiting and vasomotor centers, and therefore deals with the autonomic functions of breathing, heart rate and blood pressure as well as the sleep wake cycle.
The glossopharyngeal nerve, also known as the ninth cranial nerve, cranial nerve IX, or simply CN IX, is a cranial nerve that exits the brainstem from the sides of the upper medulla, just anterior to the vagus nerve. Being a mixed nerve (sensorimotor), it carries afferent sensory and efferent motor information. The motor division of the glossopharyngeal nerve is derived from the basal plate of the embryonic medulla oblongata, whereas the sensory division originates from the cranial neural crest.
The somatic nervous system (SNS), or voluntary nervous system is the part of the peripheral nervous system associated with the voluntary control of body movements via skeletal muscles.
The pharyngeal reflex or gag reflex is a reflex muscular contraction of the back of the throat, evoked by touching the roof of the mouth, the back of the tongue, the area around the tonsils, the uvula, and the back of the throat. It, along with other aerodigestive reflexes such as reflexive pharyngeal swallowing, prevents objects in the oral cavity from entering the throat except as part of normal swallowing and helps prevent choking, and is a form of coughing. The pharyngeal reflex is different from the laryngeal spasm, which is a reflex muscular contraction of the vocal cords.
A reflex arc is a neural pathway that controls a reflex. In vertebrates, most sensory neurons do not pass directly into the brain, but synapse in the spinal cord. This allows for faster reflex actions to occur by activating spinal motor neurons without the delay of routing signals through the brain. The brain will receive the input while the reflex is being carried out and the analysis of the signal takes place after the reflex action.
In the human brainstem, the solitary nucleus, also called nucleus of the solitary tract, nucleus solitarius, and nucleus tractus solitarii, is a series of purely sensory nuclei forming a vertical column of grey matter embedded in the medulla oblongata. Through the center of the SN runs the solitary tract, a white bundle of nerve fibers, including fibers from the facial, glossopharyngeal and vagus nerves, that innervate the SN. The SN projects to, among other regions, the reticular formation, parasympathetic preganglionic neurons, hypothalamus and thalamus, forming circuits that contribute to autonomic regulation. Cells along the length of the SN are arranged roughly in accordance with function; for instance, cells involved in taste are located in the rostral part, while those receiving information from cardio-respiratory and gastrointestinal processes are found in the caudal part.
The nucleus ambiguus is a group of large motor neurons, situated deep in the medullary reticular formation named by Jacob Clarke. The nucleus ambiguus contains the cell bodies of neurons that innervate the muscles of the soft palate, pharynx, and larynx which are associated with speech and swallowing. As well as motor neurons, the nucleus ambiguus contains preganglionic parasympathetic neurons which innervate postganglionic parasympathetic neurons in the heart.
The myenteric plexus provides motor innervation to both layers of the muscular layer of the gut, having both parasympathetic and sympathetic input, whereas the submucous plexus has only parasympathetic fibers and provides secretomotor innervation to the mucosa nearest the lumen of the gut.
The accommodation reflex is a reflex action of the eye, in response to focusing on a near object, then looking at a distant object, comprising coordinated changes in vergence, lens shape (accommodation) and pupil size. It is dependent on cranial nerve II, superior centers (interneuron) and cranial nerve III. The change in the shape of the lens is controlled by ciliary muscles inside the eye. Changes in contraction of the ciliary muscles alters the focal distance of the eye, causing nearer or farther images to come into focus on the retina; this process is known as accommodation. The reflex, controlled by the parasympathetic nervous system, involves three responses: pupil constriction, lens accommodation, and convergence.
A cranial nerve nucleus is a collection of neurons in the brain stem that is associated with one or more of the cranial nerves. Axons carrying information to and from the cranial nerves form a synapse first at these nuclei. Lesions occurring at these nuclei can lead to effects resembling those seen by the severing of nerve(s) they are associated with. All the nuclei except that of the trochlear nerve supply nerves of the same side of the body.
The dorsal nucleus of vagus nerve is a cranial nerve nucleus for the vagus nerve in the medulla that lies ventral to the floor of the fourth ventricle. It mostly serves parasympathetic vagal functions in the gastrointestinal tract, lungs, and other thoracic and abdominal vagal innervations. These functions include, among others, bronchoconstriction and gland secretion. The cell bodies for the preganglionic parasympathetic vagal neurons that innervate the heart reside in the nucleus ambiguus.
Gastrointestinal physiology is the branch of human physiology that addresses the physical function of the gastrointestinal (GI) tract. The function of the GI tract is to process ingested food by mechanical and chemical means, extract nutrients and excrete waste products. The GI tract is composed of the alimentary canal, that runs from the mouth to the anus, as well as the associated glands, chemicals, hormones, and enzymes that assist in digestion. The major processes that occur in the GI tract are: motility, secretion, regulation, digestion and circulation. The proper function and coordination of these processes are vital for maintaining good health by providing for the effective digestion and uptake of nutrients.
The basal or basic electrical rhythm (BER) or electrical control activity (ECA) is the spontaneous depolarization and repolarization of pacemaker cells known as interstitial cells of Cajal (ICCs) in the smooth muscle of the stomach, small intestine, and large intestine. This electrical rhythm is spread through gap junctions in the smooth muscle of the GI tract. These pacemaker cells, also called the ICCs, control the frequency of contractions in the gastrointestinal tract. The cells can be located in either the circular or longitudinal layer of the smooth muscle in the GI tract; circular for the small and large intestine, longitudinal for the stomach. The frequency of contraction differs at each location in the GI tract beginning with 3 per minute in the stomach, then 12 per minute in the duodenum, 9 per minute in the ileum, and a normally low one contraction per 30 minutes in the large intestines that increases 3 to 4 times a day due to a phenomenon called mass movement. The basal electrical rhythm controls the frequency of contraction but additional neuronal and hormonal controls regulate the strength of each contraction.
The gustatory nucleus is the rostral part of the solitary nucleus located in the medulla. The gustatory nucleus is associated with the sense of taste and has two sections, the rostral and lateral regions. A close association between the gustatory nucleus and visceral information exists for this function in the gustatory system, assisting in homeostasis - via the identification of food that might be possibly poisonous or harmful for the body. There are many gustatory nuclei in the brain stem. Each of these nuclei corresponds to three cranial nerves, the facial nerve (VII), the glossopharyngeal nerve (IX), and the vagus nerve (X) and GABA is the primary inhibitory neurotransmitter involved in its functionality. All visceral afferents in the vagus and glossopharyngeal nerves first arrive in the nucleus of the solitary tract and information from the gustatory system can then be relayed to the thalamus and cortex.
A vagal maneuver is an action used to stimulate the parasympathetic nervous system by activating the vagus nerve. The vagus nerve is the longest nerve of the autonomic nervous system and helps regulate many critical aspects of human physiology, including heart rate, blood pressure, sweating, and digestion through the release of acetylcholine. Common maneuvers that activate the vagus nerve include the Valsalva maneuver and carotid sinus massage, which can serve diagnostic or therapeutic functions.
The nervous system, and endocrine system collaborate in the digestive system to control gastric secretions, and motility associated with the movement of food throughout the gastrointestinal tract, including peristalsis, and segmentation contractions.
Binder, Henry. "Organization of the Gastrointestinal System."Medical Physiology. 1st ed. 2002. Print.
