Parasympathetic stimulation evokes a good flow of saliva. In contrast, sympathetic stimulation may produce a small flow of saliva or none at all, leading to a dry mouth. Of all the parasympathetic cranial nerves, the vagus nerve, which contains about 80 percent of all parasympathetic fibers in the body, is the most important.
Fibers from the vagus nerve pass all over the body, influencing almost every organ below the neck -- including the heart, lungs, esophagus, trachea, stomach, small intestine, first part of the colon, liver, gallbladder, pancreas and ureters. Parasympathetic activation of the vagus nerve therefore has a broad range of effects, including reducing the heart rate and blood pressure, increasing the production of stomach acid, stimulating the movement of food through the intestines and keeping the larynx open for breathing while constricting the air passages in the lungs.
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Monitor the health of your community here. More Articles. Family Health. The ophthalmic and maxillary nerves are purely sensory. The mandibular nerve has both sensory and motor functions. The three branches converge on the trigeminal ganglion that is located within the trigeminal cave in the brain; it contains the cell bodies of incoming sensory nerve fibers.
The trigeminal ganglion is analogous to the dorsal root ganglia of the spinal cord, which contain the cell bodies of incoming sensory fibers from the rest of the body. Areas of the face innervated by the trigeminal nerve : The ophthalmic nerve branch V1 innervates the bright red area, the maxillary nerve branch V2 innervates the light red area, and the mandibular nerve branch V3 innervates the yellow area.
From the trigeminal ganglion, a single large sensory root enters the brainstem at the level of the pons. Immediately adjacent to the sensory root, a smaller motor root emerges from the pons at the same level. Motor fibers pass through the trigeminal ganglion on their way to peripheral muscles, but their cell bodies are located in the nucleus of the trigeminal nerve, deep within the pons.
The sensory function of the trigeminal nerve is to provide tactile, proprioceptive, and nociceptive afferents to the face and mouth. The motor component of the mandibular division V3 of the trigeminal nerve controls the movement of eight muscles, including the four muscles of mastication: the masseter, the temporal, and the medial and lateral pterygoids.
The other four muscles are the tensor veli palatini, the mylohyoid, the anterior belly of the digastric, and the tensor tympani. With the exception of the tensor tympani, all of these muscles are involved in biting, chewing and swallowing, and all have bilateral cortical representation.
The abducens nerve cranial nerve VI controls the lateral movement of the eye through innervation of the lateral rectus muscle. The abducens nerve cranial nerve VI is a somatic efferent nerve that, in humans, controls the movement of a single muscle: the lateral rectus muscle of the eye that moves the eye horizontally.
In most other mammals it also innervates the musculus retractor bulbi, which can retract the eye for protection. Homologous abducens nerves are found in all vertebrates except lampreys and hagfishes.
Abducens nerve : Schematic of cranial nerves showing cranial nerve VI, the abducens nerve. The abducens nerve leaves the brainstem at the junction of the pons and the medulla, medial to the facial nerve. In order to reach the eye, it runs upward superiorly and then bends forward anteriorly. The nerve enters the subarachnoid space when it emerges from the brainstem. It runs upward between the pons and the clivus, and then pierces the dura mater to run between the dura and the skull.
At the tip of the petrous temporal bone, it makes a sharp turn forward to enter the cavernous sinus. In the cavernous sinus it runs alongside the internal carotid artery. It then enters the orbit through the superior orbital fissure and innervates the lateral rectus muscle of the eye. The long course of the abducens nerve between the brainstem and the eye makes it vulnerable to injury at many levels. For example, fractures of the petrous temporal bone can selectively damage the nerve, as can aneurysms of the intracavernous carotid artery.
Mass lesions that push the brainstem downward can damage the nerve by stretching it between the point where it emerges from the pons and the point where it hooks over the petrous temporal bone.
The facial nerve cranial nerve VII determines facial expressions and the taste sensations of the tongue. The facial nerve : Illustration of the facial nerve and its branches. The facial nerve is the seventh cranial nerve VII of the 12, paired cranial nerves. It emerges from the brainstem between the pons and the medulla and controls the muscles of facial expression.
It also functions in the conveyance of taste sensations from the anterior two-thirds of the tongue and oral cavity, and it supplies preganglionic parasympathetic fibers to several head and neck ganglia. The motor part of the facial nerve arises from the facial nerve nucleus in the pons, while the sensory part of the facial nerve arises from the nervus intermedius. The motor and sensory parts of the facial nerve enter the petrous temporal bone into the internal auditory meatus intimately close to the inner ear , then runs a tortuous course including two tight turns through the facial canal, emerges from the stylomastoid foramen, and passes through the parotid gland, where it divides into five major branches.
Although it passes through the parotid gland, it does not innervate the gland this is the responsibility of cranial nerve IX, the glossopharyngeal nerve. The facial nerve forms the geniculate ganglion prior to entering the facial canal.
Voluntary facial movements, such as wrinkling the brow, showing teeth, frowning, closing the eyes tightly inability to do so is called lagophthalmos , pursing the lips, and puffing out the cheeks, all test the facial nerve.
There should be no noticeable asymmetry. In an upper motor neuron lesion, called central seven central facial palsy , only the lower part of the face on the contralateral side will be affected due to the bilateral control to the upper facial muscles frontalis and orbicularis oculi. This can be tested with a swab dipped in a flavored solution, or with electronic stimulation similar to putting your tongue on a battery.
In regards to the corneal reflex, the afferent arc is mediated by the general sensory afferents of the trigeminal nerve. The efferent arc occurs via the facial nerve. The reflex involves the consensual blinking of both eyes in response to stimulation of one eye. Thus, the corneal reflex effectively tests the proper functioning of both cranial nerves V and VII.
The vestibulocochlear nerve also known as the auditory vestibular nerve and cranial nerve VIII has axons that carry the modalities of hearing and equilibrium. It consists of the cochlear nerve that carries information about hearing, and the vestibular nerve that carries information about balance. This is the nerve along which the sensory cells the hair cells of the inner ear transmit information to the brain.
It emerges from the pons and exits the inner skull via the internal acoustic meatus or internal auditory meatus in the temporal bone. The vestibulocochlear nerve consists mostly of bipolar neurons and splits into two large divisions: the cochlear nerve and the vestibular nerve. The cochlear nerve travels away from the cochlea of the inner ear where it starts as the spiral ganglia. Processes from the organ of Corti the receptor organ for hearing conduct afferent transmission to the spiral ganglia.
It is the inner hair cells of the organ of Corti that are responsible for activating the afferent receptors in response to pressure waves reaching the basilar membrane through the transduction of sound. The vestibular nerve travels from the vestibular system of the inner ear. The vestibular ganglion houses the cell bodies of the bipolar neurons and extends processes to five sensory organs.
Three of these are the cristae, located in the ampullae of the semicircular canals. Hair cells of the cristae activate afferent receptors in response to rotational acceleration. The other two sensory organs supplied by the vestibular neurons are the maculae of the saccule and utricle. Hair cells of the maculae activate afferent receptors in response to linear acceleration.
The vestibulocochlear nerve has axons that carry the modalities of hearing and equilibrium. Damage to the vestibulocochlear nerve may cause hearing loss, vertigo, a false sense of motion, loss of equilibrium in dark places, nystagmus, motion sickness, and gaze-evoked tinnitus. A benign primary intracranial tumor of vestibulocochlear nerve is called a vestibular schwannoma also called acoustic neuroma.
The glossopharyngeal nerve cranial nerve IX serves many distinct functions, including providing sensory innervation to various head and neck structures. The glossopharyngeal nerve is the ninth of 12 pairs of cranial nerves. It exits the brainstem out from the sides of the upper medulla, just rostral closer to the nose to the vagus nerve.
Glossopharyngeal nerve : Image of head structures including the glossopharyngeal nerve. The motor division of the glossopharyngeal nerve is derived from the basal plate of the embryonic medulla oblongata, while the sensory division originates from the cranial neural crest. There are a number of functions of the glossopharyngeal nerve. It controls muscles in the oral cavity and upper throat, as well as part of the sense of taste and the production of saliva.
Along with taste, the glossopharyngeal nerve relays general sensations from the pharyngeal walls. The various functions of the glossopharyngeal nerve are that:. The vagus nerve cranial nerve X is responsible for parasympathetic output to the heart and visceral organs. The vagus nerve, also known as the pneumogastric nerve or cranial nerve X, is the tenth of twelve paired cranial nerves.
Upon leaving the medulla between the medullary pyramid and the inferior cerebellar peduncle, it extends through the jugular foramen, then passes into the carotid sheath between the internal carotid artery and the internal jugular vein below the head, to the neck, chest and abdomen, where it contributes to the innervation of the viscera. Vagus nerve : Diagram demonstrating the course of the vagus nerve. The vagus nerve supplies motor parasympathetic fibers to all the organs, except the suprarenal adrenal glands, from the neck down to the second segment of the transverse colon.
The vagus also controls a few skeletal muscles, most notably:. This means that the vagus nerve is responsible for such varied tasks as heart rate, gastrointestinal peristalsis, sweating, and quite a few muscle movements in the mouth, including speech via the recurrent laryngeal nerve , swallowing, and keeping the larynx open for breathing via action of the posterior cricoarytenoid muscle, the only abductor of the vocal folds. This explains why a person may cough when tickled on the ear such as when trying to remove ear wax with a cotton swab.
Afferent vagus nerve fibers that innervate the pharynx and back of the throat are responsible for the gag reflex. In addition, 5-HT3 receptor-mediated afferent vagus stimulation in the gut due to gastroenteritis and other insults is a cause of vomiting. Parasympathetic innervation of the heart is partially controlled by the vagus nerve and is shared by the thoracic ganglia.
This occurs commonly in cases of viral gastroenteritis, acute cholecystitis, or in response to stimuli such as the Valsalva maneuver or pain. Excessive activation of the vagal nerve during emotional stress can also cause vasovagal syncope due to a sudden drop in cardiac output, causing cerebral hypoperfusion. The accessory nerve cranial nerve XI controls the sternocleidomastoid and trapezius muscles of the shoulder and neck.
It begins in the central nervous system CNS and exits the cranium through a foramen. Unlike the other 11 cranial nerves, the accessory nerve begins outside the skull. In fact, most of the fibers of the nerve originate in neurons situated in the upper spinal cord.
The accessory nerve : Upon exiting the skull via the jugular foramen, the spinal accessory nerve pierces the sternocleidomastoid muscle before terminating on the trapezius muscle.
The fibers that make up the accessory nerve enter the skull through the foramen magnum and proceed to exit the jugular foramen with cranial nerves IX and X. Due to its unusual course, the accessory nerve is the only nerve that enters and exits the skull. Traditional descriptions of the accessory nerve divide it into two components: a spinal component and a cranial component. However, more modern characterizations of the nerve regard the cranial component as separate and part of the vagus nerve.
Therefore, in contemporary discussions of the accessory nerve, it is common to disregard the cranial component when referencing the accessory nerve and assume reference to the spinal accessory nerve. They exit the brain in an orderly fashion. Two, olfactory and optic, are located in the telencephalon. Two, occulomotor and trochlear, are located in the midbrain.
Four, trigeminal, abducens, facial and acoustovestibular, are located in the pons. The remaining four, glossopharyngeal, vagus, accessory and hypoglossal are located in the medulla.
It is most useful however to discuss them, not related to location, but rather as to function. There are three cranial nerves with primarily sensory function. Link to Sensory. There are four cranial nerves with primarily motor function. Link to Corticobulbar. CN IV, Trochlear, innervates the superior oblique muscle of the eyeball.
CN VI, Abducens, innervates the lateral rectus muscle of the eyeball and abducts the eye. CN XI, Accessory, innervates the trapezius muscle in the back and shoulders. Five cranial nerves have mixed sensory, motor and parasympathetic function.
These five will be discussed in detail here. The occulomotor nerve exits the brain at the midbrain in the middle of the cerebral peduncle.
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