Cranial Nerves Essay

Published: 2020-02-14 21:40:04
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The human body is a unique and fascinating entity. There is not much notice taken of the features the human body is capable of. The brain is necessary to perform day-to-day actions, such as the ability to speak, and see amongst us. This brain is made up of simple mater (Pia mater, Arachnoid mater, Dura mater) and the cranial surface to protect the brain. We live our daily lives without acknowledging the importance of this organ, the brain, unless youre a medical student of course!

Despite that people go on with their daily activities using the human natural senses. Looking at the world through the eyes, watching for any danger around us; ears for hearing the sounds detecting something that may call for danger, the nose to smell the natural environment, touching and feeling surfaces to become familiar with the surrounding, and to taste the different foods that provide nourishment to stay healthy. All these senses are essential for survival, although what happens if one or more of these senses are taken away?

Surviving the everyday world becomes just a little tougher and even impossible in other societies. The cranial nerves in the brain control these senses along with other bodily functions necessary to survive. There are 12 pairs of cranial nerves that emerge from the brain out of various foramina or fissures from the cranium. Each cranial nerve contains sensory or motor fibers or a combination of these fivers to carry impulses from the brain to the various locations allowing the individual to perform normal human functions.

The 12 pairs of cranial nerves (Olfactory, Optic, Oculomotor, Trochlear, Trigeminal, Abducent, Facial, Vestibulocochlear, Glossopharyngeal, Vagus, Spinal Accessory, and Hypoglossal) can carry one or more of the five functional components of the motor (efferent) or sensory (afferent) fibers. The motor (efferent) fibers can innervate voluntary (stratified) muscle or it can be involved in innervating glands and involuntary (smooth) muscle. There are 3 types of sensory (afferent) fibers. These fibers can convey sensation from the viscera, transmit general sensation, and transmit unique sensation.

The somatic motor axons innervate the striated muscles in the orbit, tongue, external muscles of the neck, and muscles of the face. The motor fibers innervate glands and the involuntary smooth muscles such as viscera and blood vessels have visceral motor axons that make up the cranial outflow of the parasympathetic division of the autonomic nervous system. Sensory fibers that convey sensation from the viscera include visceral sensory fibers that transmit information from the carotid body and sinus, pharynx, larynx, trachea, bronchi, lungs, heart and gastrointestinal tract.

Sensations of temperature, pressure and touch are sensed by the somatic sensory fibers that transmit general sensations from the skin and mucous membranes. Sensory fibers that transmit unique sensations use special sensory fibers to convey taste, smell, vision, hearing, and balance. The cranial nerve pairs are numbered I-XII. The first cranial nerve is olfactory. The main function of cranial number one is to smell. Olfactory enters the cranium via the foramina in cribiform plate of the ethmoid bone.

The nerve cell bodies of the olfactory nerve are located on the nasal septum and the medial wall of the superior nasal concha. These special sensory fibers enter into the olfactory bulb and synaps with the mitral cells in the olfactory bulb, which transmits impulses to the brain. There are also a few nerve dysfunctions that are associated with the olfactory nerve. These dysfunctions include Anosmia, loss of smell; hyposmia, decreased sense of smell; parosmia, prevision of the sense of smell; cocosmia, awareness of a disagreeable or offensive odor that does not exist.

These nerve dysfunctions maybe caused due to injury to the neurofibrils, compression of the olfactory bulb by hemorrhage and edema or contusion and abrasion, injury to the central pathways of olfactory, injury to the nasal passage, non-traumatic causes such as upper respiratory infections, rhinitis, allergic sinusitis, chronic polyposis, depression, and medication. The second cranial nerve is optic nerve. Optic nerves main function is vision, one of the important senses of the human body. Allowing oneself to see around them.

The cell bodies of the optic nerve are located in the Retina (ganglion cells). Optic nerve begins with unmyelinated axons of the rentinal ganglion cells, which later become myelinated in the optic disc. CN II enters the cranium via the optic canal. The retina has bipolar cells that are connected to the special sensory fibers (rods and cone cells). When light hits the rod and cone cells, electrical impulse are relayed and transmitted to the bipolar cells. That is when the bipolar cells transmit electrical activity to the CNS through the optic nerve.

Loosing the sense of vision can be very detrimental. Some of the dysfunctions that occur with CN II are immediate monocular blindness (partial or complete), visual field deficits, blurring, scotomata, and monocular diplopia. There can be many possible causes for these dysfunctions to name a few, immediate loss of vision is due to injury to optic nerve due to ischemia or death, delayed vision loss is due to infarction of the optic nerve or less frequently by hematoma surrounding the nerve. Complete monocular blindness is usually due to non-organic disorders.

Blurring and scotomata are due to trauma to the cornea, vitreous tears, traumatically induced contaracts, retinal hemorrhage, retinal detachment, or intrabulbar hemorrhage. Visual impairment, including blindness, and associated secondary damage to the eye can be caused by intraocular hemorrhage. Oculomotor nerve is the third cranial nerve. CN III serves a motor function. The oculomotor nerve originates from motor neurons in the oculomotor and visceral motor nuclei in the brainstem. CN III is the chief nerve to the ocular and extra-ocular muscles.

The nerve cell bodies of the oculomotor nerve arise from axons that exit the ventral surface of the brainstem. The nerve than passes through the two layers of the dura mater including the later wall of the cavernous sinus and then enters the superior orbital fissure to innervate the orbit. The somatomotor component to the nerve dives into a superior and inferior division. The superior division supplies the levator palpebrae superioris and superior rectus muscles. The inferior division supplies the medial rectus inferior rectus and inferior oblique muscles.

The visceromotor which is also the parasympathetic component of the oculomotor nerve travels along with the inferior division that sends branches which enter the ciliary ganglion where a synapse is formed with the with the ganglion cells. The ganglion cells send nerve fibers into the back of the eye where they travel to ultimately innervate the ciliary muscle and the constrictor pupillae muscle. Oculomotor nerve being the chief nerve to innervate the ocular and extra-ocular muscle any dysfunction to this nerve can lead to ocularmotor nerve palsy, including isolated and bilateral oculomotor nerve palsies.

It can be caused by injury to the oculomotor nerve and possibly an oculomotor blowout. Sign of this dysfunction can be notised by outward and downward deviation of the eye, and dilation of the ipsilateral pupil in complete palsy. There is a syndrome that mimics an oculomotor nerve dysfunction called parinaud syndrome, which causes a paralysis of upward gaze that is caused by an injury to the dorsal midbrain but not due to the peripheral oculomotor nerves. Cranial nerve number four is Trochlear nerve.

The trochlear nerve is purely a motor nerve and is the only cranial nerve to exit the brain dorsally. The trochlear nerve is also the smallest cranial nerve that supplies one muscle, the superior oblique. The cell bodies of the cranial nerve IV are located in the ventral nucleus cross to the other side of the brainstem just prior to exiting the brainstem that are coming from the opposite side. The trochlear nerve fibers curve forward and enter the dura mater at the angle between the free and attached border of the tentorium cerebelli.

The nerve travels in the lateral wall of the cavernous sinus and then enters the orbit via the superior orbital fissure. The nerve travels medially and diagonally across the levator palpebrae superioris and superior rectus muscle to innervate the superior oblique muscle. Injury to the tochlear nerve can cause vertical diplopia on locking downward with contra-lateral head tilt and worsen with ipsi-lateral head tilt. Cranial nerve number five is the Trigeminal nerve. The trigeminal nerve is composed of three branches. The first branch is ophthalmic nerve (CN V1), which has somatic sensory fibers.

The second branch is the maxillary nerve (CN V2), which is also composed of somatic sensory fibers. The third branch is the mandibular nerve (CN V3) that has both somatic sensory and somatic motor fibers. The large sensory root and the smaller root leave the brainstem at the mid-lateral surface of pons. The ophthalmic and maxillary branches travel in the wall of the cavernous sinus just prior to leaving the cranium. The ophthalmic nerve branch travels through the superior orbital fissure to enter the cranium to reach the skin of the forehead and the top of the head.

The ophthalmic nerve branches out into several other branches (tentorial nerve, lacrimal nerve, communicating branch from zygomatic nerve, frontal nerve, nasocilliary nerve) which supplies the cornea, upper conjunctiva, mucosa of anterosuperior nasal cavity, frontal and ethmoidal sinuses, anterior and supratentonal dura mater, skin of dorsum of external nose, superior eyelid, forehead, and scalp. The maxillary nerve enters the cranium through the foramen rotundum via the pterygopalatine fossa.

The maxillary nerve also branches into the meningeal branch, zygomatic nerve, ganglionic branches to pterygopalatine ganglion, posterior superior alveolar branchesm greater palatine nerves, lesser palatine nerves, posterior superior lateral nasal branches, nasopalatine nerve, pharyngeal nerve. These branches supplies the dura mater of the anterior part of superior oral vestibule, maxillary teeth, and skin of lateral external nose, inferior eyelid, anterior check, and upper lip. The mandibular nerve has sensory and motor fibers, which pass through the foramen ovale.

Mandibular nerves sensory fibers branch into other fibers such as the meningeal branch, buccal nerve, auriculotemporal nerve, lingual nervem inferior alveolar nerve, nerve to mylohyoidm, inferior dental plexus, mental nerve. These sensory fibers of the mandibular nerve supply sensory innervation to mucosa of anterior two thirds of tongue, floor of mouth, and posterior and anterior inferior oral vestibule, mandibular teeth, and skin of lower lip, buccal, parotidm and temporal regions of the face, and external ear.

The manidubular nerves somatic motor fibers branch out to the masseter, temporal, medial and lateral pterygoidsm mylohyiod, anterior belly of digastric, tensor tympani, tensor veli palatine. These motor fibers supply innervation to the muscles of mastication, mylohoid, anterior belly of digastric, tensor tympani, and tensor veli palatini. Injuries to the trigeminal nerve and its branches can be caused by trauma, tumors, aneurysms, or meningeal infections which would leave into possible paralysis of the muscles of mastication, corneal drying, abrasions, and/or pain, and decreased salivation.

Cranial number six is the abducent nerve. The main function of the abducent nerve is to contract the lateral rectus, which results in abduction of the eye. The abducent nerve is purely somatomotor. CN VI originates from neuronal cell bodies located in the ventral pons. The nerve of each side then travels anteriorly where it pieces the dura lateral to the dorsum sellae. The nerve continues forward and bends over the ridge of the petrous part of the temporal bone and enters the cavernous sinus. The nerve passes lateral to the carotid artery prior to entering the orbit through the superior orbital fissure.

When there is a complete injury to the abducent nerve the affected eye is turned medially. When there is partial injury to this nerve the affected eye is seen at midline at rest but cannot deviate the eye laterally. The seventh cranial nerve is the facial nerve. Which has both sensory and motor components. The nerve emerges from the brainstem from the junction of the pons and the medulla. The nerve enters the internal auditory meatus where the sensory part of the nerve forms the geniculate ganglion. In the internal auditory meatus is where the greater petrosal nerve branches from the facial nerve.

The facial nerve continues in the facial canal where the chorda tympani branches from the facial nerve leaving the skull via the stylomastoid foramen. The chorda tympani passes through the petrotympanic fissure before entering the infatemporal fossa giving off the posterior auricular branch entering into the parotid gland forming the parotid plexus. The parotid plexus gives ride to the five terminal motor branches the temporal, zygomatic, buccal, marginal mandibular, and cervical. The somatomotor fibers of the facial nerve supply the muscles of facial expression.

The somatomotor component originates from neursons in the facial motor nucleus located in the ventral pons. The parasympathetic or visceral motor fibers provide presynaptic parasympathetic fibers to the pterygopalatine ganglion that innervates the lacrimal, nasal, pharyngeal and palatine glands, and submandibular ganglion for innervation of the sublingual and submandibular salivary glands. There are two sensory (special and general) components of the facial nerve both of which originate from the cell bodies in the geniculate ganglion.

The special sensory component carries information from the taste buds in the tongue. The general sensory component conducts sensation from the skin in the external auditory meatus, a small area behind the ear, and external surface of the tympanic membrane. Injuries to the facial nerve can result in complete or partial paralysis of the face, hyperacusis, and/ or unusual or impaired sense of taste. Any of these dysfunctions can be caused by cranial trauma or dysfunction at any level of the auditory system. The eighth cranial nerve is vestibulocochlear nerve.

The vestibulocochlear nerve is a sensory nerve that serves two special senses, hearing and balance. The receptor cells for these special senses are located in the membranous labyrinth, which is embedded in the petrous part of the temporal bone. The vestibulocochlear nerve enters the cranium via the external acoustic meastus where it separates into the vestibular and cochlear nerves. The vestibular nerves are associated with balance. Movement causes fluid vibration resulting in hair cell displacement that activates the vestibular part of the vestibulocochlear nerve. The cochlear nerve is associated with hearing.

The cochlear duct is the organ that is connected to the three bony ossicles, which transmit sound waves into fluid movement in the cochlea. This results in movement of hair cells, which activate the cochlear nerve of the vestibulocochlear nerve. Injuries to cranial nerve VIII can lead to hearing loss or even deafness. These injuries can be caused by longitudinal fractures, transverse fractures, cochlear concussion or ossicular chain disruption. Cranial number nine is the glossopharyngeal nerve that comes out from the lateral aspect of the medulla and passes anterolaterally leaving the cranium via the jugular foramen.

Cranial nerve IX has sensory and motor compartments. The viceromotor or parasympathetic part of the glossopharyngeal nerve originates in the inferior salivatory nucleus. The presynaptic parasympathetic fibers associate with the otic ganglion that innervates the parotid gland, an important salivary gland. The branchial motor component supplies the stylopharyngeas muscle, which elevates the phaynx during swallowing and talking. There are two sensory fibers connected to the ninth cranial nerve.

The general sensory components from the skin of the external ear, inner surface of the tympanic membrane, posterior one-third of the tongue and upper pharynx join either the superior or inferior glossopharyngeal ganglia. The visceral sensory nerve fibers originate from the carotid body and carotid sinus. The visceral sensory nerve components connect to the inferior glossopharyngeal ganglion. The central process extends from the ganglion and enters the brain stem to terminate in the nuclear solitarius. Taste from the posterior one third of the tongue travels via nerve fibers that enter the inferior glossopharnygeal anglion. Injuries to cranial nerve IX can result in weakness on the ipsilatteral side, dysphagia and dysarthria. Some of the symptoms of dysphagia are drooling, difficulty initiating swallowing, nasal regurgitation, difficulty manaing secretions, choke/cough episodes while feeling and food sticking in the throat. The tenth cranial nerve is the vagus nerve. It is the longest cranial nerve sensory and motor functions. The vagus nerve exits the cranium via the jugular foramen. The branchial motor component of the vagus nerve originates in the medulla in the nucleus ambiguus.

The nucleus ambiguus contributes to the vagus nerve as three major branches, which leave the nerve distal to the jugular foramen. The pharyngeal branch travels between the internal and external carotid arteries and enters the pharynx at the upper border of the middle constrictor muscle. It supplies the all the muscles of the pharynx and soft palate except the stylopharyngeas and tensor palati. These include the three constrictor muscles, levator veli palatini, salpingopharyngeus, palatopharyngeus and palatoglossal muscles. The superior laryngeal nerve branches distal to the pharyngeal branch and descends lateral to the pharynx.

It divides into an internal and external branch. The external branch travels to the cricothyroid muscle, which it supplies. The third branch is the recurrent branch of the vagus nerve and it travels a different path on the left and right sides of the body. On the right side the recurrent branch leave the vagus anterior to the subclavian artery and wraps back around the artery to ascend posterior to it. The right recurrent branch ascends to a groove between the trachea and esophagus. The left recurrent branch leaves the vagus nerve on the aortic arch and loops posterior to the arch to ascend through the superior mediastinum.

The left recurrent branch ascends along a groove between the esophagus and trachea. Both recurrent branches enter the larynx below the inferior constrictor and supply intrinsic muscles of larynx excluding the cricothyroid. The visceromotor or parasympathetic component of the vagus nerve originates from the dorsal motor nucleus of the vagus in the dorsal medulla. These cells give rise to axons that travel in the vagus nerve. The visceromotor part of the vagus innervates ganglionic neurons, which are located in or adjacent to each target organ.

The target organs in the head-neck include glands of the pharynx and larynx (via the pharyngeal and internal branches). In the thorax branches go to the lungs for bronchoconstriction, the esophagus for peristalsis and the heart for slowing of heart rate. In the abdomen branches enter the stomach, pancreas, small intestine, large intestine and colon for secretion and constriction of smooth muscle. The viscerosensory components of the vagus are derived from nerves that have receptors in the abdominal viscera, esophagus, heart and aortic arch, lungs, bronchia and trachea.

Nerves in the abdomen and thorax join the left and right vagus nerves to ascend beside the left and right common carotid arteries. Sensations from the mucous membranes of the epiglottis, base of the tongue, aryepiglottic folds and the upper larynx travel via the internal laryngeal nerve. Sensation below the vocal folds of the larynx is carried by the recurrent laryngeal nerves. The cell bodies that give rise to the peripheral processes of the visceral sensory nerves of the vagus are located in the inferior vagal ganglion.

The general sensory components of the tenth cranial nerve conduct sensation from the larynx, pharynx, skin the external ear and external auditory canal, external surface of the tympanic membrane, and the meninges of the posterior cranial fossa. Sensation from the larynx travels via the recurrent laryngeal and internal branches of the vagus to reach the inferior vagal ganglion. Sensory nerve fibers from the skin and tympanic membrane travel with auricular branch of the vagus to reach the superior vagal ganglion.

There are many injuries/dysfunctions that can occur due to the vagus nerve since the vagus nerve innervates many various locations of the body. The eleventh cranial nerve is the spinal accessory nerve. The spinal accessory nerve originates from neuronal cell bodies located in the cervical spinal cord and caudal medulla. Most are located in the spinal cord and ascend through the foramen magnum and exit the cranium through the jugular foramen. The branchiomotor fibers help with the function and innervation of the sternocleidomastoid and trapezius muscles in the neck and back.

The cranial root of the accessory nerve originates from cells located in the caudal medulla. They are found in the nucleus ambiguus and leave the brainstem with the fibers of the vagus nerve and join the spinal root to exit the jugular foramen. They rejoin the vagus nerve and distribute to the same targets as the vagus. Most consider the cranial part of the eleventh cranial nerve to be functionally part of the vagus nerve. Injuries to the eleventh cranial nerve can result in weakness and atrophy of the trapezius and impair rotary movements of the neck and chin to the opposite side. The twelfth and last cranial nerve is the hypoglossal nerve.

This nerve can be found below the tongue. It is a somatomotor nerve that innervates all the intrinsic and all but one of the extrinsic muscles of the tongue. The neuronal cell bodies that originate the hypoglossal nerve are found in the dorsal medulla of the brain stem in the hypoglossal nucleus. This nucleus gives rise to axons that exit as rootlets that emerge in the ventrolateral sulcus of the medulla between the olive and pyramid. The rootlets come together to form the hypoglossal nerve and exit the cranium via the hypoglossal canal. The nerve passes laterally and inferiorly between the internal carotid artery and internal jugular vein.

The twelfth cranial nerve travels lateral to the bifurcation of the common carotid and loops anteriorly above the greater horn of the hyoid bone to run on the lateral surface of the hyoglossus muscle. It then travels above the edge of the mylohyoid muscle. The hypoglossal nerve then separates into branches that supply the intrinsic muscles and three of the four extrinsic muscles of the tongue. Injuries to cranial nerve XII can result in dysarthria and difficulty in swallowing. Cranial nerves are very important for our everyday functions. It would be difficult to eat, smell, hear, or see if there are dysfunctions to these cranial nerves.

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