The Cranial Nerves - Essay Example

Introduction This paper aims to review the cranial nerves involved in dysphagia and to clearly determine their individual contribution to the swallowing process. Firstly, the cranial nerves involved in swallowing will be identified and their functions will be delineated with regard to swallowing. Secondly, methods of measuring the activity of these nerves during swallowing will be presented. Finally a conclusion will synthesize the main points of this paper and provide support for continuing investigation of the individual functional capabilities of the cranial nerves with regard to swallowing. It is anticipated that this review will contribute to the self-learning of the author, as well as extending on insights and understandings of dsyphagia within the literature at this present time. Cranial Nerves Involved in Swallowing Trigeminal (V) The trigeminal nerve is the largest of the cranial nerves. It is very important in swallowing due to its afferent control of general sensation to the face, teeth, gum, muscles of mastication and the anterior two thirds of the tongue (Miller, 2006). Its efferent control is of the muscles for mastication. Innervating the tensor velar palatine, the trigeminal is partly responsible for the flattening and tensing of the soft palate. Innervations of the extrinsic laryngeal muscle results in the nerve’s support for the upward and anterior movements of larynx. Facial (VII) The facial nerve’s main function is its motor portion; although, its afferent portion is involved in taste sensation from the anterior two thirds of the tongue (Miller, 2006). It provides motor innervations to the sublingual and submaxillary salivary glands. More specifically, it is involved in swallowing by regulating the lip sphincter and the buccal muscles, which allows food to be held inside the mouth and also assists in pulling the larynx up and back. Glossopharyngeal (IX) The glossopharyngeal (GPN) is composed of the lingual branch of the GPN (GPN-li) and the GPN-ph. Its sensory portion transits inputs from the posterior third of the tongue, the velum and the pharynx, which includes the tonsils (Miller, 2006). The glossopharygeal nerve gathers sensation from the fauces, the palatine tonsil, upper pharynx, and the back third of the tongue. Sensory fibers carry taste information from the posterior one third of the tongue. Afferent input is received from receptors located in the larynx and these are carried primarily to the superior laryngeal nerve, also known as the cough center of the medulla. The motor portion communicates outputs to the middle pharyngeal constrictor muscle and also innervates the stylopharyngeal muscle, which, together with the palatopharyngeus muscle (X nerve), elevates the palate. The stylopharyngeus muscle dilates the pharynx laterally and contributes to the elevation of pharynx and larynx. Vagus (X) The vagus nerve (X) is the most dominant cranial nerve during the swallowing process. The vagus nerve is the major efferent for the pharyngeal constrictors and is the major afferent for the middle and inferior portions of pharynx (Miller, 2006). It supplies also main efferent innervations to the palatal muscles. Branches of the vagus nerve attach to muscles of the larynx and pharynx. It is also known as the nucleus ambiguous, and it innervates branchial arch muscles of the pharynx and larynx as well as the muscles of the upper esophagus and uvula. Branches also extend to the, glossopalantine, levator veli palatine and the palatoglossus muscles making it primarily responsible for palatal functioning. One of the motor nuclei transmits motor output to the pharyngeal constrictor muscles and in this way can completely control the intrinsic musculature of the larynx. Moreover, taste buds on the root of the tongue and on the epiglottis contribute special visceral afferent fibers to the superior laryngeal branch. General visceral afferent fibers convey sensation from the lower pharynx, larynx, trachea, and esophagus. Together with the glossopharyngela nerve the motor portion of the vagus nerve contributes to the elevation of the palate, gag reflex and gestation. The superior laryngeal nerve separates from the main trunk of the vagus just outside the jugular foramen (Miller, 2006). The main internal laryngeal nerve enters the thyrohyoid membrane through a hiatus. It then divides into three set of branches (ascending, transverse and descending), which communicate with the recurrent laryngeal nerve posterior to the cricoid cartilage; also referred to as the ansa galeni. The motor portion supplies the external laryngeal nerve which supplies the inferior constrictor muscle and the cricothyroid (CT) muscle. While the sensory portion supplies the main internal laryngeal nerve supplies sensory innervations to the epiglottis, the pyriform sinus, and the larynx as far down as the vocal folds. Accessory (XI) The accessory nerve is mostly a motor nerve, which innervates the palatopharyngeus muscle which depresses the velum and constricts the pharynx. Also, it innervates the muscularis uvula which tenses the velum (Miller, 2006). It branches into the sternocleidomastoid muscle, which tilts the head toward one shoulder with an upward rotation of the face to the opposite side, and the trapezius muscle, which stabilizes and elevates (or shrugs) the shoulder. The accessory nerve aids the vagus nerve in the innervations of some of the muscles of the larynx and innervates the levator veli palatine; as well as innervating cervical aspects of the sternocleidomastoid and trapezius muscles. Hypoglossal (XII) The hypoglossal nerve is purely a motor nerve that innervates the extrinsic and intrinsic musculature of the tongue. Hypoglossal fibers end in intrinsic tongue muscles, which modify the shape of the tongue (e.g., tongue shorting, concaving or turning the tip up and lateral margins upward, narrowing, elongating, and flattening) (Miller, 2006). As well, the hypoglossal innervates the extrinsic muscles that are responsible for tongue protrusion (genioglossus), drawing the tongue upward and backward (styloglossus), and retraction and depression of the tongue (hyoglossus). The hypoglossal also aids the chondroglossus to elevate the hyoid bone too. Methods of Measurement Measurement of the cranial nerves responsible for swallowing can be done using several procedures. A popular method is videofluoroscopy which has traditionally been considered the “gold standard” for evaluation of a swallowing disorder as it can provide a wide range of in-depth information about the structures used in swallowing. In a videofluoroscopic swallow study (VFSS), the patient swallows a variety of liquids and foods mixed with barium, as a radiologist takes video X-rays of the mouth and throat (Ajemian, Nirmul, Anderson, Zirlen, & Kwasnik, 2001; Amri, Car, & Roman, 2004). These images show how food passes from the mouth through the throat and into the esophagus. However, it is not considered efficient and easy to use for all clinical situations. Another method is fiberoptic endoscopic evaluation of swallowing (FEES) that provides a visual display that patients can use almost like biofeedback to learn better swallowing methods. A flexible endoscope is inserted transnasally to the hypopharynx and from here the laryngeal and pharyngeal structures can be viewed (Amri, Car, & Roman, 2004; Schroter-Morash, Bartolome, Troppmann, & Ziegler, 1999). The patient is instructed in tasks that provide sensory and motor information about the mechanisms in these structures. Boluses are provided to the patient so that as they swallow the integrity of the pharyngeal swallow can be evaluated. The data collected informs about airway protection, as well as the ability for protection to be sustained over a few seconds. FEES also informs about the patient’s ability to initiate a prompt swallow without any spillover into the hypopharynx, whilst informing about the timing and direction of movement of bolus through the structure. Physicians can also establish if there is pooling or residue of bolus as the swallow occurs through the hypopharynx, if the airway is protected and the sensitivity of the pharyngeal/laryngeal structures and how the anatomy is affected during swallowing. An alternate method is fiberoptic endoscopic evaluation of swallowing with sensory testing (FEESST). A flexible endoscope is introduced into the oropharynx, and a sensory stimulator is used to quantify stimuli that is pulses of are that are gradually increased to initiate a laryngeal adductor reflex (American Medical Association, 2005; McCaffrey, 1998-2001). The motor evaluation occurs by providing the patient with different food consistencies to determine transit time, inhibition of swallowing, laryngeal elevation, spillage and residue. As well, FEEST can inform about the condition of the swallow, laryngeal closure, reflux, aspiration and the patient’s ability to clear residue. However, the laryngopharyngeal sensory testing technique is controversial, as some suggest that there is a lack of support for FEESST for the assessment of dysphagia (Veterans Health Administration Department of Defense, 2003; SIGN, 2004). Instead, FEES is recommended by many guidelines for evaluation of persistent dysphagia (SIGN, 2004). Turning now to electromyography (EMG), this method tests the electrical activity of the muscles used in swallowing. A nerve conduction study may also be done to measure the conduction function of nerves. The EMG aids in identifying and describing the electrical properties of the muscles, such as of the middle pharyngeal constrictor (MPC) or the cricophayngeus (CP) (Palmer, 1989). The external needles are inserted through cervical skin, at rest and during swallowing. Usually, there is a dysfunction found between the coordinated contractions of the MPC and the relaxation of the CP during swallowing. Hence, EMG may indicate myotomy of the CP. Also, it is a safe and easy to perform procedure. Due to its ability to evaluate individual muscles, the EMG is an excellent method for assessing swallowing disorders. In conclusion, a number of cranial nerves are involved in the processes of swallowing. These include the trigeminal, facial, glossopharyngeal, vagus, accessory, and hyperglossal. There are also several methods available to measure dysphagia, or problems with swallowing. However, it appears that electromyography (EMG) may be the most efficient in terms of time, patient comfort, and its ability to measure individual nerve activation. References Ajemian, M. S., Nirmul, G. B., Anderson, M. T., Zirlen, D. M., & Kwasnik, E. M. (2001). Routine fiberoptic endoscopic evaluation of swallowing following prolonged intubation: Implications for management. Archives of Surgery, 136(4), 434-437. American Medical Association. (2005). Fiberoptic endoscopic evaluation of swallowing (FEES)/Fiberoptic endoscopic evaluation of swallowing with sensory testing (FEESST). Retrieved December 4, 2006 from http://www.aetna.com/cpb/data/CPBA0248.html Amri, M, Car, A., & Roman, C. (2004). Axonal branching of medullary swallowing neurons projecting on the trigeminal and hypoglossal motor nuclei: demonstration by electrophysiological and fluorescent double labeling techniques. Experimental Brain Research, 81(2), 384-390. Kahrilas, P. J, (1994). Current investigation of swallowing disorders, Baillieres Clinical Gastroenterology, 8(4), 651-664. Miller, A. J. (2006). Neurophysiological basis of swallowing. Dysphagia, 1(2), 1432-0460. McCaffrey, P. (1998-2001). The neuroscience on the web series: CMSD 620 neuroanatomy of speech, swallowing and language. Retrieved December 4, 2006 from http://www.csuchico.edu/~pmccaff/syllabi/CMSD%20320/362unit9.html Palmer, J. B. (1989). Electromyography of the muscles of oropharyngeal swallowing: basic concepts. Dysphagia, 3(4), 192-198. Scottish Intercollegiate Guidelines Network (SIGN). Management of patients with stroke: Identification and management of dysphagia. A National Clinical Guideline. Guideline No. 78. Edinburgh, UK. Retrieved December 4, 2006 from http://www.sign.ac.uk/guidelines/published/index.html#CHD Veterans Health Administration, Department of Defense. (2003). VA/DoD clinical practice guideline for the management of stroke rehabilitation in the primary care setting. Washington, DC: Department of Veteran Affairs. Retrieved December 4, 2006 from http://www.guideline.gov/summary/summary.aspx?doc_id=3846 Schroter-Morash, H., Bartolome, G., Troppmann, N., & Ziegler, W. (1999). Values and limitations of pharyngolaryngoscopy (transnasal, transoral) in patients with dysphagia. Dysphagia, 51(4-5), 172-182. Read More