Cervical Segmentof SLN

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The SLN is the second major branch of the tenth cranial nerve [2]. The SLN branches from the vagus at or below the inferior cervical (nodose) ganglion. The SLN travels from the medial aspect of the vagus nerve, superficial to the superior sympathetic cervical ganglion from which it receives sympathetic contribution [13]. The SLN courses deep to the internal carotid artery as it emerges in an anteromedial direction towards the larynx. In this region it supplies branches posteriorly to the carotid body [14]. The average length of the SLN trunk is 1.9 cm in males and 1.4 cm in females [15]. The main trunk of SLN divides into the internal branch (inSLN) and external branch (exSLN) prior to supplying the larynx.

The internal branch of the SLN is predominantly a sensory nerve, though there may be a small degree of motor input [15-17]. After dividing from the SLN trunk, the internal branch of SLN runs parallel and medial to the superior laryngeal artery [15]. As it passes the greater cornu of the hyoid it turns medially, passing deep to thyrohyoid muscle [10,15]. The inSLN then enters the larynx through the thyrohyoid membrane just superior to the superior border of the inferior pharyngeal constrictor muscle [10]. Prior to entering the larynx, this branch runs deep or superior to the superior laryngeal artery which ramifies from the superior thyroid artery [10, 13]. After entrance into the larynx, the inSLN branches into three main divisions: superior; middle; and inferior. Sanders and Mu

[16] studied the course of these branches using the Sihler neurospecific staining technique to demonstrate the fine terminal branches of the nerves which are often too delicate for anatomical dissection [18]. Occasionally, after the inSLN enters the larynx it will anastomose with the exSLN through the thyroid foramen in the superoposterior aspect of the thyroid cartilage

The superior division of inSLN ramifies into two to three secondary branches. The most su perior branch innervates the lingual aspect of epiglottis. The next branch provides sensory input to the lateral aspect of the glossoepiglottic fold. The caudal branch of the superior division courses through the aryepiglottic fold to innervate the laryngeal aspect of the epiglottis. The laryngeal mucosa of the epiglottis has a richer nerve supply than the lingual aspect [16].

The middle division of the inSLN divides into four to six branches; these innervate the aryepiglottic fold, the mucosa superior to the true vocal fold, the vestibule, and the posterior aspect of the arytenoid. The most dense sensory supply of the middle division is to the true and false vocal folds, and the aryepiglottic folds [16].

The descending division of inSLN is the largest branch of the three. This divides into eight to ten secondary branches. The main trunk of this inferior division courses inferiorly along the medial aspect of the piriform sinus supplying branches medially. The first branch travels to the posterior aryepiglottic fold and the outer aspect of the tip of arytenoid [16]. The next few branches pass to the posterior surface of the arytenoid and to the interarytenoid muscle. These interarytenoid branches divide further to supply the interarytenoid muscle, posterior glottic mucosa, medial aspect of arytenoid, and posterior subglottic mucosa. The terminal branches of the inSLN continue inferiorly as they pass over the posterior aspect of posterior cricoarytenoid (PCA) muscle to provide sensation to the anterior hypopharyngeal mucosa. The inSLN also provides the branch which communicates with RLN via Galen's anastomosis [16, 17, 19]. Communication between the inferior and superior laryngeal nerves is discussed in greater detail below.

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