Rats and Pigs' Sympathetic Fiber Origin of the Laryngeal Nerve

Abstract

The external laryngeal nerve (ELN) is a slender branch of the superior laryngeal nerve (SLN) which primarily innervates the cricothyroid and inferior pharyngeal constrictor muscles. However, the exact nature of fibers which are contained in this nerve has not been ascertained yet. To this purpose, gross dissection was carried out on 50 (100 sides) human cadavers, to isolate the nerve and identify its course and branches. 10 dog (Canis lupus familiaris) cadavers were also dissected to isolate the ELN and to obtain specimens for histo-chemical analysis. In dogs the ganglion nodosum of vagus nerve and the Superior Cervical Sympathetic Ganglion (SCSG) were completely or partially fused, and the SCSG gave off 1 or more communicating twigs, that connected to the ganglion nodosum. In human beings the SCSG communicated with the SLN and its branches. In most cases the SCSG connected with the ELN (86 %). Hence, statistically, the ELN is a looped nerve. In both dog and human dissections every one of the ELN/ELN loops without exception gave off a branch to the thyroid gland. The Acetylcholinesterase (AChE) staining showed that the dog ELN contained AChE-positive myelinated, AchE-negative myelinated and AChE-positive unmyelinated nerve fibers indicating the presence of somatic motor fibers, somatic sensory fibers and post-ganglionic sympathetic fibers. This proves that the ELN is a mixed nerve. Thus, damage to the nerve during thyroid surgeries could result in functional impairment to not only voice, but also gland function and sensation in the region.