Morfometría de los nervios laríngeos recurrentes de la rata

Somatotopy of the neurons innervating the cricothyroid, posterior cricoarytenoid, and thyroarytenoid muscles of the rat's larynx

Neurons innervating the intrinsic muscles of the larynx are located within the nucleus ambiguus but the precise distribution of the neurons for each muscle is still a matter for debate. The purpose of this study was to finely determine the position and the number of the neurons innervating the intrinsic laryngeal muscles cricothyroid, posterior cricoarytenoid, and thyroarytenoid in the rat. The study was carried out in a total of 28 Sprague Dawley rats. The B subunit of the cholera toxin was employed as a retrograde tracer to determine the locations, within the nucleus ambiguus, of the neurons of these intrinsic laryngeal muscles following intramuscular injection. The labelled neurons were found ipsilaterally in the nucleus ambiguus grouped in discrete populations with reproducible rostrocaudal and dorsoventral locations among the sample of animals. Neurons innervating the cricothyroid muscle were located the most rostral of the three populations, neurons innervating the posterior cricoarytenoid were found more caudal, though there was a region of rostrocaudal overlap between these two populations. The most caudal were the neurons innervating the thyroarytenoid muscle, presenting a variable degree of overlap with the posterior cricoarytenoid muscle. The mean number (±SD) of labelled neurons was found to be 41 ± 9 for the cricothyroid, 39 ± 10 for the posterior cricoarytenoid and 33 ± 12 for the thyroarytenoid.

Morphological and functional asymmetry of the human recurrent laryngeal nerve

PURPOSE

The aim of the current study was to analyze some parameters that are involved in nerve conduction in the right and left recurrent laryngeal nerves (RLNs) in humans. We have studied two morphological parameters such as the length of the two nerves, and the total intraperineural area (TIPA) from necropsies, and one functional parameter such as the latency of the thyroarytenoid muscle from operated patients.

METHODS

The morphological parameters of six white female adult cadavers were analyzed. The RLNs were totally removed on both sides with the vagus nerve from their origin to the entrance in the larynx at the cricothyroid joint. The lengths were measured with the help of a digitalized caliper. Selected sections were obtained from each third of the RLNs for histological analysis. The TIPA was estimated at each selected level using digitized images of nerve sections and a manual micrometer. All the measurements were carried out by specialists in histology. The intraoperative vagal nerve stimulations were conducted using the NIM3 Medtronic(®) monitoring system (Medtronic Xomed Inc., Jacksonville, FL, USA) in ten adult patients without laryngeal or nervous disease during thyroid gland surgery. The evocated laryngeal electromyography was performed with the Medtronic bipolar needle electrode directly inserted into the thyroarytenoid muscle. The direct vagal nerve stimulation was achieved with the Medtronic stimulation from 0.5 to 1 mA. The latency of the thyroarytenoid muscle was recorded on the NIM-response monitor.

RESULTS

The mean lengths of the left and right RLNs were, respectively, 136.6 and 75.0 mm with a mean difference of 61.6 mm (range 50-75 mm). The quantitative comparison of the TIPA between proximal segments of the left and right RLN showed no significant difference. The distal and proximal TIPA ratio demonstrated differences between the left and right RLN, respectively, 53.76 and 38.88 % without any statistical meaning. The comparison of the TIPA of the distal segments of the RLNs showed no significant difference. The intraoperative vagal nerve stimulation (0.5 and 1 mA) displayed a mean latency of the right and left thyroarytenoid muscle, respectively, of 3.55-3.68 and 5.90-5.98 ms with a mean difference of 2.35-2.30 ms (range 1.75-3.30 ms).

CONCLUSION

If length and latency asymmetry of the right and left RLNs in humans can be demonstrated, the synchronicity of the vocal folds requiring precise controlled variations within intrinsic laryngeal muscles needs further investigations.

The central projections of the laryngeal nerves in the rat

The larynx serves respiratory, protective, and phonatory functions. The motor and sensory innervation to the larynx controlling these functions is provided by the superior laryngeal nerve (SLN) and the recurrent laryngeal nerve (RLN). Classical studies state that the SLN innervates the cricothyroid muscle and provides sensory innervation to the supraglottic cavity, whereas the RLN supplies motor innervation to the remaining intrinsic laryngeal muscles and sensory innervation to the infraglottic cavity, but recent data suggest a more complex anatomical and functional organisation. The current neuroanatomical tracing study was undertaken to provide a comprehensive description of the central brainstem connections of the axons within the SLN and the RLN, including those neurons that innervate the larynx. The study has been carried out in 41 adult male Sprague-Dawley rats. The central projections of the laryngeal nerves were labelled following application of biotinylated dextran amines onto the SLN, the RLN or both. The most remarkable result of the study is that in the rat the RLN does not contain any afferent axons from the larynx, in contrast to the pattern observed in many other species including man. The RLN supplied only special visceromotor innervation to the intrinsic muscles of the larynx from motoneurons in the nucleus ambiguus (Amb). All the afferent axons innervating the larynx are contained within the SLN, and reach the nucleus of the solitary tract. The SLN also contained secretomotor efferents originating from motoneurons in the dorsal motor nucleus of the vagus, and special visceral efferent fibres from the Amb. In conclusion, the present study shows that in the rat the innervation of the larynx differs in significant ways from that described in other species.

Histological asymmetry of the human recurrent laryngeal nerve

Histological studies of the human recurrent laryngeal nerves (RLNs) have described differences in fiber length and thickness between the right and left RLNs. This asymmetry is probably involved in the different times of arrival of the stimuli to the laryngeal musculature controlled by each nerve. Histological and structural differences between the right and left RLNs could explain the synchronicity of laryngeal musculature contraction despite the differing nerve lengths. The purpose of this investigation was to shed some light on this paradigm by obtaining estimates of some morphometric parameters, such as intraperineural area, intraperineural perimeter, fiber area, fiber perimeter, fiber density (number of fibers/mm(2)), and total number of fibers in the right and left RLN of humans. Thus, the right and left RLNs were studied in a total sample of eight human specimens obtained from necropsies. The nerves were analyzed using histology, and the morphometric parameters were measured using Image Pro Plus Software 4.1 (Media Cybernetics, Silver Spring, MD, USA). No statistical differences between the two RLNs were observed in the intraperineural area, intraperineural perimeter, density (number of fibers/mm(2)), and total number of fibers. However, the area and perimeter of fibers of the right RLN were statistically larger when compared with those of the left RLN--21% and 11%, respectively. In conclusion, we show that in humans, the area and perimeter of the right RLN are larger than those of the left RLN. This morphological finding is probably related to the different time of arrival of the stimulus to the laryngeal musculature.