Reorganization of laryngeal motoneurons after crush injury in the recurrent laryngeal nerve of the rat

Abstract

Motoneurons innervating laryngeal muscles are located in the nucleus ambiguus (Amb), but there is no general agreement on the somatotopic representation and even less is known on how an injury in the recurrent laryngeal nerve (RLN) affects this pattern. This study analyzes the normal somatotopy of those motoneurons and describes its changes over time after a crush injury to the RLN. In the control group (control group 1, n = 9 rats), the posterior cricoarytenoid (PCA) and thyroarytenoid (TA) muscles were injected with cholera toxin-B. In the experimental groups the left RLN of each animal was crushed with a fine tip forceps and, after several survival periods (1, 2, 4, 8, 12 weeks; minimum six rats per time), the PCA and TA muscles were injected as described above. After each surgery, the motility of the vocal folds was evaluated. Additional control experiments were performed; the second control experiment (control group 2, n = 6 rats) was performed labeling the TA and PCA immediately prior to the section of the superior laryngeal nerve (SLN), in order to eliminate the possibility of accidental labeling of the cricothyroid (CT) muscle by spread from the injection site. The third control group (control group 3, n = 5 rats) was included to determine if there is some sprouting from the SLN into the territories of the RLN after a crush of this last nerve. One week after the crush injury of the RLN, the PCA and TA muscles were injected immediately before the section of the SLN. The results show that a single population of neurons represents each muscle with the PCA in the most rostral position followed caudalwards by the TA. One week post-RLN injury, both the somatotopy and the number of labeled motoneurons changed, where the labeled neurons were distributed randomly; in addition, an area of topographical overlap of the two populations was observed and vocal fold mobility was lost. In the rest of the survival periods, the overlapping area is larger, but the movement of the vocal folds tends to recover. After 12 weeks of survival, the disorganization within the Amb is the largest, but the number of motoneurons is similar to control, and all animals recovered the movement of the left vocal fold. Our additional controls indicate that no tracer spread to the CT muscle occurred, and that many of the labeled motoneurons from the PCA after 1 week post-RLN injury correspond to motoneurons whose axons travel in the SLN. Therefore, it seems that after RLN injury there is a collateral sprouting and collateral innervation. Although the somatotopic organization of the Amb is lost after a crush injury of the RLN and does not recover in the times studied here, the movement of the vocal folds as well as the number of neurons that supply the TA and the PCA muscles recovered within 8 weeks, indicating that the central nervous system of the rat has a great capacity of plasticity.

Differential expression of glial-derived neurotrophic factor in rat laryngeal muscles during reinnervation

“…Despite the reinnervation, the movement of the vocal folds is never fully restored because of the nonselective synkinetic reinnervation of regenerating axons. The somatotopic map of the laryngeal motoneurons in the nucleus ambiguus, as well as electromyographic data of the laryngeal muscles following regeneration of the RLN, show a strong evidence that the laryngeal motor neurons survive and reinnervate the larynx but that the continued absence of vocal fold motion is due to nonfunctional synkinetic reinnervation …”

“…As a consequence of an injury to the RLN in rat, the ipsilateral laryngeal fold becomes paralyzed . The injury activates a regenerative state of the nerve with newly sprouting axons from the intact proximal end of the nerve growing toward and reinnervating the denervated laryngeal muscles . Despite the reinnervation, the movement of the vocal folds is never fully restored because of the nonselective synkinetic reinnervation of regenerating axons.…”

Innervation of the human posterior cricoarytenoid muscle by the external branch of the superior laryngeal nerve

“…However, these changes in somatotropic organization have been attributed to misdirected regrowth of axons in the postlesion nerve stump and of collateral sprouting. The first phase of laryngeal nerve regeneration seems to include some collateral reinnervation and axonal sprouting, but these early connections are lost once the original axons contact and mature . Woodson demonstrated a strong propensity for reinnervation after RLN injury in cats.…”

“…The first phase of laryngeal nerve regeneration seems to include some collateral reinnervation and axonal sprouting, but these early connections are lost once the original axons contact and mature. 26 Woodson 27 demonstrated a strong propensity for reinnervation after RLN injury in cats. When the RLN is visually intact, positive EMG activity of the laryngeal muscles in patients with vocal cord paralysis may be determined by IONM.…”

GFAP immunoreactivity within the rat nucleus ambiguus after laryngeal nerve injury

“…Such a phenomenon would also explain why, after a RLN crush injury, retrograde tracers applied to RLN innervated muscles label motor neuron somata in the region of the Amb that gives rise to the SLN and innervates the posterior cricoarytenoid muscle (Hernández‐Morato et al. ).…”