Neurophysiological effects of recurrent laryngeal and thoracic vagus nerves on mediating the neurogenic inflammation of the trachea, bronchi, and esophagus of rats

Avoiding the Esophageal Branches of the Recurrent Laryngeal Nerve During Retractor Placement: Precluding Postoperative Dysphagia During Anterior Approaches to the Cervical Spine

STUDY DESIGN

Anatomical cadaver study.

OBJECTIVES

Postoperative dysphagia is a significant complication following anterior approaches to the cervical spine and the etiology of this complication is poorly understood. Herein, we studied the esophageal branches of the recurrent laryngeal nerves to improve understanding of their anatomy and potential involvement in dysphagia.

METHODS

Ten fresh frozen cadaveric human specimens were dissected (20 sides). All specimens were adults with no evidence of prior surgery of the anterior neck. The recurrent laryngeal nerves were identified under a surgical microscope and observations and measurements of their esophageal branches made.

RESULTS

For each recurrent laryngeal nerve, 5-7 (mean 6.2) esophageal branches were identified. These branches ranged from 0.8 to 2.1 cm (mean 1.5 cm) in length and 0.5 to 2 mm (mean 1 mm) in diameter. They arose from the recurrent laryngeal nerves between vertebral levels T1 and C6. They all traveled to the anterior aspect of the esophagus. No statistical differences were seen between left and right sides or between sexes.

CONCLUSION

The esophageal branches of the recurrent laryngeal nerve have been poorly described and could contribute to complications such as swallowing dysfunction following anterior cervical discectomy and fusion procedures. Therefore, a better understanding of their anatomy is important for spine surgeons. Our study revealed that these branches are always present on both sides and the anterior surface of the esophagus should be avoided while retracting it in order to minimize the risk of postoperative dysphagia.

Thoracic Vagal Efferent Nerve Stimulation Evokes Substance P-Induced Early Airway Bronchonstriction and Late Proinflammatory and Oxidative Injury in the Rat Respiratory Tract

Electrical stimulation of efferent thoracic vagus nerve (TVN) evoked neurogenic inflammation in respiratory tract of atropine-treated rats by an undefined mechanism. We explored whether efferent TVN stimulation via substance P facilitates neurogenic inflammation via action of nuclear factor-kappaB (NF-kappaB) activation and reactive oxygen species (ROS) production. Our results showed that increased frequency of TVN stimulation concomitantly increased substance P-enhanced hypotension, and bronchoconstriction (increases in smooth muscle electromyographic activity and total pulmonary resistance). The enhanced SP release evoked the appearance of endothelial gap in silver-stained leaky venules, India-ink labeled extravasation, and accumulations of inflammatory cells in the respiratory tract, contributing to trachea plasma extravasation as well as increases in blood O (2)(-) and H(2)O(2) ROS amount. L-732138 (NK(1) receptor antagonist), SR-48968 (NK(2) receptor antagonist), dimethylthiourea (H(2)O(2) scavenger) or catechins (O (2)(-) and H(2)O(2) scavenger) pretreatment reduced efferent TVN stimulation-enhanced hypotension, bronchoconstriction, and plasma extravasation. Increased frequency of TVN stimulation significantly upregulated the expression of nuclear factor-kappaB (NF-kappaB) in nuclear protein and intercellular adhesion molecule-1 (ICAM-1) in total protein of the lower respiratory tract tissue. The upregulation of NF-kappaB and ICAM-1 was attenuated by NK receptor antagonist and antioxidants. In conclusion, TVN efferent stimulation increases substance P release to trigger NF-kappaB mediated ICAM-1 expression and O (2)(-) and H(2)O(2) ROS production in the respiratory tract.

Inducible nitric oxide synthase evoked nitric oxide counteracts capsaicin-induced airway smooth muscle contraction, but exacerbates plasma extravasation

The contribution of nitric oxide (NO) to capsaicin-evoked airway responses was investigated in rats. The measurement of plasma NO level, airway dynamics, airway smooth muscle electromyogram, and plasma extravasation by India ink and Evans blue leakage technique was adapted. Capsaicin-evoked hypotension, bronchoconstriction, trachea plasma extravasation as well as increases in plasma NO level in a dose-dependent manner. L-732138 (NK1 receptor antagonist) or SR-48968 (NK2 receptor antagonist) pretreatment reduced capsaicin-enhanced hypotension, bronchoconstriction, plasma extravasation, and plasma NO level. N(G)-nitro-L-Arginine methyl ester (L-NAME, 10 mg/kg, i.v.), a non-selective NO synthase (NOS) inhibitor, or aminoguanidine (10 mg/kg, i.v.), a selective inducible NOS (iNOS) inhibitor, reduced capsaicin-induced increases in plasma NO level and protected against capsaicin-induced plasma extravasation, whereas L-arginine (150 mg/kg, i.v.), a NO precursor, enhanced capsaicin-evoked plasma NO level and plasma extravasation. L-Arginine pretreatment ameliorated capsaicin-induced bronchoconstriction, whereas L-NAME and aminoguanidine exaggerated capsaicin-induced bronchoconstriction. In summary, NK1 and NK2 receptors and iNOS play a role in NO formation and on capsaicin-induced bronchoconstriction and plasma extravasation. NO generated by iNOS counteracts tachykinin-mediated bronchoconstriction, but exacerbates tachykinin-mediated plasma extravasation.

Local capsaicin application to the stellate ganglion and stellatectomy attenuate neurogenic inflammation in rat bronchi

The present study investigated the contributions of vagal and nonvagal sensory nerve fibers on neurogenic inflammation in rat bronchial airways. A surgical procedure was developed via the rat mediastinum ventral intercostal space to prepare an intercostal opening without causing pneumothorax for performing stellate ganglionectomy alone, thoracic vagus nerve section alone, and stellatectomy plus thoracic vagotomy, and for injecting capsaicin (2 microl, 10 mg/ml) and 6-hydroxydopamine (2 microl, 50 mg/ml) into the ganglion. One week later in our procedure, we investigated if neurogenic inflammation induced by an intravenous injection of capsaicin (300 nmol/ml/kg) and innervation density of substance P-immunoreactive sensory axons could be decreased after chronic denervation in the rat lower airways. The major findings were that surgical removal of the right stellate ganglion and local capsaicin application resulted in a significant attenuation of neurogenic plasma extravasation in the right bronchial tree evoked by systemic capsaicin application. Reduction of neurogenic plasma extravasation was totally abolished by combined stellatectomy and thoracic vagotomy. The number of substance P-containing axons was also greatly decreased following these surgical and capsaicin treatments. It is concluded that sensory nerve fibers from both vagal source and nonvagal (spinal) source, which associated with the stellate ganglion, contributed significantly to neurogenic inflammation in the bronchial airways with a slightly higher contribution from the vagus nerve.