Abstract
Previous research has indicated that differences in sensitivities to muscle relaxants exist between facial nerve- and somatic nerve-innervated muscles. Here, we report that the 50% inhibitory concentration (IC50) values for rocuronium were significantly larger in the normal orbicularis oris than those in the gastrocnemius. Increased IC50 values and reduced twitch tension were observed after facial nerve injury. The normal orbicularis oris had a smaller muscle fiber cross-sectional area (CSA) and a larger ratio of endplate surface area (ESA) to muscle fiber CSA (ESA/CSA), but no difference was found in the density of nicotinic acetylcholine receptor (nAChR) subunits on endplates between normal orbicularis oris and gastrocnemius. Expression of the nAChR α1, β1, δ, ε, and γ subunits increased significantly on the postsynaptic membranes of endplates and extra-junctional muscle membranes after facial nerve injury. Our results suggest that facial nerve-innervated muscle was less sensitive than somatic nerve-innervated muscle, and the mechanisms underlying this result may be related to muscle fiber CSA and the ESA/CSA ratio, but not to the density of nAChR subunits on endplates. Facial nerve injury caused the resistance to neuromuscular blockers and reduced twitch tension, which was related to qualitative, quantitative, and locational changes in nAChR subunits.
Highlights
Evoked electromyography (EEMG) is widely used during neurosurgery, otorhinolaryngology, and skull base surgery to detect the location of the facial nerve, which can reduce the incidence of iatrogenic facial nerve injury[1]
We aimed to provide theoretical evidence on the usage of partial neuromuscular blockade (PNMB) in the patients with different degrees of latent pre-surgical facial nerve injury referring intraoperative facial nerve EEMG monitoring in general anesthesia
The results showed that the IC50 of the orbicularis oris was higher than that of the gastrocnemius, suggesting that the responses to rocuronium were less sensitive in the facial nerve-innervated muscles compared with those in the tibial nerve-innervated muscles, which was in accordance with our previous experimental studies[6, 11,12,13]
Summary
Evoked electromyography (EEMG) is widely used during neurosurgery, otorhinolaryngology, and skull base surgery to detect the location of the facial nerve, which can reduce the incidence of iatrogenic facial nerve injury[1]. In our previous clinical study, partial neuromuscular blockade (PNMB) was used to achieve sufficient muscle relaxation to ensure body immobility, while maintaining sufficient facial nerve neuromuscular conduction to allow the use of EEMG2. These results are in accordance with other reports[3, 4] showing that muscles innervated by the facial and somatic nerves might have different sensitivities to muscle relaxants. Γ-AChR), which have different electrophysiological characteristics, resulting in pharmacodynamic changes in NDMRs. the different sensitivities to muscle relaxants between facial nerve- and somatic nerve-innervated muscles might be associated with quantitative or qualitative differences in nAChR subunits at the NMJ. We aimed to provide theoretical evidence on the usage of PNMB in the patients with different degrees of latent pre-surgical facial nerve injury referring intraoperative facial nerve EEMG monitoring in general anesthesia
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