Characteristics of Glottic Closure Reflex in a Canine Model Fig. 2. Presence of R2 at 0.5 MAC. Late response (R2) was detected at lowest level of anesthesia in one experimental animal. also appears to support the notion that a facilitated adductor reflex is likely responsible for a crossed R1 in the awoken state, whereas its sensitivity to pharmacologic sedation would imply that relevant facilitatory mechanisms are located central to motor neurons of nucleus ambiguus.22 The reason why the overall latencies obtained in this study are bigger than those of previous studies with canine model4,6 can be explained by the fact that we used younger dogs that are not fully mature.23 Furthermore, the longer latency of the left rather than right ipsilateral evoked response understandably reflects the longer course of the left recurrent laryngeal nerve.6 After electrical stimulation of iSLN, we can expect the swallowing reflex as well as the laryngeal responses. The esophageal muscle of dogs is entirely composed of striated fibers, and therefore is controlled by cranial motoneurons.24 Accordingly, we can expect the existence of esophageal motility during checking the laryngeal EMG when we stimulate the iSLN. However, a typical rhythmic pattern of swallowing was elicited during long-lasting repetitive stimulation of iSLN.24 Thus, we could not only exclude the possibilities of electrical interruption of esophageal peris- talsis, but also avoid the aspiration which could exaggerate the laryngeal responses mimicking the laryngospasm as happens in gastroesophageal reflux. Interestingly, the latency of contralateral evoked response was about 1.5 to 2 msec longer than that of ipsilateral response. Assuming that nerve conduction velocity appro- ximates 5 cm/msec,25 that the average length of neural cir- cuitry from larynx to brain stem is 20 cm, and that each synaptic delay is 1.5 msec,25 the data support the following organizational model. In our model, we propose that the Fig. 3. Organizational model of the crossed adductor reflex pathway in a dog. iSLN projects to motor neurons of nucleus ambiguus through at least two synapses, the first within ipsilateral nucleus tractus solitarius, and the second likely within nucleus ambiguus ipsilaterally and contralaterally within the reti- cular formation in a manner supported by Sessle’s observa- tions.26 In recent studies, it was suggested that the pig model has equivalent numbers of ipsilateral and contralateral interneurons,22 and that humans have 2 to 3 more contrala- teral interneurons compared to ipsilateral side.27 However, in our current study with canine model, the fact that the contralateral latency was about 2 msec longer than ipsilateral one leads us to suggest the presence of one more interneuron possibly in the reticular formation (Fig. 3). The presence of R2 found in our study, which has not been observed in anesthetized animal models,1 can be a valuable data to investigate the level of awakening to affect the laryngeal reflex. In summary, the glottic closure response appears bilaterally when the contralateral response is supported by central facilitation in awoken state, but anesthesia abolishes the crossed response, thus restricting the response to an ipsila- teral one. Therefore, the anesthetic loss of crossed adductor response is expected to weaken the glottic closing force even under the normal conditions of physiologic stimulation when stimuli are delivered bilaterally and concurrently to each iSLN. A complete suppression of crossed response under anesthesia may exert a clinically important effect. Pharmacologic suppression of supramedullary facilitation gives us a clear explanation for several important clinical findings, including the increased incidence of life threaten- ing aspiration among sedated ICU patients or institutio- nalized psychiatric patients under heavy psychotropic control where aspiration pneumonia represents a highly significant risk. The results of this study are expected to provide the neurophysiologic basis for preventive measures in high risk patients and could, for example, alter dosing schedules of psychotropic medications, improve respiratory Yonsei Med J http://www.eymj.org Volume 50 Number 3 June 2009 383