Abstract
The fact that auditory stimuli elicited an evoked potential in the efferent activity of the recurrent laryngeal nerve was reported in the cat, and this reflex action was termed the “auditory laryngeal reflex”. In the present study, it was reported that a comparable auditory laryngeal reflex action was experimentally identified in the rabbit. Moreover, the relationships between the intensity of the auditory laryngeal reflex action and the following parameters of sound stimuli: sound pressure; sound frequency: and the pattern of sound stimuli (train and single of tone burst) were described. Furthermore, the effects of the depth of anesthesia using thiamylal or urethan and chloralose on the reflex action were studied. The experiments were performed on rabbits anesthetized with diethyl ether, paralyzed with pancronium bromide and artificially ventilated under monitoring of the end-tidal CO2 levels. Biphasic evokcd potential in the recurrent laryngeal nerve was identified of the averaging computation with 500 times triggered by sound stimuli. The first deflection of the evoked potential was positive, and the second one was negative. The peak time of the first peak was 12-14 msec. and that of the second was 20-23 msec. Within a range around the threshold of the evoked potential in response to sound stimului as measured by the sound pressure, both the amplitude of each of the biphasic evoked potential depended on the intensity of sound stimuli. The threshold of the evoked potential was about 25 dB higher than that of the auditory brain stem response (ABR) recorded at the same time. Comparable biphasic evoked potential as induced by the sound stimuli with frequencies ranging from 250 Hz to 8 kHz. Continuous sound stimuli of a tone burst (3 kHz; 110 dB; duration of tone, 5 msec.; interstimulus interval 238 msec.) for 20 minutes did not result in any distinct change of the waveform of the evoked potential, and no adaptation to the sound stimuli was noticed. Intravenous administration of thiamylal (4 mg/ kg) abolished the evoked potential due to the auditory laryngeal reflex and depressed concurrently the spontaneous nerve activity of the recurrent laryngeal nerve. As the anesthesic level become less, the abolished reflex action as well as the depressed spontaneous nerve activity gradually recovered. Power spectral analysis of the recurrent laryngeal nerve activity revealed two spectral peak of 75 Hz and 110 Hz. These components were identified to be analogous to those described as the “high frequency oscillations” in the phrenic nerve activity in the rabbits. The sound stimuli (3 kHz, 110 dB, tone burst, duration of tone 5 msec., interstimulus interval 238 msec.) did not exert any significant influence on the respiratory modulation of the nerve activity and the activity of the high frequency oscillations in the recurrent laryngeal nerve.
The administration of chloralose at a dose of 50 mg/kg reduced and at higher doses abolished the two spectral peaks representing the high frequency oscillations in the order of the first spectral peak at 75 Hz and then that at 110 Hz. The shortness of the latency of the evoked potential in the recurrent laryngeal nerve suggested that the auditory laryngeal reflex was mediated by certain neural organizations in the brain stem. It was concluded that the auditory laryngeal reflex may play a significant role as an important component in the auditory phonatory feedback system subserving speech and singing in man.
