Time Course of the Suppression Effect on Transient Evoked Otoacoustic Emissions by Prolonged Contralateral Acoustic Stimulation

BACKGROUND AND OBJECTIVES: Although the suppressive effect of the medial efferent acoustic reflex is well known, the time course of this effect over prolonged periods has yet to be fully evaluated. We assessed time-dependent change in the suppression of transient evoked otoacoustic emissions (TEOAEs) by the medial efferent acoustic reflex over a relatively long period. SUBJECTS AND METHODS: We measured TEOAEs in the right ear before contralateral acoustic stimulation (CAS), and then measured serial TEOAEs in the right ear at four intervals during a total of 16 minutes of continuous CAS, followed by three more recordings after termination of CAS. RESULTS: TEOAE amplitudes were reduced with CAS during a certain period (from the immediate period to 10 minutes depending on frequency) and subsequently recovered. TEOAE suppression values in the mean amplitudes for overall frequency were 0.76 dB at the initial recording, 0.35 dB at 5 minutes, 0.44 dB at 10 minutes, and 0.33 dB at 15 minutes during CAS. The initial suppression value was significantly larger than other suppression values of 5, 10, and 15 minutes (p<0.05). In recordings obtained after CAS, TEOAE amplitude exceeded pre-acoustic amplitudes at 1 kHz, 1.5 kHz, and 2 kHz. CONCLUSIONS: The present results show the existence of the medial efferent acoustic reflex and demonstrate the time course that TEOAE suppressions present initially after CAS, showing fatigue over time. Overshooting of TEOAE was observed in recordings at several frequencies after termination of CAS.

Time Course of the Suppression Effect on Transient Evoked Otoacoustic Emissions by Prolonged Contralateral Acoustic Stimulation

BACKGROUND AND OBJECTIVES: Although the suppressive effect of the medial efferent acoustic reflex is well known, the time course of this effect over prolonged periods has yet to be fully evaluated. We assessed time-dependent change in the suppression of transient evoked otoacoustic emissions (TEOAEs) by the medial efferent acoustic reflex over a relatively long period. SUBJECTS AND METHODS: We measured TEOAEs in the right ear before contralateral acoustic stimulation (CAS), and then measured serial TEOAEs in the right ear at four intervals during a total of 16 minutes of continuous CAS, followed by three more recordings after termination of CAS. RESULTS: TEOAE amplitudes were reduced with CAS during a certain period (from the immediate period to 10 minutes depending on frequency) and subsequently recovered. TEOAE suppression values in the mean amplitudes for overall frequency were 0.76 dB at the initial recording, 0.35 dB at 5 minutes, 0.44 dB at 10 minutes, and 0.33 dB at 15 minutes during CAS. The initial suppression value was significantly larger than other suppression values of 5, 10, and 15 minutes (p<0.05). In recordings obtained after CAS, TEOAE amplitude exceeded pre-acoustic amplitudes at 1 kHz, 1.5 kHz, and 2 kHz. CONCLUSIONS: The present results show the existence of the medial efferent acoustic reflex and demonstrate the time course that TEOAE suppressions present initially after CAS, showing fatigue over time. Overshooting of TEOAE was observed in recordings at several frequencies after termination of CAS.

Expression of c-fos in the Myenteric Plexus of Rat Small Intestine Following Electrical Vagal Stimulation: Quantitative Analysis on the Vagally Activated Enteric Neurons / 대한해부학회지

Activity of the enteric nervous system (ENS) is controlled by the autonomic nerves under the normal physiological condition, even though ENS has been regarded to be independent from the central nervous system. However, the relation between myenteric neurons and vagus nerves has not been fully clarified. For the defining of topographical and functional relationship between these two nervous systems, we analyzed how many myenteric neurons are activated after electrical vagal stimulation in the rat. Bilateral cervical vagi were electrically stimulated (10 V, 5 msec, 40 Hz) for a duration of 30 minutes, and then each part of the small intestine was obtained. Fos, as a functional marker for neuronal activation, immunohistochemistry was used for the detection of vagally activated myenteric neurons. Total number of myenteric neurons was obtained using cuprolinic blue stained samples, and was calculated as 12,819+/-1,514, 14,261+/-1,452, 15,411+/-2,380 per unit area (1 cm2) in duodenum, jejunum and ileum, respectively. Fos-positive myenteric neurons were scarcely observed in the normal control group. After the electrical vagal stimulation, Fos-immunoreactive (IR) neurons were detected as 31+/-17%, 17+/-9%, 16+/-10% of total number of myenteric neurons in duodenum, jejunum and ileum, respectively. These data demonstrate that only some (16~31%) of myenteric neurons are regulated by vagal efferent input, and the duodenum receives much more vagal input functionally than other distal regions. Furthermore, these findings can be applied to trials defining the functional circuit of the myenteric nervous system linked to the vagus nerves, since Fos-positive nuclei can be easily double-labeled with various neurotransmitters existing in the myenteric neurons.