Development of spontaneous activity and response properties of primary lagenar neurons in the chick.

Here, we report for the first time developmental changes in spontaneous activity and in response properties of single nerve fibers from the macular chick lagena. Such aspects are important in order to get insight into the functional role of the lagena which remains undetermined. For this purpose, we used intracellular and extracellular single-unit recording techniques in an isolated inner ear preparation from the chicken at ages E15 and P1. At E15, afferent fibers displayed a low irregular spontaneous discharge rate (41 +/- 14 spikes/s, CV = 1.17 +/- 0.1), which was replaced by regular high frequency spontaneous activity at P1 (CV = 0.48 +/- 0.8, 89 +/- 27 spikes/s). During the developmental period including E15, the percentage of silent neurons was 60% while that of P1 was 40%. The synaptic activity was higher at E15 than at P1. The action potential waveform generated at E15 had small amplitude and derivative depolarization, and consequently, a large duration in correlation with respect to action potential waveform at P1 (respectively: 53 +/- 2 vs. 65 +/- 3 mV, 60 +/- 11 vs. 109 +/- 20 mV/ms, 3.6 +/- 0.4 vs. 1.1 +/- 0.12 ms). In addition, we recognized two response dynamics to the injection of current steps: phasic, or rapidly adapting neurons and tonic, or slowly adapting neurons. Our results indicate similar developmental processes for the lagena as described for the vestibular system in other species, in agreement with the known morphological characteristics of this otholitic end organ. The presence of more than one subtype of afferent neuron also correlates with previous reports on vestibular afferents with analogous electrophysiological properties, strongly suggesting the vestibular nature of the lagena.

Internal auditory canal: pre- and postnatal growth.

OBJECTIVE: The purpose of this study was to describe the size and shape of the internal auditory canal (IAC) during development. STUDY DESIGN: The study was conducted in 98 dried temporal bones from 7-month-old fetuses to adults. Silicone mould casts of the IAC were used. METHODS: The length of the IAC was measured on the moulds corresponding four walls as the diameter on the fundus, middle portion, and porus. RESULTS: Although size and shape vary considerably among individuals, the data showed no significant differences between the right and left sides of the same individual. The values of the wall length and porus diameter increase as a function of age, especially for the upper wall and horizontal diameter. Both middle portion diameters suffer variations at different ages, increasing and decreasing until reaching the final shape. The diameter of the fundus, near the inner ear, remains stable. CONCLUSIONS: Our results showed that during development, the shape of the IAC adapts to cranial growth, with those of adults and fetuses having a similar shape but different dimensions, which was quite evident near the porus.

Efectos de la gentamicina y kanamicina a dosis terapéuticas en el desarrollo del oído interno de la ratas / Gentamicin and Kanamicin effects on the development of the internal ear in rat

Los aminoglucósidos son antibióticos de acción bactericida a nivel ribosomal en la subunidad 30s, en donde se inhiben la síntesis proteica y disminuye la fidelidad de la traslación del RNAm. La ototoxicidad es el resultado de la destrucción progresiva de células sensoriales a nivel coclear y vestibular. El objetivo del estudio fue determinar los cambios histopatológicos en el desarrollo del oído interno en embriones de rata al administrarse gentamicina a dosis de 3.5 mg/Kg/día y 7 mg/Kg/día por 21 días y kanamicina a dosis de 7 mg/Kg/día y 15 mg/Kg/día por 21 días. Los productos fueron sacrificados a los 7 y 14 días de gestación e inmediatamente después de termianr el período de gestación. Se observó un detenimiento del desarrollo y crecimiento de las estructuras del oído interno. La gentamicina causó mayores daños a nivel vestibular y la kanamicina a nivel coclear. Los principales cambios cocleares encontrados fueron degeneración de las células externas e internas hacía la porción basal, así como colapso de las rampas vestibular y timpánica. Los principales cambios vestibulares fueron: falta de desarrollo de la cresta ampular, ausencia de células ciliadas e hipodesarrollo de canales semicirculares