Semicircular canal structure during postnatal development in cat and guinea pig

Insights into Inner Ear Function and Disease Through Novel Visualization of the Ductus Reuniens, a Seminal Communication Between Hearing and Balance Mechanisms

The sensory end-organs responsible for hearing and balance in the mammalian inner ear are connected via a small membranous duct known as the ductus reuniens (also known as the reuniting duct (DR)). The DR serves as a vital nexus linking the hearing and balance systems by providing the only endolymphatic connection between the cochlea and vestibular labyrinth. Recent studies have hypothesized new roles of the DR in inner ear function and disease, but a lack of knowledge regarding its 3D morphology and spatial configuration precludes testing of such hypotheses. We reconstructed the 3D morphology of the DR and surrounding anatomy using osmium tetroxide micro-computed tomography and digital visualizations of three human inner ear specimens. This provides a detailed, quantitative description of the DR's morphology, spatial relationships to surrounding structures, and an estimation of its orientation relative to head position. Univariate measurements of the DR, inner ear, and cranial planes were taken using the software packages 3D Slicer and Zbrush. The DR forms a narrow, curved, flattened tube varying in lumen size, shape, and wall thickness, with its middle third being the narrowest. The DR runs in a shallow bony sulcus superior to the osseus spiral lamina and adjacent to a ridge of bone that we term the "crista reuniens" oriented posteromedially within the cranium. The DR's morphology and structural configuration relative to surrounding anatomy has important implications for understanding aspects of inner ear function and disease, particularly after surgical alteration of the labyrinth and potential causative factors for Ménière's disease.

A mathematical model of human semicircular canal geometry: a new basis for interpreting vestibular physiology

We report a precise, simple, and accessible method of mathematically measuring and modeling the three-dimensional (3D) geometry of semicircular canals (SCCs) in living humans. Knowledge of this geometry helps understand the development and physiology of SCC stimulation. We developed a framework of robust techniques that automatically and accurately reconstruct SCC geometry from computed tomography (CT) images and are directly validated using micro-CT as ground truth. This framework measures the 3D centroid paths of the bony SCCs allowing direct comparison and analysis between ears within and between subjects. An average set of SCC morphology is calculated from 34 human ears, within which other geometrical attributes such as nonplanarity, radius of curvature, and inter-SCC angle are examined, with a focus on physiological implications. These measurements have also been used to critically evaluate plane fitting techniques that reconcile many of the discrepancies in current SCC plane studies. Finally, we mathematically model SCC geometry using Fourier series equations. This work has the potential to reinterpret physiology and pathophysiology in terms of real individual 3D morphology.

Aging Effects on Vestibulo-Ocular Responses in C57BL/6 Mice: Comparison with Alteration in Auditory Function

Age-related changes in auditory function are well documented in animal models; however, this is not the case as regards vestibular function. In this study, we evaluated age-related changes in vestibulo-ocular responses in C57BL/6 mice that are considered as a model of presbycusis. The functional data were substantiated by the findings of histological analysis of vestibular and auditory peripherals. The gain in vestibulo-ocular reflex, which reflects functionality of the vestibular system, increased in an age-dependent manner until 12 weeks and exhibited limited functional loss due to aging after 24 weeks. By contrast, no alteration in the thresholds of the auditory brainstem response (ABR) was observed from 3 to 12 weeks of age; however, ABR thresholds were significantly elevated from age 24 weeks and onwards. Histological analysis demonstrated that the degeneration of auditory peripherals was closely related with functional loss due to aging. Vestibular peripherals also exhibited age-related degeneration morphologically, although age-related dysfunction was not apparent. Age-related changes in the vestibular function of C57BL/6 mice followed a different time course when compared to changes in auditory function. These findings indicate that mechanisms for age-related changes in vestibular function differ from those of auditory function.

Morphometry of the vestibular organ in neonate and adult African mole-rats Cryptomys species

The dimensions (length and cross-sectional area) of the semicircular canal of the inner ear in Cryptomys (a rodent with long gestation, and altricial young with slow, extended postnatal growth) did not change after birth. This is further evidence for the relative morphometric postnatal stability of semicircular structures in mammals.

Dimensions of the horizontal semicircular duct, ampulla and utricle in rat and guinea pig

The dimensions of the membranous labyrinth partly determine the mechanical operation of the semicircular canal system. This study provides, for the first time, extensive measures in individual specimens of the sizes, cross-sectional shapes and areas of the horizontal semicircular duct, ampulla and utricle in the rat and the guinea pig. The membranous labyrinths were fixed in Karnovsky's fixative, exposed, photographed, sectioned, oriented perpendicular to the line of sight and then measured using a calibrated graticule in the eye piece of an operating microscope. As well as the expected size differences between these species, there are major differences in the shape of the utricle.

The response of primary horizontal semicircular canal neurons in the rat and guinea pig to angular acceleration

In rats and guinea pigs, primary afferent neurons from the horizontal semicircular canal were divided into two categories, regular and irregular, on the basis of the regularity of their resting activity. Regular neurons tend to have higher average resting rates than irregular neurons and in response to a constant angular acceleration stimulus of 16.7 deg/s2 regular neurons tended to have lower sensitivity and longer time constants than irregular cells. Some irregular neurons are more sensitive to incremental accelerations than to decremental accelerations of the same magnitude, whereas regular neurons tend to show symmetrical sensitivity. In response to sinusoidal angular acceleration stimuli (fixed frequencies) in the range 0.01-1.5 Hz, cells which fired regularly at rest tended to have smaller gain and longer phase lag re acceleration at most frequencies than irregular cells. Transfer functions were obtained for averaged data for regular and irregular neurons separately in both species. In both species there is evidence of systematic variation between neurons within each category, and this systematic variation is obscured by averaging across neurons.