Maturation of medial efferent system function in humans

Prevalence and ultrastructural morphology of axosomatic synapses on spiral ganglion cells in humans of different ages

Axosomatic synapses were found on human spiral ganglion cells (HSGCs). Ultrastructural characterization and calculation of the prevalence of these synapses were performed by electron microscopic semi-serial sections of both type I and type II HSGCs, in specimens from subjects of ages 1 day, 14 days, 21 years and 51 years. Synapses on type I HSGCs were extremely rare. In contrast, axosomatic synapses were present on approximately 50% of type II HSGCs of a young adult. This prevalence seemed to vary by age. Thus, no synapses were found in a 1-day old neonate, few in a 14-day old, and on approximately 15% of the type II SGCs from a 51-year old specimen. The origin of the nerve fibers synapsing on the type II HSGCs could not be determined. In view of the fact that some of the fibers projected from the intraganglionic spiral bundle, which is known to contain olivocochlear efferents, these fibers may represent an efferent pathway to the spiral ganglion. However, since there was morphological evidence of more than one type of nerve fiber synapsing on type II HSGCs, other neural origins must be considered. Although the physiological function of these synapses is unknown, they may mediate pre-synaptic neural modulation of the type II HSGCs at the level of the spiral ganglion.

Distortion product otoacoustic emission (2f1-f2) amplitude growth in human adults and neonates

Distortion product otoacoustic emissions (DPOAEs) are thought to be by-products of an active amplification process in the cochlea and thus serve as a metric for evaluating the integrity of this process. Because the cochlear amplifier functions in a level-dependent fashion, DPOAEs recorded as a function of stimulus level (i.e., a DPOAE growth function) may provide important information about the range and operational characteristics of the cochlear amplifier. The DPOAE growth functions recorded in human adults and neonates may provide information about the maturation of these active cochlear processes. Two experiments were conducted. Experiment I included normal-hearing adults and term-born neonates. The 2f1-f2 DPOAE growth functions were recorded for both age groups at three f2 frequencies. Experiment II was an extension of the first experiment but added a subject group of premature neonates. The results of these studies indicate that DPOAE growth functions most often show amplitude saturation and nonmonotonic growth for all age groups. However, premature neonates show monotonic growth and the absence of amplitude saturation more often than adults. Those premature neonates who do show saturation also show an elevated threshold for amplitude saturation relative to adults. In contrast, term neonates are adultlike for most measures except that they show a larger percentage of nonsaturating growth functions than adults. These results may indicate immaturity in cochlear amplifier function prior to term birth in humans. Outer hair cell function and/or efferent regulation of outer hair cell function are hypothesized sources of this immaturity, although some contribution from the immature middle ear cannot be ruled out.

The human olivocochlear system: organization and development

The goals of the present study were to identify olivocochlear neurons in the human brainstem, to establish the time course of their early development and to compare the organization of the human olivocochlear system to that of other mammals. To accomplish these goals, we used immunohistochemistry for choline acetyltransferase (ChAT) and calcitonin gene-related peptide (CGRP) in postmortem brainstems of human subjects ranging in age from 16 fetal weeks to 17 years. By immunostaining, we identified two classes of cells in the superior olivary complex: both classes were seen to be present from the twenty-first fetal week to the seventeenth year. Neurons which are immunostained only for ChAT are located primarily in the dorsomedial, ventral and ventrolateral sectors of the periolivary region. These neurons are predominantly bipolar or multipolar cells, and are probably homologous to medial olivocochlear neurons in other species. A second population of cells is immunoreactive for both ChAT and CGRP. This population includes a cluster of mostly small oval neurons, located on the dorsal edge of the olivary complex, and a variable number of cells found along the margin of the lateral olivary nucleus. These ChAT- and CGRP-immunoreactive cells are likely to be homologous to the lateral olivocochlear system in other mammals. With increasing age, the dorsal cluster of small cells shifts from its original cap-like position over the lateral olivary nucleus to become an extended column of cells lying among the fibers of the olivocochlear bundle.