科研之友
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摘要
Afferent fibres in the auditory nerve of the barn owl were counted and their diameters measured, using a semi-automated image analysis of light-microscopical sections. Since fibres of both the basilar papilla and the lagenar macula run in the auditory nerve, but cannot be sharply distinguished, an independent evaluation of the lagenar fibres was obtained from sections near the apical end of the cochlear duct. Average numbers of 31,142 afferents from the basilar papilla and 1,342 lagenar fibres were found. Papillar axons were, on average, considerably larger than lagenar axons. Analysis of serial sections at regular intervals along the cochlea showed that less than 20% of all papillar afferents derive from regions of the basilar papilla corresponding to frequencies below 2 kHz. Above 2 kHz, about equal numbers of afferents were counted per octave. While this reflects an unusually heavy emphasis on high frequencies among birds, the afferent fibre supply is less focused on a narrow frequency band than the cochlear space map implies. Axon diameters increased systematically with frequency up to approximately 7 kHz and then decreased again towards the base of the papilla. This pattern would be suited to exaggerate latency differences between frequencies in the cochlear nucleus. However, larger axons could also be an adaptation to the increasing demands for temporal accuracy in phase locking at high frequencies. Myelination of papillar afferents was studied in ultrathin sections and found to be very uniform, with an average sheath thickness of 0.58 mu m, regardless of axon diameter.
- 出版日期1997
