Zook J M, Leake P A
Department of Zoological and Biomedical Sciences, Ohio University, Athens 45701.
J Comp Neurol. 1989 Dec 8;290(2):243-61. doi: 10.1002/cne.902900206.
The goals of this study were to describe the cochlear frequency map of the mustache bat, Pteronotus parnellii, and to relate the organization of cochlear primary afferents to that of the second-order projections from the cochlear nucleus to the superior olivary complex. Small deposits of horseradish peroxidase (HRP) were placed in the cochlear nucleus at sites that were physiologically characterized with respect to unit-best frequency. From the deposits, labeled fibers were traced in the retrograde direction to the cochlea and in the anterograde direction to the superior olive. Cochleas from both experimental and control animals were examined with light and electron microscopy. The peripheral axons of spiral ganglion neurons were counted in order to quantify the unusual variation in the innervation density along the cochlear spiral of the mustache bat. Regions of increased innervation density were found at the beginning and end of the basal turn of the cochlea. The highest cochlear innervation density consistently occurred in the upper basal turn. In horseradish peroxidase tracing experiments, this region contained labeled fibers only when HRP deposits were made at sites within the cochlear nucleus with unit-best frequencies around 60 kHz. These findings support the suggestion (Kössl and Vater, '85) that the cochlear sites of increased innervation density are related to the neural and behavioral emphasis that this echolocating bat places upon the analysis of the 60 kHz frequency band. The general arrangement of tonotopic maps within the cochlea, cochlear nucleus, and superior olive was consistent with previous observations in this bat and other mammalian species. At all three levels, there was evidence of a disproportionately large representation of frequencies around 60 kHz, similar to the enlarged representation reported within the inferior colliculus and auditory cortex of the mustache bat. In all cases there was a consistent relation between the size of the HRP deposit and the number and distribution of retrogradely labeled neurons in the cochlea. For most cases there was a similar relation between the size of the deposit and the terminal arborization field of anterogradely labeled fibers in the superior olive. However, in cases with deposits associated with the 60 kHz frequency band, the size of the labeled arborization field was more than twice as large as expected from the size of the deposits and from the extent of labeling in the cochlea. These cases suggest that the representation of frequencies around 60 kHz, already overrepresented in both the cochlea and cochlear nucleus, may be further expanded at the level of the superior olivary complex.
本研究的目的是描绘髯蝠（Pteronotus parnellii）的耳蜗频率图谱，并将耳蜗初级传入神经的组织与从耳蜗核到上橄榄复合体的二级投射的组织联系起来。将小量辣根过氧化物酶（HRP）置于耳蜗核中，这些位置已根据单位最佳频率进行了生理学特征描述。从这些沉积部位开始，逆行追踪标记纤维至耳蜗，顺行追踪至橄榄体。对实验动物和对照动物的耳蜗进行了光镜和电镜检查。对螺旋神经节神经元的外周轴突进行计数，以量化髯蝠耳蜗螺旋上神经支配密度的异常变化。在耳蜗基底转的起始和末端发现了神经支配密度增加的区域。耳蜗神经支配密度最高的区域始终出现在基底转上部。在辣根过氧化物酶追踪实验中，只有当在耳蜗核内单位最佳频率约为60kHz的部位进行HRP沉积时，该区域才含有标记纤维。这些发现支持了如下观点（科斯尔和瓦特，1985年）：神经支配密度增加的耳蜗部位与这种回声定位蝙蝠在分析60kHz频段时所给予的神经和行为重点有关。耳蜗、耳蜗核和上橄榄体内的音频定位图谱的总体排列与此前在这种蝙蝠和其他哺乳动物物种中的观察结果一致。在所有这三个层面上，都有证据表明60kHz左右的频率有不成比例的大量表征，类似于在髯蝠的下丘和听觉皮层中所报道的扩大表征。在所有情况下，HRP沉积的大小与耳蜗中逆行标记神经元的数量和分布之间都存在一致的关系。在大多数情况下，沉积大小与上橄榄体中顺行标记纤维的终末分支场之间也存在类似的关系。然而，在与60kHz频段相关的沉积情况下，标记分支场的大小比根据沉积大小和耳蜗中的标记范围所预期的大两倍以上。这些情况表明，在耳蜗和耳蜗核中已经过度表征的60kHz左右的频率，在上橄榄复合体层面可能会进一步扩大。
