Merchán M A, Saldaña E, Plaza I
Departamento de Biología Celular y Patología, Facultad de Medicina, Universidad de Salamanca, Spain.
J Comp Neurol. 1994 Apr 8;342(2):259-78. doi: 10.1002/cne.903420209.
A basic principle of organization in auditory centers is the topographic-tonotopic order. Whether this applies to the dorsal nucleus of the lateral lemniscus (DNLL), however, is still debated. To clarify this problem, we have utilized the neuroanatomical tracers horseradish peroxidase (HRP) and biotinylated dextran (BD) injected into different regions of the central nucleus of the inferior colliculus (CNIC) in the rat. After large injections of HRP that included most of the CNIC, retrogradely labelled neurons were found all across the ipsi- and contralateral DNLL, showing that all parts of this nucleus innervate the CNIC bilaterally. More neurons were seen consistently on the side contralateral to the injection site. Labelled fibers, however, were abundant ipsilaterally, but scarce in the contralateral DNLL. Single, small injections of HRP or BD into the CNIC resulted in labelling in restricted areas of the ipsi- and contralateral DNLL. In coronal sections, the neurons and fibers labelled in the ipsilateral DNLL formed a well-defined, ring-shaped structure made of dendrites and axons oriented parallel to each other, which we termed "annular band." The observation of serial sections revealed that the annular band seen in any individual section represents a slice through a more or less complete three-dimensional, hollow, ovoid structure oriented rostrocaudally. The position and diameter of the annular band changed as the injection site was shifted along the tonotopic axis of the CNIC. Single injections placed in the ventromedial, high-frequency region of the CNIC produced a large annular band along the periphery of the DNLL. After injections placed in progressively more dorsolateral, lower-frequency regions of the CNIC, the annular band became smaller in diameter and occupied a successively more central position in the DNLL. Double injections along the tonotopic axis of the CNIC resulted in two roughly concentric annular bands. The labelled neurons and fibers in the contralateral DNLL systematically occupied a position symmetric to the annular band seen ipsilaterally. These findings indicate that the rat DNLL is primarily composed of neurons with flattened dendritic arbors and flattened fields of terminal fibers. These two elements intermingle, forming concentric layers around the geometric center of the nucleus. The axons of neurons within corresponding layers on the two sides converge onto the CNIC of both sides in a strict topographic fashion: the peripheral layers project to the ventromedial, high-frequency region of the CNIC, and the central layers project to the dorsolateral, low-frequency region. These results suggest that the concentric arrangement of the DNLL is the substrate of its tonotopic organization.
听觉中枢组织的一个基本原则是拓扑 - 音频拓扑顺序。然而，这一原则是否适用于外侧丘系背核（DNLL）仍存在争议。为了阐明这个问题，我们利用神经解剖学示踪剂辣根过氧化物酶（HRP）和生物素化葡聚糖（BD），将其注射到大鼠下丘中央核（CNIC）的不同区域。在大量注射包含大部分CNIC的HRP后，在同侧和对侧DNLL中均发现了逆行标记的神经元，这表明该核的所有部分均双侧支配CNIC。在注射部位对侧始终能看到更多的神经元。然而，标记纤维在同侧丰富，而在对侧DNLL中稀少。向CNIC单次小剂量注射HRP或BD会导致同侧和对侧DNLL的受限区域出现标记。在冠状切片中，同侧DNLL中标记的神经元和纤维形成了一个界限分明的环形结构，由相互平行排列的树突和轴突组成，我们将其称为“环形带”。连续切片观察显示，在任何单个切片中看到的环形带代表了一个或多或少完整的三维、中空、卵形结构的切片，该结构沿头尾方向排列。随着注射部位沿CNIC的音频拓扑轴移动，环形带的位置和直径会发生变化。在CNIC腹内侧高频区域进行单次注射会在DNLL周边产生一个大的环形带。在将注射部位逐渐移至CNIC背外侧低频区域后，环形带直径变小，并在DNLL中占据越来越靠中心的位置。沿CNIC音频拓扑轴进行两次注射会产生两个大致同心的环形带。对侧DNLL中标记的神经元和纤维系统地占据了与同侧所见环形带对称的位置。这些发现表明，大鼠DNLL主要由具有扁平树突分支和平坦终末纤维场的神经元组成。这两个元素相互交织，围绕核的几何中心形成同心层。两侧相应层内神经元的轴突以严格的拓扑方式汇聚到两侧的CNIC：外周层投射到CNIC的腹内侧高频区域，中央层投射到背外侧低频区域。这些结果表明，DNLL的同心排列是其音频拓扑组织的基础。
