Iyengar S, Viswanathan S S, Bottjer S W
Department of Biology, University of Southern California, Los Angeles, California 90089-2520, USA.
J Neurosci. 1999 Jul 15;19(14):6037-57. doi: 10.1523/JNEUROSCI.19-14-06037.1999.
Refinement of topographic maps during sensitive periods of development is a characteristic feature of diverse sensory and motor circuits in the nervous system. Within the neural system that controls vocal learning and behavior in zebra finches, axonal connections of the cortical nucleus lMAN demonstrate striking functional and morphological changes during vocal development in juvenile males. These circuits are uniquely important for song production during the sensitive period for vocal learning, and the overall size of these brain regions and their patterns of axonal connectivity undergo dramatic growth and regression during this time. Axonal connections to and from lMAN are topographically organized in adult males that have already learned song. We wondered whether the large-scale changes seen in lMAN circuitry during the time that vocal behavior is being learned and refined could be accompanied by the emergence of topographic mapping. However, results presented herein demonstrate that most of these song-control circuits show the same broad patterns of axonal connectivity between subregions of individual nuclei at the onset of song learning as seen in adult birds. Thus, coarse topographic organization is not dependent on the types of experience that are crucial for vocal learning. Furthermore, this maintenance of topographic organization throughout the period of song learning is clearly not achieved by maintenance of static axonal arbors. In fact, because the volumes of song-control nuclei are growing (or regressing), topography must be maintained by active remodeling of axonal arbors to adapt to the changes in overall size of postsynaptic targets. A salient exception to this pattern of conserved topography is the projection from lMAN to the motor cortical region RA: this pathway is diffusely organized at the onset of song learning but undergoes substantial refinement during early stages of song learning, suggesting that remodeling of axonal connections within this projection during the period of vocal learning may signify the production of increasingly refined vocal utterances.
在发育的敏感期对地形图进行细化是神经系统中各种感觉和运动回路的一个典型特征。在控制斑胸草雀发声学习和行为的神经系统中，皮质核lMAN的轴突连接在幼年雄性的发声发育过程中表现出显著的功能和形态变化。这些回路在发声学习的敏感期对歌曲产生具有独特的重要性，并且这些脑区的整体大小及其轴突连接模式在此期间经历了显著的生长和消退。在已经学会唱歌的成年雄性中，与lMAN之间的轴突连接是按地形图组织的。我们想知道，在发声行为被学习和细化期间，lMAN回路中看到的大规模变化是否可能伴随着地形图映射的出现。然而，本文给出的结果表明，在歌曲学习开始时，大多数这些歌曲控制回路在各个核的子区域之间显示出与成年鸟类相同的广泛轴突连接模式。因此，粗略的地形图组织并不依赖于对发声学习至关重要的经验类型。此外，在整个歌曲学习期间这种地形图组织的维持显然不是通过维持静态轴突分支来实现的。事实上，由于歌曲控制核的体积在增大（或缩小），地形图必须通过轴突分支的主动重塑来维持，以适应突触后靶标整体大小的变化。这种保守地形图模式的一个显著例外是从lMAN到运动皮质区域RA的投射：这条通路在歌曲学习开始时是分散组织的，但在歌曲学习的早期阶段经历了大量细化，这表明在发声学习期间该投射内轴突连接的重塑可能意味着产生越来越精细的发声。
