Brodal P, Bjaalie J G
Department of Anatomy, University of Oslo, Norway.
Prog Brain Res. 1997;114:227-49. doi: 10.1016/s0079-6123(08)63367-1.
Recent studies of the primate corticopontine projection show that the neocerebellum--in addition to connections from motor and sensory areas--receives connections from various association areas of the cerebral cortex, some of which are thought to be primarily engaged in cognitive tasks. The quantities of such connections in relation to those from more clearly motor-related parts of the cortex need to be more precisely determined, however. Furthermore, the anatomic data on origin of corticopontine fibers needs to be supplemented with physiological experiments to clarify their functional properties at the single-cell level. For example, nothing is known of the functional role of the large input from the cingulate gyrus, nor is the input from the posterior parietal cortex physiologically characterized. Finally, the scarcity of corticopontine connections from the prefrontal cortex in the monkey (and probably also in man) may not seem readily compatible with a prominent role of the neocerebellum in certain cognitive tasks. We discuss data--in particular from three-dimensional reconstructions--indicating that both corticopontine projects and pontocerebellar neurons are arranged in a lamellar pattern. Corticopontine and pontocerebellar lamellae have similar shapes and orientations but appear to differ in other respects. Corticopontine terminal fields are sharply delimited, apparently without gradual overlap between projections from different sites in the cortex, whereas pontocerebellar lamellae are more fuzzy and exhibit gradual overlap of neuronal populations projecting to different targets. In spite of the sharpness of the corticopontine projection, there may be many opportunities for convergence of inputs from different parts of the cortex. Thus, the wide divergence of corticopontine projections produces many sites of overlap, and extensive interfaces between different terminal fields enabling convergence of inputs onto each neuron. We suggest that the lamellar arrangement of corticopontine terminal fields and of pontocerebellar neurons serve to create diversity of pontocerebellar neuronal properties. Thus, each small part of the cerebellar cortex would receive a specific combination of messages from many different sites in the cerebral cortex. The spatial arrangement of cerebrocerebellar connections have to be understood both in terms of fairly simple large-scale, gradual topographic relationships and an apparently highly complex pattern of divergence and convergence. Developmental studies of corticopontine and of pontocerebellar projections together with three-dimensional reconstructions in adults suggest that the highly complex adult connectional pattern may be created by simple rules operating during development.
最近对灵长类动物皮质脑桥投射的研究表明，新小脑——除了与运动和感觉区域有联系外——还接收来自大脑皮质各个联合区域的联系，其中一些区域被认为主要参与认知任务。然而，与来自皮质更明确的运动相关部分的联系相比，此类联系的数量需要更精确地确定。此外，关于皮质脑桥纤维起源的解剖学数据需要通过生理学实验来补充，以在单细胞水平上阐明它们的功能特性。例如，扣带回大量输入的功能作用尚不清楚，后顶叶皮质的输入在生理学上也没有特征描述。最后，猴子（可能人类也是如此）前额叶皮质的皮质脑桥联系稀少，这似乎与新小脑在某些认知任务中起重要作用不太相符。我们讨论了一些数据——特别是来自三维重建的数据——表明皮质脑桥投射和脑桥小脑神经元都呈层状排列。皮质脑桥层和脑桥小脑层形状和方向相似，但在其他方面似乎有所不同。皮质脑桥终末场界限分明，不同皮质部位的投射之间显然没有逐渐重叠，而脑桥小脑层则更模糊，投射到不同靶点的神经元群体存在逐渐重叠。尽管皮质脑桥投射界限分明，但来自皮质不同部位的输入仍可能有许多汇聚的机会。因此，皮质脑桥投射的广泛发散产生了许多重叠位点，不同终末场之间有广泛的界面，使输入能够汇聚到每个神经元上。我们认为，皮质脑桥终末场和脑桥小脑神经元的层状排列有助于创造脑桥小脑神经元特性的多样性。因此，小脑皮质的每个小部分都会从大脑皮质的许多不同部位接收特定的信息组合。大脑小脑连接的空间排列必须从相当简单的大规模、逐渐的拓扑关系以及明显高度复杂的发散和汇聚模式两方面来理解。对皮质脑桥和脑桥小脑投射的发育研究以及对成体的三维重建表明，高度复杂的成体连接模式可能是由发育过程中起作用的简单规则形成的。
