Shipp Stewart
Department of Anatomy, University College of London, London WC1E 6BT, UK.
Philos Trans R Soc Lond B Biol Sci. 2005 Apr 29;360(1456):797-814. doi: 10.1098/rstb.2005.1630.
The agranular cortex is an important landmark-anatomically, as the architectural flag of mammalian motor cortex, and historically, as a spur to the development of theories of localization of function. But why, exactly, do agranularity and motor function go together? To address this question, it should be noted that not only does motor cortex lack granular layer four, it also has a relatively thinner layer three. Therefore, it is the two layers which principally constitute the ascending pathways through the sensory (granular) cortex that have regressed in motor cortex: simply stated, motor cortex does not engage in serial reprocessing of incoming sensory data. But why should a granular architecture not be demanded by the downstream relay of motor instructions through the motor cortex? The scant anatomical evidence available regarding laminar patterns suggests that the pathways from frontal and premotor areas to the primary motor cortex actually bear a greater resemblance to the descending, or feedback connections of sensory cortex that avoid the granular layer. The action of feedback connections is generally described as "modulatory" at a cellular level, or "selective" in terms of systems analysis. By contrast, ascending connections may be labelled "driving" or "instructive". Where the motor cortex uses driving inputs, they are most readily identified as sensory signals instructing the visual location of targets and the kinaesthetic state of the body. Visual signals may activate motor concepts, e.g. "mirror neurons", and the motor plan must select the appropriate muscles and forces to put the plan into action, if the decision to move is taken. This, perhaps, is why "driving" motor signals might be inappropriate-the optimal selection and its execution are conditional upon both kinaesthetic and motivational factors. The argument, summarized above, is constructed in honour of Korbinian Brodmann's centenary, and follows two of the fundamental principles of his school of thought: that uniformities in cortical structure, and development imply global conservation of some aspects of function, whereas regional variations in architecture can be used to chart the "organs" of the cortex, and perhaps to understand their functional differences.
无颗粒皮质在解剖学上是一个重要的标志——作为哺乳动物运动皮质的结构标志，在历史上，它也是功能定位理论发展的一个推动因素。但确切地说，为什么无颗粒状态与运动功能会联系在一起呢？要回答这个问题，应该注意到，运动皮质不仅缺少颗粒层第四层，其第三层也相对较薄。因此，主要构成通过感觉（颗粒状）皮质的上行通路的这两层在运动皮质中已经退化：简单地说，运动皮质不参与对传入感觉数据的串行再处理。但是，为什么通过运动皮质的运动指令的下游传递不需要颗粒状结构呢？关于层状模式的现有解剖学证据很少，这表明从额叶和运动前区到初级运动皮质的通路实际上与感觉皮质的下行或反馈连接更相似，这些连接避开了颗粒层。反馈连接的作用在细胞水平上通常被描述为“调节性的”，或者从系统分析的角度来说是“选择性的”。相比之下，上行连接可能被标记为“驱动性的”或“指导性的”。在运动皮质使用驱动性输入的情况下，它们最容易被识别为指示目标视觉位置和身体动觉状态的感觉信号。视觉信号可能会激活运动概念，例如“镜像神经元”，并且如果做出了运动的决定，运动计划必须选择合适的肌肉和力量来将计划付诸行动。也许，这就是为什么“驱动性”运动信号可能不合适的原因——最佳选择及其执行取决于动觉和动机因素。上述论点是为纪念科比尼安·布罗德曼诞辰一百周年而构建的，并且遵循了他的思想流派的两个基本原则：皮质结构和发育的一致性意味着功能的某些方面在全球范围内得到保留，而结构上的区域差异可以用来描绘皮质的“器官”，也许还能理解它们的功能差异。
