Thomson A M, Bannister A P
Department of Physiology, Royal Free Hospital School of Medicine, London, UK.
Neuroscience. 1998 Jun;84(3):669-83. doi: 10.1016/s0306-4522(97)00557-5.
Paired intracellular recordings in slices of adult rat neocortex with biocytin filling of synaptically connected neurons were used to investigate the pyramidal targets, in layer V, of layer III pyramidal axons. The time-course and sensitivity of excitatory postsynaptic potentials to current injected at the soma, and locations of close appositions between presynaptic axons and postsynaptic dendrites, indicated that the majority of contributory synapses were located in layer V. Within a "column" of tissue, radius < or = 250 microm, the probability that a randomly selected layer III pyramid innervated a layer V pyramid was 1 in 4 if the target cell was a burst firing pyramid with an apical dendritic tuft in layers II/I. If, however, the potential target was a regular spiking pyramid, the probability of connectivity was only 1 in 40, and none of the 13 anatomically identified postsynaptic layer V targets had a slender apical dendrite terminating in layers IV/III. Morphological reconstructions indicated that layer III pyramids select target layer V cells whose apical dendrites pass within 50-100 microm of the soma of the presynaptic pyramid in layer III and which have overlapping apical dendritic tufts in the superficial layers. The probability that a layer V cell would innervate a layer III pyramid lying within 250 microm of its apical dendrite was much lower (one in 58). Both presynaptic layer III pyramids and their large postsynaptic layer V targets could therefore access similar inputs in layers I/II, while small layer V pyramids could not. One prediction from the present data would be that neither descending layer V inputs to the striatum or thalamus, nor transcallosal connections would be readily activated by longer distance cortico-cortical "feedback" connections that terminated in layers I/II. These could, however, activate corticofugal pathways to the superior colliculus or pons, both directly and via layer III.
在成年大鼠新皮质切片中，通过对突触连接神经元进行生物胞素填充来进行成对细胞内记录，以研究Ⅲ层锥体轴突在Ⅴ层的锥体靶细胞。兴奋性突触后电位对注入胞体电流的时间进程和敏感性，以及突触前轴突与突触后树突紧密邻接的位置，表明大多数起作用的突触位于Ⅴ层。在半径≤250微米的组织“柱”内，如果靶细胞是在Ⅱ/Ⅰ层有顶端树突簇的爆发式放电锥体，则随机选择的Ⅲ层锥体支配Ⅴ层锥体的概率为四分之一。然而，如果潜在靶细胞是规则发放的锥体，则连接概率仅为四十分之一，并且在13个经解剖学鉴定的突触后Ⅴ层靶细胞中，没有一个有纤细的顶端树突终止于Ⅳ/Ⅲ层。形态学重建表明，Ⅲ层锥体选择的Ⅴ层靶细胞，其顶端树突在Ⅲ层突触前锥体胞体的50 - 100微米范围内通过，并且在表层有重叠的顶端树突簇。Ⅴ层细胞支配位于其顶端树突250微米范围内的Ⅲ层锥体的概率要低得多（五十八分之一）。因此，突触前Ⅲ层锥体及其大的突触后Ⅴ层靶细胞都可以在Ⅰ/Ⅱ层获得相似的输入，而小的Ⅴ层锥体则不能。根据目前的数据可以预测，终止于Ⅰ/Ⅱ层的远距离皮质 - 皮质“反馈”连接不会轻易激活下行至纹状体或丘脑的Ⅴ层输入，也不会轻易激活胼胝体连接。然而，这些连接可以直接并通过Ⅲ层激活至上丘或脑桥的皮质传出通路。
