DeFelipe Javier, Alonso-Nanclares Lidia, Arellano Jon I
Instituto Cajal (CSIC), Avenida Dr. Arce, 37, 28002 Madrid, Spain.
J Neurocytol. 2002 Mar-Jun;31(3-5):299-316. doi: 10.1023/a:1024130211265.
The appearance of the neocortex, its expansion, and its differentiation in mammals, represents one of the principal episodes in the evolution of the vertebrate brain. One of the fundamental questions in neuroscience is what is special about the neocortex of humans and how does it differ from that of other species? It is clear that distinct cortical areas show important differences within both the same and different species, and this has led to some researchers emphasizing the similarities whereas others focus on the differences. In general, despite of the large number of different elements that contribute to neocortical circuits, it is thought that neocortical neurons are organized into multiple, small repeating microcircuits, based around pyramidal cells and their input-output connections. These inputs originate from extrinsic afferent systems, excitatory glutamatergic spiny cells (which include other pyramidal cells and spiny stellate cells), and inhibitory GABAergic interneurons. The problem is that the neuronal elements that make up the basic microcircuit are differentiated into subtypes, some of which are lacking or highly modified in different cortical areas or species. Furthermore, the number of neurons contained in a discrete vertical cylinder of cortical tissue varies across species. Additionally, it has been shown that the neuropil in different cortical areas of the human, rat and mouse has a characteristic layer specific synaptology. These variations most likely reflect functional differences in the specific cortical circuits. The laminar specific similarities between cortical areas and between species, with respect to the percentage, length and density of excitatory and inhibitory synapses, and to the number of synapses per neuron, might be considered as the basic cortical building bricks. In turn, the differences probably indicate the evolutionary adaptation of excitatory and inhibitory circuits to particular functions.
新皮层在哺乳动物中的出现、扩张及其分化，是脊椎动物大脑进化过程中的主要事件之一。神经科学的一个基本问题是，人类的新皮层有何特殊之处，以及它与其他物种的新皮层有何不同？很明显，不同的皮层区域在同一物种和不同物种内都表现出重要差异，这导致一些研究人员强调相似性，而另一些人则关注差异。一般来说，尽管有大量不同的元素构成新皮层回路，但人们认为新皮层神经元是围绕锥体细胞及其输入-输出连接组织成多个小的重复微回路。这些输入来自外在传入系统、兴奋性谷氨酸能棘状细胞（包括其他锥体细胞和棘状星状细胞）以及抑制性γ-氨基丁酸能中间神经元。问题在于，构成基本微回路的神经元元素分化为亚型，其中一些在不同的皮层区域或物种中缺失或高度改变。此外，不同物种的皮质组织离散垂直柱中所含神经元的数量也有所不同。此外，研究表明，人类、大鼠和小鼠不同皮层区域的神经毡具有特定层的突触学特征。这些差异很可能反映了特定皮层回路的功能差异。皮层区域之间以及物种之间在兴奋性和抑制性突触的百分比、长度和密度以及每个神经元的突触数量方面的层特异性相似性，可能被视为基本的皮层构建单元。反过来，这些差异可能表明兴奋性和抑制性回路对特定功能的进化适应。
