Synapses are involved in the communication of information from one neuron to another. However, a systematic analysis of synapse density in the neocortex from a diversity of species is lacking, limiting what can be understood about the evolution of this fundamental aspect of brain structure. To address this, we quantified synapse density in supragranular layers II–III and infragranular layers V–VI from primary visual cortex and inferior temporal cortex in a sample of 25 species of primates, including humans. We found that synapse densities were relatively constant across these levels of the cortical visual processing hierarchy and did not significantly differ with brain mass, varying by only 1.9-fold across species. We also found that neuron densities decreased in relation to brain enlargement. Consequently, these data show that the number of synapses per neuron significantly rises as a function of brain expansion in these neocortical areas of primates. Humans displayed the highest number of synapses per neuron, but these values were generally within expectations based on brain size. The metabolic and biophysical constraints that regulate uniformity of synapse density, therefore, likely underlie a key principle of neuronal connectivity scaling in primate neocortical evolution.

中文翻译：

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灵长类动物新皮质进化中的不变突触密度和神经元连接缩放。

突触参与从一个神经元到另一个神经元的信息交流。然而，缺乏对来自不同物种的新皮层突触密度的系统分析，限制了对大脑结构这一基本方面进化的理解。为了解决这个问题，我们量化了包括人类在内的 25 种灵长类动物样本中初级视觉皮层和颞下皮层的颗粒上层 II-III 和颗粒下层 V-VI 中的突触密度。我们发现突触密度在皮层视觉处理层次的这些级别上相对恒定，并且与大脑质量没有显着差异，跨物种仅变化 1.9 倍。我们还发现神经元密度与大脑增大有关。最后，这些数据表明，在灵长类动物的这些新皮质区域，每个神经元的突触数量随着大脑扩张而显着增加。人类每个神经元的突触数量最多，但这些值通常在基于大脑大小的预期范围内。因此，调节突触密度均匀性的代谢和生物物理限制可能是灵长类动物新皮质进化中神经元连接缩放的关键原则的基础。