The biophysical properties of neurons are the foundation for computation in the brain. Neuronal size is a key determinant of single neuron input–output features and varies substantially across species^1,2,3. However, it is unknown whether different species adapt neuronal properties to conserve how single neurons process information^4,5,6,7. Here we characterize layer 5 cortical pyramidal neurons across 10 mammalian species to identify the allometric relationships that govern how neuronal biophysics change with cell size. In 9 of the 10 species, we observe conserved rules that control the conductance of voltage-gated potassium and HCN channels. Species with larger neurons, and therefore a decreased surface-to-volume ratio, exhibit higher membrane ionic conductances. This relationship produces a conserved conductance per unit brain volume. These size-dependent rules result in large but predictable changes in somatic and dendritic integrative properties. Human neurons do not follow these allometric relationships, exhibiting much lower voltage-gated potassium and HCN conductances. Together, our results in layer 5 neurons identify conserved evolutionary principles for neuronal biophysics in mammals as well as notable features of the human cortex.

神经元的生物物理特性是大脑计算的基础。神经元大小是单个神经元输入-输出特征的关键决定因素，并且在物种^1,2,3之间存在很大差异。然而，尚不清楚不同物种是否会适应神经元特性以保存单个神经元如何处理信息^4,5,6,7. 在这里，我们描述了 10 个哺乳动物物种的第 5 层皮质锥体神经元，以确定控制神经元生物物理学如何随细胞大小变化的异速生长关系。在 10 个物种中的 9 个中，我们观察到控制电压门控钾和 HCN 通道电导的保守规则。具有较大神经元的物种，因此具有降低的表面体积比，表现出更高的膜离子电导。这种关系产生每单位脑体积的守恒电导。这些尺寸相关的规则导致体细胞和树突综合特性发生大但可预测的变化。人类神经元不遵循这些异速生长关系，表现出低得多的电压门控钾和 HCN 电导。一起，