Accelerated signal propagation speed in human neocortical microcircuits

Abstract

Human-specific cognitive abilities depend on information processing in the cerebral cortex, where neurons are significantly larger and sparser compared to rodents. We found that, in synaptically-connected layer 2/3 pyramidal cells (L2/3 PCs), soma-to-soma signal propagation delay is similar in humans and rodents. Thus, to compensate for the increase in neurons’ size, membrane potential changes must propagate faster in human axons and/or dendrites. Dual somato-dendritic and somato-axonal patch recordings show that action potentials (APs) propagation speed is similar in human and rat axons, but the forward propagation of the EPSPs and the back-propagating APs are ∼ 26 and 47% faster in human dendrites respectively. Faithful biophysical models of human and rat L2/3 PCs, combined with pharmacological manipulations of membrane properties, showed both the larger diameter of the apical dendrite and the larger conductance load imposed by the basal tree in human, combined with differences in cable properties, underlie the accelerated signal propagation in human cortical circuits.