Externalization of neuronal somata as an evolutionary strategy for energy economization

Figure 1. Signal attenuation in neurons with central or externalized soma location. (A) Distinct morphology of neurons in the central nervous system of various invertebrates and vertebrates: in the former, the soma is externalized, while in the latter, a central location of the soma predominates (examples from blowfl y and rat). (B) Left: circuit diagrams representing the analytical, simplifi ed models. Right: voltage response to injected current pulses. The build-up of depolarization is initially faster for externalized somata, rendering them well adapted for the transmission of brief stimuli or high frequencies. (C) Color-coded morphology (either externalized in green, or central in blue) that is advantageous for signal attenuation as a function of the stimulus duration and the soma-to-neurite ratio for passive analytical models. Curves depict the critical soma-to-neurite ratios: analytical solution (solid curve), multicompartmental models with purely passive conductances (dashed), active models including spike generation (dotted). Above the critical soma-to-neurite ratio, externalization enhances energy effi ciency. For illustration, the red box marks the soma-toneurite interval corresponding to a biologically relevant range of stimulus durations (0.1m – 0.4m). (D) Experimental data on the soma-to-neurite ratio for neurons from various species (each vertical bar corresponding to one cell type; for details see Supplemental Information). Top: based on axial resistances as measured in dendrites or axons of the respective neurons, average soma-to-neurite ratio is larger in cells with externalized soma than in those with central soma. Bottom: assuming a higher axial resistance in the stem neurite (model prediction) increases this trend. Externalization of neuronal somata as an evolutionary strategy for energy economization