Externalization of neuronal somata as an evolutionary strategy for energy economization

Janina Hesse, Susanne Schreiber

Abstract: Neuronal morphology of vertebrates and many invertebrates differs in a fundamental aspect: the location of neuronal cell bodies (somata) relative to their dendritic and axonal trees. The somata of most vertebrate neurons are located centrally between dendrites and axon. In contrast, neurons of various invertebrates, such as arthropods and cephalopods, typically externalize their somata to the end of a single process called a ‘stem neurite’ (Figure 1A). While this difference has been related to advantages of a spatial separation of neuropil and externalized somata [1–5], we here propose that the right soma location also reduces signal attenuation and consequently the energetic cost of signaling. Neurons commonly transfer signals from their dendrites to the axon, such that signals depolarize a centrally located soma before reaching the axon. The signal attenuation resulting from leakage through the soma membrane can be decreased through externalization of the soma, resulting in a reduction of the depolarized membrane area. In the light of evolutionary pressure towards energy-efficient signaling [6,7], we argue that an externalization of the soma is advantageous for relatively large somata. We support this hypothesis on the basis of compartmental models and previously published experimental data.