Morphological details contribute to neuronal response variability within the same cell type
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A large body of literature offers an explanation on how biophysical diversity and variable branching patterns of neurons contribute to degeneracy, and therefore enable multiple solutions for a characteristic neuronal response. The specific influence of finer morphological details, such as diameter and length of dendritic branches, on response variability is yet unclear. In this study, we address this question using a model database approach with spatially extended, conductance-based compartmental models to study variability of response features, such as resting membrane potential, input resistance, spike count, first spike latency, spike height, and spike width. Using 15 reconstructed morphologies of leech touch cells with fixed branching patterns, we identified several thousands of parameter sets that reproduced the experimentally measured response features of all the tested morphologies. Even when the biophysical parameters were kept equal across reconstructed morphologies, variability in response features arose from the morphological details. Systematically varying the distribution of ion channels across the neuronal membrane revealed that all spike response features are influenced by the location of spike initiation zones with higher conductance density. Nevertheless, biologically plausible responses can arise from different locations of spike initiation zones and even homogeneous distribution of ion channels. Furthermore, comparing the simulated spike responses from two morphological subtypes of leech touch cells revealed that the previously published systematic differences cannot be explained by the morphological differences alone. A larger total conductance was required to reproduce the experimental finding of an increased spike count and a larger spike amplitude in a morphological subtype with a larger membrane area. In conclusion, biophysical properties, morphological details, and ion channel distribution across the membrane all interact in their contribution to the functionality and response variability of neurons of the same cell type.
Author summary
Neurons of the same type can exhibit differences in their shape or their biophysical properties. This variability, known as degeneracy, allows the nervous system to function reliably through many alternative mechanisms. While differences in ion channel properties and coarse branching patterns are well-understood contributors to response variability, the influence of fine morphological details remains less clear. Using computational models of touch-sensitive neurons in the leech, we examined how neuronal shape, electrical properties, and the location of spike initiation zones affect their spiking activity. Based on 15 reconstructed neurons, we created thousands of models that reproduced experimentally observed activity. Even with identical electrical properties, we found that small morphological differences alone produced variability in spike timing, size, and number. Furthermore, realistic responses were obtained with different spike initiation sites and from uniformly distributed ion channels. Finally, to reproduce the systematic differences observed between two neuronal subtypes, our models required not only morphological differences but also distinct total membrane conductances. Our results reveal how morphology, ion channel distribution, and electrical properties interact to generate variability in neuronal responses, highlighting multiple pathways by which nervous systems achieve reliable function.
