CNS*2026 Halifax

Introduction

Recent work [1] on the ctenophore M. leidyi has shown that the standard architecture of excitable neurons connected by neurites terminated by synapses is not universal. The ctenophore subepithelial nerve net (SNN) is a continuous cytoplasmic network lacking chemical or electrical synapses. Electron micrographs revealed a nerve net with a beaded or “pearls-on-a-string” morphology in which spherical swellings alternate with thin cylindrical constrictions [1]. This morphology appears to be heterogeneous, with varying degrees of pearls (or beads) and connectors between them. The overall function and electrical measurements of the M. leidyi SNN are not yet known, inviting investigation into basic models of possible signal propagation modes.



Methods

We model the structure of the SNN as consisting of a variable number of polygons embedded on a circular disk, respecting the organism’s physical dimensions. The aboral organ and the comb rows are modeled as excitable tissue with the help of a standard diffusively coupled neuron model. The neurites connecting neurons are modelled as only partially excitable, with two parameters controlling their excitability, reflecting the special ‘blebbed’ morphology of the SNN. We then study activity spread on the computational domain by numerically solving the underlying partial differentials equations. We also derive an effective cable equation taking into account the blebbed morphology.


Results

A central function of the SNN is the conduction of a swimming or reversal signal from the aboral organ to all eight comb rows [2]. We examine the conditions under which this can occur using our model, and derive constraints on neurite heterogeneity. We discuss neuropeptides and other biophysical stimuli as drivers for the beaded morphology. Using simulations of excitable and diffusive elements on a network, we parameterize the model with a modified cable equation and compare the conduction speed and directionality in such networks to the observed ciliary beating. We use our modified cable equation to gauge the accuracy of parameters used in our simulations and suggest that the SNN uses neurite morphology to adjust propagation delays.


Discussion

Taken together we theoretically investigate SNN neurite structure and heterogeneity for its consequences for signal propagation on multiple scales. Notably, the same organism at a later developmental stage possesses a second syncytial nerve net - the mesogleal nerve net - whose neurites display a distinct cylindrical morphology [4]. The coexistence of two nerve nets with morphology in the same animal suggests a link between SSN morphology and function. Indeed, in mammalian central nervous system axons, remodelling of an analogous beaded morphology fine-tunes action potential conduction velocity and overall neuronal function was recently discussed [3]. Yet the connection to the synapse-free ctenophore SNN has not been exploreduntil now. 


References

1. Burkhardt, P., et al (2023). Syncytial nerve net in a ctenophore adds insights on the evolution of nervous systems. Science, 380(6642), 293–297. https://doi.org/10.1126/science.ade5645
   
2. Tamm, S. L. (2014). Cilia and the life of ctenophores. Invertebrate Biology, 133(1), 1–46. https://doi.org/10.1111/ivb.12042
   
3. Griswold, J. M., et al (2025). Membrane mechanics dictate axonal pearls‑on‑a‑string morphology and function. Nature Neuroscience, 28(1), 49–61. https://doi.org/10.1038/s41593-024-01813-1
   
4. Jokura, K., Jasek, S., Niederhaus, L., Burkhardt, P., & Jékely, G. (2026). Neural connectome of the ctenophore statocyst. eLife, 14, e108420. https://doi.org/10.7554/eLife.108420
   

Acknowledgement

JS acknowledges funding by the European Union (ERC, SYNNEURO, 101163768). Views and opinions expressed are however those of the author(s) only and do not necessarily reflect those of the European Union or the European Research Council. Neither the European Union nor the granting authority can be held responsible for them.