Syncytial Nets vs. Chemical Signaling: Emerging Properties of Alternative Integrative Systems

Here, we critically evaluate the communication and integrative mechanisms in ctenophores or comb jellies, the descendants of the earliest animal lineage. Ctenophores convergently evolved their neuro-muscular systems compared to other animals. Besides the canonical neural organization with distinct synapses, recent 3D electron microscopy reconstruction revealed unique syncytial-type connectivity in some neuronal populations, providing additional support for the independent origins of neurons in ctenophores. Furthermore, it was proposed that non-synaptic transmission is the distinct characteristic of ctenophores, in contrast to other metazoans and Cajal’s neuronal doctrine, highlighting Golgi’s reticular theory. However, the astonishing diversity and higher information capacity of classical synapses and volume transmission indicate that chemical signaling is the hallmark of neural and other integrative systems regardless of their origins.