"The Four Channels"

Neurons don't communicate only through synapses. Extrasynaptic signaling — neurotransmitters and neuromodulators released into the extracellular space rather than across a synaptic cleft — carries a parallel stream of information that operates on different timescales and follows different network topology. In C. elegans, where the complete connectome is known, Sunil, Benali, and Moutuou apply a thermodynamic framework to map both channels simultaneously.

Four communication regimes emerge. First: topology-dependent circuits where synaptic architecture directly determines function, reinforcing motor control pathways. The wiring diagram is the function. Second: a modulatory layer where extrasynaptic signaling tunes and regulates behavioral states — not executing specific movements but setting the context in which movements occur. Third: purely extrasynaptic networks supporting homeostatic regulation — maintaining internal state without fast synaptic transmission. Fourth: rapid synaptic pathways mediating sensorimotor responses where speed is essential.

The structural insight is complementarity. Synaptic and extrasynaptic signaling aren't redundant — they're optimized for different operational requirements. Speed versus modulation. Precision versus robustness. Reaction versus regulation. The nervous system runs multiple communication architectures in parallel, each suited to a different functional demand, layered on the same physical substrate.

This matters beyond C. elegans because the same distinction — fast point-to-point transmission versus slow volume signaling — exists in every nervous system. The four-regime decomposition may be a general principle of neural architecture: different communication modes don't just coexist, they partition the functional space into complementary domains that together cover what no single mode could handle alone.