Ionic Plasmon-Polaritons in Neural Signaling II: Control Role of the Myelin over Frequency and Speed of Stimulus in Axons

This work presents a mechanism for controlling stimulus speed within the plasmon-polariton model of fast stimulus kinetics during saltatory conduction in myelinated axons. Myelin thickness plays a crucial role, but the related mechanism differs from that in the conventional cable model. The thickness of the myelin sheath precisely tunes the plasmon-polariton frequency and the associated signal velocity. This frequency is limited by the timescale of triggering the opening of sodium channels at the nodes of Ranvier, allowing for the initiation of the Huxley-Hodgkin cycles. Synchronization of the plasmon-polariton stimulus frequency with these cycles at consecutive nodes of Ranvier ensures the balance of Ohmic losses and enables arbitrarily long-range axon firing. The myelin layer thickness assessed by the model, considering the molecular structure of sodium channels, is consistent with observations. This model of plasmon-polariton stimulus control in myelinated axons completes the theory of saltatory conduction presented in Ionic Plasmon-Polaritons in Neural Signaling I: Structure and Dynamics of Plasmon-Polaritons in Myelinated Axons, Plasmonics 20, 4195-4220 (2025), https://doi.org/10.1007/s11468-024-02694-7 .