Stimulation-induced ectopicity and propagation windows in model damaged axons

Area of Science:

• Neuroscience
• Computational Biology
• Biophysics

Background:

• Neural tissue injuries disrupt voltage-gated sodium channels (Nav), altering neuronal excitability and causing neuropathic pain.
• Membrane damage induces a hyperpolarizing shift in Nav channel kinetics (coupled left-shift, CLS), creating persistent subthreshold conductance.

Purpose of the Study:

• To investigate if mild Nav channel damage (CLS) in myelinated axons renders them vulnerable to further impairment under normal physiological conditions.
• To explore the mechanisms by which CLS contributes to hyperexcitability, ectopic activity, and altered action potential propagation.

Main Methods:

• Utilized a 10-node myelinated axon computational model to simulate scenarios of incipient diffuse axonal injury with varying degrees of CLS.
• Analyzed the impact of CLS on ion homeostasis, axonal excitability, and action potential (AP) burst generation.
• Investigated signal propagation fidelity through simulated ectopic nodes under different input frequencies and noise conditions.

Main Results:

• Mild CLS can transform quiescent, damage-restabilized nodes into ectopic signal generators, stressing Na+/K+ gradients and altering AP thresholds.
• Ectopic firing sites exhibit a 'propagation window,' allowing faithful signal transmission only when input frequencies exceed the ectopic firing frequency.
• This phenomenon remains robust despite Gaussian noise and jitter, highlighting vulnerability in diffuse axonal injury.

Conclusions:

• Incipient axonal injury, characterized by mild CLS, can induce ectopic activity and impair signal propagation, contributing to neuropathic pain and channelopathies.
• The 'propagation window' mechanism suggests specific conditions under which neuronal signals can be reliably transmitted through damaged axonal segments.
• Findings offer insights into acquired sodium channelopathies and the amplification of sensory inputs in neuropathic conditions.