Dynamics of Excitability in Axonal Trees
We report that axons of cortical neurons, structurally intricate excitable media, maintain somatic spike timing with high fidelity during spontaneous network activity, even at short (2--3 msec) inter-spike intervals. However, long series of external stimulation at physiological frequencies expose a vulnerability that varies depending on distance and branching. At 10 Hz, propagation delays increase within seconds, and failures accumulate over time, particularly in distal axonal branches. These effects are negligible at frequencies of 1--4 Hz. We attribute this phenomenon to the gradual and cumulative inactivation of sodium channels at vulnerable sites. Simulations incorporating this mechanism replicate the observed dynamics. Our findings underscore a limit on axonal reliability. To the extent that a sustained (seconds) drive at ca. 10 Hz or higher is physiologically feasible, axons should be regarded as active, heterogeneous structures with context-sensitive excitability.