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I simulated a fiber response to extracellular stimulation steered by a sinus wave:
- first, the optimal stimulation threshold (max amplitude of a sinus wave in mA) was found by stimulating with different amplitudes (binary search) until the chosen amplitude value in mA produces exactly one AP recorded at one of the nodes (close to the end of the fiber).
- Then, the fiber response is recorded while being stimulated for 0.5 sec with the found amplitude.

These are the results that I see:

Zoomed in:


I expected to see APs (or bursts) with some periodicity, but in reality there are 6 AP close by in the very beginning of the stimulation and nothing afterwards, though stimulation is going on until 500 ms.
How do I interpret this? Is the axons blocked? If so, for how long? What do I need to do to clarify that? is this effect moderated by by the stimulation frequency? Thanks

This is a perfect opportunity to use what you know about initiation and propagation of action potentials to formulate a hypothesis that might account for your observation. Then design, perform, and interpret a computational experiment that tests this hypothesis.