Myo-axonal ephaptic responses and their F waves in case of chronic denervation.

Twenty-two ephaptic responses (ERs) were observed in chronically denervated muscles. The ER is a late potential, either evoked by every stimulus or inconstant, with most often an unstable latency. ERs are not artefacts since they are recorded with surface electrodes and again some days later. The ER seems to result from the ephaptic excitation of an axon by the synchronized late potential (LP) of a contiguous reinnervated motor unit. Both the ER and the LP present the same latency and excitability. Some ERs disappear in the presence of the direct response of the post-ephaptic axon or when applying a second stimulus. Ten ERs were accompanied by F waves. The origin of F1 was on the post-ephaptic axon, at the stimulation site. F1 never followed the ER, being cancelled by its antidromic wave. The pre-ephaptic origin of F2 was at the stimulation site. Its recurrent wave reached the post-ephaptic axon through the ephapse. The origin of the antidromic wave of F3 was on the post-ephaptic axon at the site of the ephapse. F3 followed the ER with a constant interval regardless of the stimulation site, and its latency increased with more proximal stimulation.