Double discharges of distal origin: influence on the firing rhythm.

Double discharges, or doublets, following the direct M response after peripheral nerve stimulation had a distal origin on the axon and were accompanied bay a back-response. This antidromic wave was demonstrated by collision technique or by its occasional recurrent F wave. Double discharges (DD) with a long interval between the two components had their origin in one subunit and were transmitted to another by motor axon reflex. Increse of the post-doublet interval (PDI) duration during EMG was best explained by invasion of the alpha cell by the back-response and the resetting of a new cycle. This invasion was sometimes followed by a recurrent F wave: a shorter PDI resulted. Normal PDI were probably due to blocking of the back-response at the myelinated-non-myelinated junction, without alpha cell invasion: weak or absent Renshaw inhibitory effect on the alpha cell concerned seemed to play a role.