Motor axon loop: an electroneurographic response.

A new kind of response to the stimulation of the median and ulnar nerves was observed in 10 patients. It is a motor unit showing a late latency in response to a weak stimulus and then suddenly a normal latency when the intensity is increased to a critical level. These two responses can be evoked at any level of a distinct nerve segment, the difference between their latencies being constant. The late response cannot be elicited below this segment and the normal response cannot be elicited above this segment. Its F-latency is delayed and does not vary when the late response changes in a normal one. The conduction velocity along the axon seems normal. We propose the hypothesis of a motor axon loop along the nerve, with the recurrent portion traveling in the " grandes ellipses" or the classical anastomosis. These kinked axons are probably congenital; their frequency is still unknown.