Preferential generation of recurrent responses by groups of motor neurons in man. Conventional and single unit F wave studies

Conventionally recorded surface F waves over the abductor digiti minimi muscle sampled a wide range of conduction velocities (CVs). Single motor unit (MU) F responses to threshold stimulation of the ulnar nerve were recorded with bipolar needle electrodes (BNE); the mean latencies of their fastest associated surface single unit M and F potentials were similar to those of the compound muscle action potential and fastest F wave, indicating that the fastest F wave indexes adequately the fastest motor fibre of the ulnar nerve. The mean surface unit F amplitude of MUs recorded by a BNE was similar to that of voluntary MUs recorded with a spike-triggered averaging technique; an estimated mean of 2-3 MUs per F wave was found by two methods. The frequency distribution of estimated F wave CVs was shifted towards faster values than expected from available studies of the distribution of CVs in single peripheral nerves; it was also higher than predicted from the expected relation between this distribution and the number of MUs per F wave, if an equal chance of activation and recording is assumed for each MU. There was a significant positive correlation between the frequency of F responses and their CV in 81 single MUs recorded by a BNE and tested with 200 threshold stimuli. These findings are consistent with preferential generation of recurrent responses by larger MUs; it may relate to a lesser chance of antidromic discharge in the smaller motor neurons and to a greater chance of collision of orthodromic (reflex) and antidromic impulses in their axons. The higher than expected percentage of all F waves that were repeater shapes or waves, and the presence of several distinct peaks in the distribution of intervals between repeaters of the same shape, suggest special, and heterogeneous, functional and anatomical arrangements in the groups of motor neurons generating them. Repeaters had greater amplitude and area, but similar latency and duration, than F wave shapes that did not repeat, suggesting that the former have a larger number of component MUs.