We have attempted to reconcile the different patterns of distribution
of interspike intervals that are found in motoneurones made to discharge
by intracellular injection of constant current in reduced animal preparations
and by voluntary control in human subjects. We recorded long spike trains
from single motor units in three human muscles made to discharge at constant
mean frequencies with the help of auditory and visual feedback. The distribution
of interspike intervals in each spike train was analysed quantitatively.
We found that the different pattern of discharge of the human motor units
could be accounted for when due allowance was made for the variability
of the drive to the human motoneurone which arose because of the feedback
process used to maintain the target frequency. A model testing this hypothesis
gave results that were qualitatively consistent with the human data.