THE SHAPE OF THE MEMBRANE-POTENTIAL TRAJECTORY IN TONICALLY-ACTIVE HUMAN MOTONEURONS

An innovative technique for estimating the shape of the membrane poten tial trajectory in voluntarily activated human motoneurons is describe d. This method involves establishing stimulus-induced interval histogr ams (SIIH) using the H-reflex algorithm, in which the common peroneal nerve was stimulated and the action potentials of voluntarily-activate d low-threshold tibialis anterior motor units were recorded using intr amuscular wire electrodes. At least two different stimulus intensities were used for each motor unit and SIIHs for both stimuli were establi shed and compared. It is proposed that the position of the mode of a S IIH (as the abscissa) and the amplitude of a given excitatory postsyna ptic potential (EPSP) or its normalized S value (as the ordinate) can be used to estimate the coordinates of a paint an the average membrane potential trajectory as it approaches the firing threshold. The new e stimated average membrane potential trajectory is at variance with the previously suggested linear trajectory. This new trajectory displayed a 'slow depolarization' phase at around 2 mV from the firing threshol d. This phase continued for about 20% of the interspike interval (ISI) and was terminated by a rapid depolarization phase, 'subthreshold rec tification'. Using this new trajectory, the previously estimated EPSP and IPSP amplitudes can be reexamined where the trajectories were assu med to approach the firing threshold linearly. The new trajectory may help explain the peak that follows an inhibitory reflex, and the diffi culty in grading the stimulus intensity for reflex studies.