The shape of the membrane potential trajectory in tonically-active human motoneurons

An innovative technique for estimating the shape of the membrane potential trajectory in voluntarily activated human motoneurons is described. This method involves establishing stimulus-induced interval histograms (SIIH) using the H-reflex algorithm, in which the common peroneal nerve was stimulated and the action potentials of voluntarily-activated low-threshold tibialis anterior motor units were recorded using intramuscular wire electrodes. At least two different stimulus intensities were used for each motor unit and SIIHs for both stimuli were established and compared. It is proposed that the position of the mode of a SIIH (as the abscissa) and the amplitude of a given excitatory postsynaptic potential (EPSP) or its normalized S value (as the ordinate) can be used to estimate the coordinates of a point on the average membrane potential trajectory as it approaches the firing threshold. The new estimated 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 threshold. This phase continued for about 20% of the interspike interval (ISI) and was terminated by a rapid depolarization phase, 'subthreshold rectification'. Using this new trajectory, the previously estimated EPSP and IPSP amplitudes can be reexamined where the trajectories were assumed to approach the firing threshold linearly. The new trajectory may help explain the peak that follows an inhibitory reflex, and the difficulty in grading the stimulus intensity for reflex studies.