Saccadic discharges evoked by intracellular stimulation of extraocular motoneurons

Saccadic eye movements are preceded by a stereotyped neural command from the participating extraocular motoneurons. Depending on the size of the saccade, motoneurons which innervate the agonist muscle transiently increase their rate of discharge to 200-800 impulses/s for 10-100 ms. This burst causes the steplike saccade and is proportional to saccadic amplitude. The burst is followed by a maintained increment in steady state discharge which is proportional to commanded eye position. The pulse step increment in frequency causes a pulse step increment in force which overcomes the viscosity of the extraocular muscle and produces the step (saccadic) change in eye position. In order to elucidate the premotoneuronal command signal which causes these stereotyped discharges, antidromically identified extraocular motoneurons in the anesthetized cat were stimulated intracellularly with various waveforms of current. The purpose was to find the simplest current waveform which would evoke discharges in the anesthetized preparation and which would be parametrically analogous to the behavior of extraocular motoneurons in unanesthetized animals. By applying exponential depolarizing current steps of varying rise time (1-20 ms), amplitude (.5-10 nA), and duration (50-400 ms), discharges were evoked in both abducens and trochlear motoneurons which were similar to those seen during saccadic eye movements. An exponential step increase in depolarizing current evoked a high frequency burst, which decayed down to a steady state. The frequency and duration of this burst were sensitive to both amplitude and rate of the intracellular current stimulation. The frequency response of extraocular motoneurons to a step of depolarizing current could be described as the algebraic sum of three exponentials having time constants of t₁ = .005, t₂ = .027, and t₃ = 1.04 s. There was a discrepancy between the frequencies at which a sustained repetitive discharge of low variability could be evoked by intracellular stimulation (200-400 impulses) and the range of discharge frequencies recorded extracellularly in relationship to eye position (10-250 impulses/s). This discrepancy was related to the amplitude and duration of the afterhyperpolarization.