Phasic and tonic firing properties in rat oculomotor nucleus motoneurons, studied in vitro

Alert-chronic studies show that ocular motoneurons (Mns) exhibit a phasic and tonic firing correlated with eye saccade-velocity and position (fixation), respectively. Differences in the phasic and tonic firing among Mns depend on synaptic inputs and/or the intrinsic membrane properties. We have used in vitro slice preparation to investigate the contribution of membrane properties to firing properties of Wistar rat oculomotor nucleus Mns. We recorded different discharge patterns and focused on Mns with sustained discharge (type I) because they were the most abundant, and their firing pattern resembles that reported in alert preparations. Various differences divided these Mns into types I-A and I-B; the afterhyperpolarization (AHP) phase of the spike was monophasic in I-A and biphasic in I-B; I-A Mns showed tonic or phasic-tonic firing depending on the current intensity, while I-B Mns showed phasic-tonic discharge; the phasic firing was higher in I-B than in I-A Mns; I-A Mns fired in a narrower range than did I-B Mns; and I-A Mns showed lower maximum frequency than did I-B Mns. In conclusion, I-A and I-B Mns show different phasic firing properties and dynamic range, supported by intrinsic membrane properties. We suggest that I-A and I-B Mns innervate fast-twitch muscle fibres with different contraction speeds, and could contribute to generating a fine phasic signal for a graded muscle contraction. Finally, we have demonstrated an inverse relationship between Mn thresholds and tonic firing gain, concluding that intrinsic membrane properties could not support the covariation between tonic firing gain and recruitment thresholds reported in alert studies.