Electrical Activity of Rat Ocular Motoneurons Recorded in Vitro

The electrophysiological properties of rat oculomotor neurons were studied in an in vitro slice preparation. Motoneurons were identified by their antidromic response to third nerve rootlet stimulation, and by their orthodromic responses to medial longitudinal fasciculus and reticular stimulations. Passive membrane properties showed the existence of an inward rectification mechanism in all the recorded motoneurons. The action potential is comprised of several distinct components. The fast initial spike, composed of an initial segment spike and a somatodendritic spike, is followed by a delayed depolarization, an afterhyperpolarization and a late afterdepolarization. The afterhyperpolarization has a maximum duration of 55 ms. The late afterdepolarization is a voltage-dependent mechanism that produces an oscillatory behavior in depolarized cells. Two types of motoneurons were distinguished on the basis of their response to long-lasting depolarizing current pulses. The intensity-frequency curves show the existence of a primary and secondary range of discharge and the study of the interspike intervals points to specific properties of the conductance underlying the afterhyperpolarization. It is concluded that large, stellate motoneurons of the brainstem maintained in vitro retain specific electrophysiological properties, comparable to those described in vivo and which differentiate the ocular motoneurons from spinal motoneurons.