Intracellular study of oculomotor neurons in the rat

The electrical and morphological properties of oculomotor neurons were investigated in the rat with intracellular recordings and intracellular horseradish peroxidase staining. Motoneurons were identified by their antidromic response to electrical stimulation of the ipsilateral medial rectus muscle. The antidromic action potential was followed by a delayed depolarization and an afterhyperpolarization of 20-50 ms in duration. The whole neuron input resistance calculated from intensity/voltage curves, was found between 4 and 15 M omega. Passive membrane properties showed the existence of anomalous rectifications. Motoneurons were studied on the basis of their responses to long-lasting depolarizing current pulses. The intensity/frequency curves suggest the existence of two ranges of discharges. The average intensity frequency slope during the steady state was 33 imp/s/nA. Ten oculomotor neurons were intracellularly labelled with horseradish peroxidase and fully reconstructed. The soma (23-33 microns in diameter) gave off five-eight primary dendrites which could extend over 600-800 microns from the soma. The oculomotor neurons were principally oriented in the sagittal plane. The soma size of oculomotor neurons was not related to the size of proximal tree. According to our observations, the morphological features of motoneurons did not allow us to predict the whole neuron input resistance. The comparison between in vivo and in vitro studies of oculomotor neurons revealed one major difference in the input resistance of the whole neuron which was three times higher in slices.