Comparison of the morphology of physiologically identified abducens motor and internuclear neurons in the cat: a light microscopic study employing the intracellular injection of horseradish peroxidase.

Abducens motoneurons and internuclear neurons were identified electrophysiologically in anesthesized, paralyzed cats and stained by intracellular injection of horseradish peroxidase. Neurons were reconstructed and surface area of selected cells measured by light microscopy. Surface area of motoneurons and internuclear neuron with similar soma size and shape were roughly comparable. Dendrites of motoneurons were highly tapered and highly branched. By contrast, dendrites of internuclear neurons were less tapered and less branched. Axons of motoneurons had no collaterals within the brainstem. Internuclear axons crossed the midline at the level of their parent somata and ascended in the medial longitudinal fasciculus toward the oculomotor nucleus. Approximately 30% of the internuclear axons branched in the contralateral medial longitudinal fasciculus sending a fine collateral caudal toward the prepositus hypoglossi nucleus. The results suggest that, on the average, structural correlates of injected neurons (i.e., soma-dendritic morphology) can account at least in part for the earlier firing and higher intraburst frequencies of internuclear neurons versus motoneurons during on-direction rapid eye movements in alert cats.