Ultrastructural observations of the abducens nucleus of the cat after injection of horseradish peroxydase into the lateral rectus muscle.

The ultrastructure of cat abducens motoneurones is described, after labelling by retrograde axonal transport of horseradish peroxydase. The motoneurones were densely covered with synaptic boutons of both the "terminal" and "en passant" types and axo-axonal synapses were observed on the initial segment of motoneurone axons. Within the population of axons forming the VIth nerve, multiple branching was encountered and axons closely apposed at the nodes of Ranvier were seen.