Figure 1. Schematic representation of the experimental design and arrangement of the horizontal oculomotor system.

The abducens (ABD) nucleus is composed of motoneurons (Mn, caged circle) that innervate the ipsilateral lateral rectus (LR) muscle through the VIth nerve, and internuclear interneurons that cross the midline and project via the medial longitudinal fasciculus (MLF) to the contralateral oculomotor (OCM) nucleus, reaching the Mn that innervate the medial rectus (MR) muscle. Teflon-coated stainless-steel coils were fixed to the scleral margin of both eyes. A bipolar stimulating electrode (St) was implanted on each VIth nerve to stimulate Mn antidromically in the ABD nucleus. A glass micropipette was introduced into each ABD nucleus to record the activity of identified Mn. The recording activity of each ABD Mn began during alertness and was maintained to the end of the next rapid eye movement sleep phase.