Anatomical study of the rabbit's corneal-VIth nerve reflex: connections between cornea, trigeminal sensory complex, and the abducens and accessory abducens nuclei.

The corneal-VIth nerve reflex of the rabbit, involving retraction of the eyeball by the retractor bulbi muscle and the correlated extension of the nictitating membrane, has been suggested to be mediated by retractor bulbi motoneurons in the accessory abducens-(ACC) nucleus but not by those in the abducens (ABD) nucleus, and to consist of both a fast, disynaptic, component and a slower component mediated by the reticular formation (RF). We, therefore, employed the anterograde and retrograde transport of horseradish peroxidase (HRP) to examine the neural connections between anatomical structures proposed to be involved in the afferent limb of the corneal VIth nerve reflex. The transganglionic transport of HRP from cornea indicated a primary projection to the ventral half of pars oralis of the trigeminal sensory complex. The retrograde transport of HRP infused into ACC resulted in a bilateral labeling of cells in ventral pars oralis with 75% of the labeled cells being ipsilateral to the side of infusion. In contrast, there was no retrograde labeling of cells in the trigeminal sensory complex after HRP infusions into ABD. Infusion of HRP into ACC and ABD also revealed retrogradely labeled cells in the RF caudal to these two nuclei and infusion of HRP into this area of the RF resulted in both the retrograde labeling of cells in ventral pars oralis and anterograde-like labeling in both ACC and ABD. These data provide anatomical support for a direct relationship of the ACC, but not ABD, to the trigeminal sensory system and for the suggested existence of two components of the corneal-VIth nerve reflex: a disynaptic component from cornea to ventral pars oralis which in turn projects only to the ACC nucleus; and a multisynaptic component consisting of projections from the ventral pars oralis to RF cells which, in turn, are premotor to the ACC and ABD nuclei.