The role of the accessory abducens nucleus in the rabbit nictitating membrane response

Electrolytic and knife-cut lesions were employed in the rabbit to examine the role of the VIth cranial nerve, and of the motoneurons in the abducens (ABD) and accessory abducens (ACC) nuclei that supply the VIth nerve, in the reflex extension of the nictitating membrane. The nictitating membrane response (NMR) was elicited by tactual stimulation of the cornea with a puff of air or by electric shock delivered to the skin over the paraorbital region of the head. Total destruction of the VIth nerve or interruption of all ACC inputs to the VIth nerve (while leaving ABD inputs intact) produced a large and comparable reduction in the magnitude of the NMR elicited by air puff, although a small residual NMR of less than 1 mm could still be detected. In contrast, the magnitude of the NMR elicited by shock was not affected by ACC isolation and only reduced by 50% after VIth nerve lesions. Total isolation of ABD inputs to the VIth nerve (while leaving ACC inputs intact) had no effect on NMR magnitude elicited by either air puff or shock. The small residual NMRs to air puff and the larger NMRs to shock remaining after total destruction of the VIth nerve were not eliminated by the removal of all extraocular muscles (while leaving the retractor bulbi muscle intact). However, knife cut lesions that interrupted all ACC inputs to the VIth nerve and transected the VIIth (facial) nerve completely eliminated NMRs elicited by both air puff and shock. The results of this study indicate that NMRs elicited by tactual stimulation of the cornea are primarily mediated by retractor bulbi motoneurons in the ACC nucleus via the VIth nerve. In contrast, NMRs elicited by electric shock delivered to the skin over the paraorbital region of the head are produced by contraction of the retractor bulbi muscle via the VIth nerve and by contraction of the orbicularis oculi muscle via the VIIth nerve which then squeezes the nictitating membrane over the cornea.